Transcript SCC CIS Radio Int Technical (Complete)

CIS Radio
Intermediate
Technical
CIS Radio Intermediate Technical – Subjects covered:-
Technical Basics
Units & Symbols – Electrical Circuits – Conductors & Insulators – Power & Resistance –
Ohms Law – Uses for resistors – Alternating Currents & Voltages – Different
frequencies – Radio Frequencies – Wavelengths – Other radio users
Transmitters
A simple transmitter – Modulation
Receivers
A simple receiver – Detector
Feeders & Antennas
Feeders – Antennas (Dipole, ½ wave, 5/8th wave. Yagi and End Fed) – Polarisation –
Matching Antennas – Antenna Tuning Unit – Standing Waves & SWR meters –
Baluns – Dummy Loads
Propagation
Spreading out – Buildings – Range – The ionosphere – Frequency & time of day
Electromagnetic Compatibility
How is interference caused – What can be done to minimise interference – Earthing –
Choice of antenna – Power and modes of transmission – Immunity – The
neighbours – Other hazards
Safety Considerations
High Voltage – High current – Mains & earths – Protective Multiple Earthing – Accidents
& emergencies – Antennas and feeders – Car batteries – Headphones – Other
hazards
Units & Symbols
Big and Small Units
You should know:-
• milli (written m) means thousandths of
– e.g. 10 mA = 10 thousandths of an Amp
• Kilo (written k) means thousands of
– e.g. 10kW = 10 thousand Watts
– e.g. 1kG = 1 thousand grams
• Mega (written M) means millions of
– e.g. 1M W = 1 million ohms
Converting between units
•
•
•
•
•
•
•
•
100mA
100mA
10mA
10mA
10M W
10M W
1k W
1k W
= ??? A
= .1 A
= ??? A
= .010 A
= ??? W
= 10,000,000 W
= ??? W
= 1,000 W
Volts
• Potential Difference is measured in Volts
– e.g. Car Battery 12 Volts
– e.g. AA Cell 1.5 Volts
– e.g. PP3 Battery 9 Volts
• Voltage is a bit like the electrical form of
“pressure” - the higher the voltage, the
more current will flow through a circuit.
• Symbol is the letter V
Amps
• Electrical current is measured in Amps
(short for Amperes)
– e.g. starter motor for car is about 100 Amps
– e.g. electric kettle is about 10 Amps
• Current is the quantity of electrons moving
through a wire every second - think of the
volume of water going through a pipe.
• Symbol is the letter I
The current is a measure of how much electricity is flowing. The unit of
measure is the amp
The current is made up of millions of small particles called electrons
which are electrically charged and move around the circuit carrying the
electricity.
These electrons need enough room to move and if a larger current is
expected to flow a larger diameter wire is needed.
The electrons get their energy from the supply or battery
The electrons have more energy when they enter a device than when
they leave it
The potential difference across a device shows this difference in
energy and is the energy being transferred to it often simply called the
‘voltage’
Potential Difference is measured in a unit called the
Volt. If more volts are measured across a device it
means the flow of electrons (current) is transferring
more energy to that device.
A 4 cell battery has a potential difference of 6V. It will
give the electrons four times as much energy as a
single cell (1.5V) and will cause the electrons to flow
Quicker.
Note: If we use mains supply the potential difference
is 230V. Enough to cause serious injury or even kill
you.
Ohms
• Electrical resistance is measured in Ohms
• Electrical resistance is the property of
material to resist the movement of
electrons.
• Think of plastics as having very high
resistance, metals as having very low
resistance.
• Symbol is the Greek letter omega W
Power
• Power is the amount of energy used each
second.
• Power is measured in Watts
• Symbol is W
• Think of:
– Light bulbs 40W, 60W, 100W
– Kettle 2000W
Symbols
You should be familiar with the following symbols -
Cell
Battery
Fuse
Lamp
Resistor
Symbols
Switch single pole
single throw
Antenna
Earth
Microphone
Loudspeaker
Resistor Coding
Resistors are coded using colours -
Examples:
A)
If the first band is green (5) the
second digit is blue (6) and the
third band is orange (3), the
value of the resistor is 56000
ohm. Because 1000 Ohm = 1 K,
we have 56k
B) red, red, yellow. So we have 2, 2,
0000 or 220.000 Ohm
Electrical Circuits
Electrical Circuits
When you load a battery into an electronic device,
you're not simply unleashing the electricity and
sending it to do a task. Negatively charged electrons
wish to travel to the positive portion of the battery
and if they have to rev up your personal electric
shaver along the way to get there, they'll do it. On a
very simple level, it's much like water flowing down a
stream and being forced to turn a water wheel to get
from point A to point B.
Whether you are using a electrical mains or a battery to produce
electricity, three things are always the same:
1. The source of electricity must have two terminals: a positive terminal and a
negative terminal.
2. The source of electricity (whether it is a generator, battery or something
else) will want to push electrons out of its negative terminal at a certain
voltage. For example, one AA battery typically wants to push electrons
out at 1.5 volts.
3. The electrons will need to flow from the negative terminal to the positive
terminal through a copper wire or some other conductor. When there is a
path that goes from the negative to the positive terminal, you have a
circuit, and electrons can flow through the wire.
A simple circuit with a variable
resistor to increase or decrease
the bulbs brightness
You can attach any type of load, such as a light bulb or motor, in the middle of
the circuit. The source of electricity will power the load, and the load will perform
whatever task it's designed to carry out, from spinning a shaft to generating light.
Electrical circuits can get quite complex, but basically you always have the
source of electricity (such as a battery), a load and two wires to carry electricity
between the two. Electrons move from the source, through the load and back to
the source.
Moving electrons have energy. As the electrons move from one point to another,
they can do work. In an incandescent light bulb, for example, the energy of the
electrons is used to create heat, and the heat in turn creates light. In an electric
motor, the energy in the electrons creates a magnetic field, and this field can
interact with other magnets (through magnetic attraction and repulsion) to create
motion
This circuit shows a switch,
motor and battery
An electric circuit is the name given to the way electrical devices are
connected. By connecting a battery to a light bulb we make a circuit
Consisting of the battery, connecting wires and the bulb
The battery provides the electrical energy. The electricity flows out of
the positive side (+) along the connecting wire and into the bulb and
back along a wire to the negative side (-). Electricity needs a complete
circuit or path to flow round. Some electronic devices are very
sensitive to which way around the battery is connected. The light bulb
is a thin filament of wire in a glass bulb where all the air has been
sucked out (a vacuum). If electricity is passed through the bulb the
filament glows white hot and gives off light and heat. It does not matter
which way the electricity flows through the bulb for it to work. It has no
polarity.
Care must be taken that the battery is of the correct
voltage for the bulb or it will be either to dim or to
bright and possibly blow.
Battery Circuits
• A battery provides voltage but no current flows from it
unless there is a circuit connecting the positive
terminal and negative terminal together.
• Battery “pushes” in a particular direction - not
important for e.g. torch bulbs but usually very
important for electronic devices.
Conductors
&
Insulators
The connecting wires we used to connect the battery to the bulb
Is called a conductor and conducts electricity because the
electrons can move freely. Metals are conductors.
The wire should have a plastic sheath or covering. This is called
an insulator and does not conduct electricity. Electrons cannot
move in an insulator. Wood, rubber, glass and ceramics are also
insulators but NOT if they get wet.
Electricity may still be able to flow through any surface water
and can still give you an electric shock. Be very careful in wet
conditions
In a conductor, electric current can flow freely, in an insulator it cannot. Metals
such as copper typify conductors, while most non-metallic solids are said to be
good insulators, having extremely high resistance to the flow of charge through
them. "Conductor" implies that the outer electrons of the atoms are loosely bound
and free to move through the material. Most atoms hold on to their electrons tightly
and are insulators. In copper, the valence electrons are essentially free and
strongly repel each other. Any external influence which moves one of them will
cause a repulsion of other electrons which propagates, "domino fashion" through
the conductor.
Simply stated, most metals are good electrical conductors, most non-metals are
not. Metals are also generally good heat conductors while non-metals are not.
Insulators
Most solid materials are classified as insulators because they offer
very large resistance to the flow of electric current. Metals are
classified as conductors because their outer electrons are not tightly
bound, but in most materials even the outermost electrons are so
tightly bound that there is essentially zero electron flow through them
with ordinary voltages. Some materials are particularly good insulators
and can be characterized by their high resistance, plastic, wood, glass
etc.
Power &
Resistance
Volts
• Potential Difference is measured in Volts
– e.g. Car Battery 12 Volts
– e.g. AA Cell 1.5 Volts
– e.g. PP3 Battery 9 Volts
• Voltage is a bit like the electrical form of
“pressure” - the higher the voltage, the
more current will flow through a circuit.
• Symbol is the letter V
Amps
• Electrical current is measured in Amps
(short for Amperes)
– e.g. starter motor for car is about 100 Amps
– e.g. electric kettle is about 10 Amps
• Current is the quantity of electrons moving
through a wire every second - think of the
volume of water going through a pipe.
• Symbol is the letter I
Ohms
• Electrical resistance is measured in Ohms
• Electrical resistance is the property of
material to resist the movement of
electrons.
• Think of plastics as having very high
resistance, metals as having very low
resistance.
• Symbol is the Greek letter omega W
Resistance
Resistance is the measure of how difficult it is for electricity to
flow.
The symbol for resistance is R and is measured in ohms
(symbol Ω)
In electrical terms a device that restricts the flow of current is
called a resistor. How much difficulty it presents to the flow of the
current is called its resistance. The higher the value the more
resistance. If a voltage remained the same then higher resistance
will mean a lower current flowing. Increasing the voltage would
therefore increase the Current.
This relationship is called V=IxR or Ohms Law
Power
• Power is the amount of energy used each
second.
• Power is measured in Watts
• Symbol is W
• Think of:
– Light bulbs 40W, 60W, 100W
– Kettle 2000W
Ohms Law
Ohms Law
V=I x R
(so the voltage across a resistance is the product of the resistance
and the current through it)
Divide both sides of the top equation by I
V/I= R
(so the resistance of a circuit is given by the voltage across the
circuit divided by the current through it)
Or divide both sides of the top equation
by R
V/R=I
(so the current through a circuit is given by the voltage
across the circuit divided by the resistance)
Uses for
Resistors
In electrical circuits it is often necessary to deliberately limit
the flow of current. A resistor is used to insert some
resistance into a circuit. A low value resistor will have little
effect on the flow of current and the bulb will glow quite
brightly. A higher value resistor will have a greater limiting
effect on the current and the bulb will be much less bright.
If the resistance is too
high then the bulb may
not glow at all.
POWER
• Power is simply a measure of how quickly
a device transfers the energy we deliver to it.
• Power is simply the voltage times the current.
• Power is measured in a unit called the watt
• The symbol is P
• So P = V x I
(watts = volts x amps)
• A 1 watt light bulb transfers 1 unit of energy every
second to heat and light
Alternating
Currents
&
Voltages
Alternating current, AC or a.c., keeps changing
direction, first one way and then the other. AC is easier to generate and
easier to change from one voltage to another. Your mains electricity is
AC. The generator at the power station is a large coil of wire rotating
around a powerful magnetic field. If we look at just one wire in the
rotating coil, it is first going up through the field, then, half a turn later,
coming down. Whilst the wire is going up the voltage and current
generated has one polarity and moments later when its coming down
the voltage and current are of opposite polarity.
The electricity produced by a bicycle dynamo is
AC for the same reason. It is easy to produce and
perfectly good for powering the light bulbs. The
only problem is when the rider stops the lights go
out unless there is a battery powered backup.
The alternator in a car also produces AC but that is no use to charge the battery or
run the many electronics like the radio. Special electronics are required to convert
the AC into DC. That recharges the battery and powers all the electrical items
including lights (which could use AC or DC)
Alternating currents and voltages do not suddenly switch from one polarity to the
other. They build up to a peak in one direction, then reduce back to zero before
building up in the opposite direction. This smooth waveform is called a sine wave.
There are two features of AC or current that we need to know.
1. The size of the voltage
2. How often the cycles occur or the frequency.
Frequency is defined as the number of cycles occurring in 1 second.
The unit of measurement is cycles per second or ‘Hertz’ Hz.
Domestic mains supply is 230 volts and 50Hz meaning there are 50
complete cycles in one second.
Alternating Current AC
• AC reverses its direction several/many times per
second.
• Advantages:
– easier to generate (e.g. dynamo on bike)
– easier to step up and down to different voltages
• 1 cycle (one direction then the other direction) per
second is called 1 Hz
• Mains supply is 50Hz
Direct Current DC
Direct current or DC is the type of electricity that is produced by
batteries, static, and lightning. A voltage is created, and possibly stored,
until a circuit is completed. When it is, the current flows directly, in one
direction. In the circuit, the current flows at a specific, constant voltage
(this is oversimplified somewhat but good enough for our needs.) When
you use a flashlight, pocket radio, portable CD player or virtually any
other type of portable or battery-powered device, you are using direct
current. Most DC circuits are relatively low in voltage. e.g. your car's
battery is approximately 12 V. That's about as high a DC voltage as
most people will ever use.
Different
Frequencies
Sound is also an alternating signal carried by the movement of air. A
sound of 50Hz is a very low note, as much felt as it is heard. Human
hearing ranges from about 100Hz (a low note) to 15kHz (a very high
note). For high quality music a full range of frequencies is desirable but
for speech a much narrower range is sufficient. Usually 300Hz to 3kHz.
Typical of a good telephone line. Electrical signals in the leads to a
loudspeaker are alternating currents and voltages and many frequencies
may be present at the same time, depending on the sounds. A clean
whistle will be a single note and a single electrical frequency whilst music
is likely to contain many notes and hence many frequencies.
Some Typical Frequencies
• Normal hearing (pressure waves in air) 100Hz - 15kHz
(Audio Frequency - AF)
• Audio communication (electrical signals in wires etc)
300Hz - 3kHz
• HF, VHF, UHF radio signals are up to 1000MHz (Radio
Frequency - RF)
Sine Wave
Sine waves are produced by oscillators - think of
a swinging pendulum.
Electromagnetic waves are waves consisting of vibrating electric
and magnetic fields. Various frequencies of theses waves are
known as the Electromagnetic Spectrum
Radio
Frequencies
Radio frequencies or RF are generated by feeding alternating electrical
signals to an antenna. These frequencies are much higher than we can hear.
While the whole of the electromagnetic wave spectrum covers a huge range
of frequencies, radio waves themselves extend over a very large range as
well. Again it is useful to be able to easily refer to different sections of the
spectrum. To achieve this different designations are given to different areas.
The frequencies that are covered are split into sections that vary by a factor
of ten, e.g. from 3 MHz to 30 MHz. Each section is allocated a name such as
high frequency and these areas are abbreviated to give terms like HF, VHF
and so forth that are often used. Often talk is heard of VHF FM, or UHF
television.
The VHF and UHF refer to
the areas of the radio
spectrum where these
transmissions take place.
For ease of reference these
frequencies are divided up
into bands.
Waves with low frequencies or long wavelengths are known as radio
waves and are produced by causing electrons to vibrate in an
antenna.
Molecular excitation produces microwave and infrared waves
which have little higher frequency than radio waves.
A higher frequency of molecules make up the visible and ultraviolet
regions of the spectrum. A very small portion of the electromagnetic
spectrum is visible to the human eye. It is the ultraviolet radiation in
sunlight that tans or burns are skin.
The next step of higher frequency waves are called X-rays.
Lastly gamma rays are the highest frequency in the spectrum.
All electromagnetic waves travel at the same speed in a
vacuum. This speed is called the speed of light and is designated by
the letter C and = 299 792 458 m/s or 186000 miles a second
Areas of the radio spectrum where our radio transmissions take place
Wavelengths
What is wavelength λ ?
A wavelength is the distance in between the repeating units of a wave, as measured
from one point on a wave to the corresponding point in the next unit. For example,
the distance from the top -- called the crest -- of one wave unit to the crest of the next
is one wavelength. Wavelength is often designated by the Greek letter lambda λ.
Wavelength is inversely proportional to the frequency of a wave. In other words, the
shorter the wavelength is, the more wave units will pass in a given amount of time.
So, the WAVELENGTH of a wave is the distance between the same point
on two consecutive cycles
In wireless systems, this length is usually specified in meters,
centimetres or millimetres. Wavelength is inversely related to
frequency. The higher the frequency of the signal, the shorter the
wavelength. If f is the frequency of the signal as measured in
megahertz, and w isthe wavelength as measured in meters, then w
= 300/f and conversely f = 300/w
How a wave is made up
Frequency to Wavelength
There is a fixed relationship between frequency and
wavelength so that for radio waves, the frequency times the
wavelength is equal to the speed of light. 186000 miles a
second or 700 million miles an hour
You can use a conversion chart to convert frequency to wavelength
To convert frequency to Wavelength, select a point on the X or
horizontal axis of the graph, say 100MHz and look vertically upwards
to the diagonal line. Now look to the Y or vertical axis and read off the
wavelength. In this case 3m
Other Radio
Users
We share the use of the radio spectrum with many
other radio user's. Sometimes these other users are
on adjacent frequencies and where it is possible to
do so, we share these frequencies with other users.
For instance, the SCC shares frequencies with the
MOD. SCL14 for example on 6992.5 kHz which is
just below the bottom end of the amateur 40m or
7Mhz band which covers 7000 – 7200 kHz.
This diagram shows where and what these ‘other users’ are
in the frequency spectrum
The Radio Frequency Spectrum
Frequency Allocation Table
FREQUENCY
USE
87.5-108 MHz
108.0-117.975 MHz
117.975-137.0 MHz
137.0-138.0 MHz
138.0-144.0 MHz
144.0-146.0 MHz
146.0-149.9 MHz
149.9-150.05 MHz
150.05-152.0 MHz
152.0-156.0 MHz
156.0-158.525 MHz
158.525-160.6 MHz
160.6-160.975 MHz
Broadcasting
Aeronautical Radionavigation
Aeronautical Mobile
Space Operations and Space Research
Land Mobile
Amateur & Amateur Satellite
Mobile (except aeronautical mobile)
Radionavigation - Satellite
Radio Astronomy
Land Mobile
Maritime Mobile
Land Mobile
Maritime Mobile
An example of a
amateur radio
frequency band
plan for
the 14Mhz or 20m
band
Transmitters
The transmitter generates the radio waves with which
we communicate. It has the potential to cause problems
and it is important you understand a bit about how it
works. A simple transmitter consists only of a method of
generating the correct radio frequency plus an antenna.
To send a message you switch it either ‘on’ or ‘off’ in an
arranged way. Morse code is the most well known code
used to send letters or the alphabet, numbers and
punctuation marks simply by switching on and off. The
need to send voice or pictures meant that a method of
superimposing the information on the radio signal had to
be devised. This device is called the modulator
Modulation
Modulation is the process of getting the radio signal to carry an audio signal.
The signal before modulation is usually called the ‘carrier’.
There are two ways of modulating the carrier. The first is to vary the
amplitude of the carrier in time with the audio signal called Amplitude
Modulation or AM. The second is to vary the frequency of the carrier in time
with the audio signal called Frequency Modulation of FM.
Audio Signal
RF Carrier
Modulated
Waveform
Frequency
Modulatio
n
Amplitude
Modulatio
n
Transmitter Block Diagram
1) Audio Stage
2) Modulator
3) Frequency Generator (or Oscillator)
4) RF Power Amplifier
+ Microphone at left and antenna at right
The signal from the microphone is quite weak and needs to be
amplified in the audio stage box 1. Box 3 is the frequency generator.
This produces the frequency which the transmitter will use to transmit
the signal. This generator must be carefully designed and made to
ensure it works on the correct frequency. If not on the right frequency
the person you are talking to will not hear you. Importantly, you may
be transmitting on someone else's frequency, even outside the band
in use! This could cause problems if it was a frequency used for
safety etc. Box 2 is the modulator which takes the radio signal from
Box 3 and mixes it with the audio signal from Box 1 to produce a
modulated radio signal. This signal is not usually powerful enough to
transmit so we use a RF Power Amplifier to amplify the signal. Box 4
This signal is then fed to your antenna.
AM
In AM the top waveform is the audio signal from the
Microphone.
The carrier is the HF (closer spaced) wave of constant
amplitude in the middle.
The process of AM produces a wave of the same
frequency as the carrier, but its amplitude varies in
Time with the audio signal seen at the bottom.
FM
The top wave form is the signal from the
microphone.
The steady, HF wave is the carrier.
FM produces a modulated wave of constant
amplitude, but with the frequency varying in time
with the signal from the microphone. The frequency
does not vary much and the radio is tuned to the
centre of the frequency variations of the signal
Care must be taken with either type of modulation not to ‘over
modulate’, that is, not to have too strong a signal from the microphone
or audio stage feeding the modulator. This is usually caused by turning
the ‘microphone gain’ up too far. The effect is similar to turning up the
volume on a receiver except the effects are heard at the distant end of
your radio contact. Shouting into the microphone can also cause the
same problem.
Excessive AM will make the peaks of the modulated carrier too large
and reduce the troughs to zero. This distorts the audio which will sound
rough at the receiver. It will also cause interference to radio receivers
tuned to adjacent channels. This is important as it is a condition of your
licence that you do not cause interference to other radio users.
Excessive FM is also to be avoided and may cause interference to
neighbouring Users as well as risking poor quality audio signals to your
intended recipient.
Transmitters in brief
•
Oscillator defines the frequency on which the transmitter operates.
•
Incorrect setting of the oscillator can result in operation outside the SCC
or amateur band and thus cause interference to other users. This risk of
interference is why Foundation Licensees cannot use home made
equipment.
•
The audio signal from the microphone or data input is added to the radio
signal by varying either the height (amplitude) of the radio wave (AM) or
the frequency of the radio wave (FM).
•
Speech can be carried by AM, SSB, or FM
•
Data by CW or FSK
RF Power Stage
RF Power stage (Box 4) amplifies the radio signal. The power output must
be connected to a correctly matched antenna. Use of the wrong antenna
can damage the transmitter.
Excessive amplitude modulation causes distorted output and interference
to adjacent channels. Excessive frequency modulation will cause
interference to adjacent channels. So you need to set microphone gain
carefully.
Receivers
The radio signal is picked up by the antenna, which converts the radio
signal into electrical signals on the feeder and fed along the feeder to the
input of the receiver Box 1 which contains the tuning which selects the
wanted signal from all the hundreds of signals picked up by the antenna
And RF amplification which amplifies the wanted signal to bring it up to a
Suitable level for Box 2 which contains the detector. This recovers the
original modulating signal. It extracts the original audio signal from the
modulated signal as the carrier is no longer required. Detection is also
called de-modulation. The audio amplifier in Box 3 ensures the audio
signal is powerful enough to drive the loudspeaker or headphones Box 4.
The wanted radio signal is then selected by tuning the receiver to the
correct frequency.
Receiver Block
Diagram
Antenna at left - Feeder from antenna to receiver
1) Tuning and RF amplifier
3) Audio Amplifier
2) Detection / Demodulation
4) Loudspeaker/Headphones
• The receiver must pick up weak radio signals, select the
right signal from the thousands of signals being
transmitted, amplify the signal to a suitable level, extract
the audio (or data or picture) from the modulated
waveform and then present it to us in a suitable form
• The tuning stage selects one frequency from the entire
radio spectrum prior to amplification and demodulation.
• Tuning is done with a “tuned circuit” consisting of a coil
(inductor) and a capacitor connected together.
Detector
The type of detector used must be suitable for the method of
modulation being used by the transmitter.
This can be demonstrated using a radio receiver by setting the mode
switch to the wrong type of modulation. The correct mode like CW,
SSB, FM etc needs to be chosen to correctly recover the original audio
signal.
If is was a data signal sent at the transmitting end then a suitable
detector is needed and the selection of the correct mode (e.g. upper
sideband USB or lower sideband LSB or FM) at the receiving end is
just as important but may be harder to determine by ear.
Feeders &
Antennas
Feeder Cable
The wire connecting a transmitter to an antenna is called the feeder.
This carries the powerful radio frequency signals which will radiate
from any wire. To prevent radiation from the feeder it is usually
made as coaxial cable. This has a centre conductor which carries
the signal and an outer screen which confines the signal within the
cable. This outer screen is usually braided to provide a good
continuous covering. The inner conductor may be a single wire or
several strands twisted together. Two ‘feeders’ are shown here –
RG58 (50 Ohm) and twin feeder (450 Ohm)
Plugs
The correct type of plug must be used
BNC Plug
PL259 Plug
Coax braid is continuous through plug’s outer metallic case.
BNC typically used at VHF and low power. PL259 at HF and
higher power. The screen of the coaxial cable must be properly
connected to the body of the plug to ensure the screen and
plug form a continuous shield for the inner conductor which
should be soldered to the centre pin of the plug.
Antennas
• The purpose of an antenna
(sometimes called an aerial) is to
convert electrical signals on the
feeder into radio waves or vice
versa.
• It needs to be designed for the
frequency or wavelength in use and
there are five that we need to
consider, the dipole, 1/4 wave
ground plane, the Yagi, the 5/8th
wave and the end-fed
Five Types of Antenna
Yagi
5/8th Wave
Ground Plane
¼ Ground Plane
Half-Wave Dipole
The Long
Wire or
End Fed
The dipole
A dipole antenna is a radio antenna that can be made of a simple wire with
a center-fed driven element. It consists of two metal conductors of rod or
wire, oriented parallel and collinear with each other (in line with each other),
with a small space between them. The radio frequency voltage is applied to
the antenna at the centre, between the two conductors. These antennas are
the simplest practical antennas from a theoretical point of view. They are
used alone as antennas, notably in traditional "rabbit ears" television
antennas and as the driven element in many other types of antennas, such
as the Yagi. Dipole antennas were invented by German physicist Heinrich
Hertz around 1886 in his pioneering experiments with radio waves.
The dipole is a basic antenna and is half a wavelength long. This means the size
of the dipole and all other antennas must be suitable for the frequency it is
intended to use it for.
If it is mounted vertically it radiates equally in all directions. If mounted horizontally,
More common at HF it radiates well from the sides but not off the ends. Given the
Choice it should be side on to the desired direction of maximum signal but this
may not always be possible
The ¼ Wave Ground Plane
This antenna gets its name from the fact that the radiating elements are ¼
wavelength long or λ/4. The radiating or active element is always vertical.
Four horizontal wires called radials form the groundplane – an earthed
surface which acts like a mirror to radio waves.
The transmitted signal is ‘omni-directional’ radiating equally in all directions. It
does not radiate vertically.
¼ Wave Ground Plane
The Yagi
Highly directional antennas
such as the Yagi are commonly
referred to as "beam antennas"
due to their high gain. However
the Yagi design only achieves
this high gain over a rather
narrow bandwidth, making it
more useful for various
communications bands
(including amateur radio) but
less suitable for traditional radio
and television broadcast bands.
Amateur radio operators
("hams") frequently employ
these for communication on HF,
VHF and UHF bands.
The Yagi or Yagi-Uda RF antenna or aerial is one of the most
successful RF antenna designs for directive applications. It is
used in a wide variety of applications where an RF antenna
design with gain and directivity is required. It has become
particularly popular for television reception, but it is used in very
many other applications where an RF antenna design is needed
that has gain.
The full name for the antenna is the Yagi-Uda antenna. It was
derives it name from its two Japanese inventors Yagi and his
student Uda. The RF antenna design concept was first outlined in
a paper that Yagi himself presented in 1928. Since then its use
has grown rapidly to the stage where today a television antenna is
synonymous with an RF antenna having a central boom with lots
of elements attached.
5/8th Wave
This is a development of the ¼-wave groundplane. It is
better at directing signals towards the horizon, rather
than up in the air. It is always mounted with the active
element vertical and is omni-directional. The vertical
element is 5/8 of a wavelength long and due to its size
it is more often used on VHF and UHF frequencies
where the wavelengths are shorter.
The coil at the base is part of the matching of the antenna
to the coaxial cable.
The 5/8 wave has a slight gain over the 1/4 wave antenna . Like the 1/4 wave, the
5/8 wave is also used vertically and gives an omni-directional radiation pattern, but
the thing to note is that the 1/4 wave has no coil, whereas the 5/8 wave requires
the coil at the base of the antenna for impedance matching.
Just like the λ/4 wave the 5/8λ antenna is a favourite choice for mobile working,
especially on 2 metres and above, due to enhanced performance over the λ/4
wave but still with a relatively small size. As with the λ/4 the ground plane radials
are replaced by the vehicles' bodywork
End fed wire (long wire)
The end fed wire is simply a random length of wire attached to the
centre of a coax feeder or, more usually, linked directly onto the rear of
a suitable ATU that can take single wire. This is a poor antenna as it is
not tuned to any particular frequency and thus generally performs
badly relative to a dipole. It is unlikely to be a ¼ or ½ wavelength long
and matching it will be a problem. A device called an ‘antenna tuning
unit’ will match the wire and enable the antenna to accept power from
the transmitter. A likely minimum length of the wire for this antenna
would be around 80 feet but is often much longer.
The antenna is often set up with the far end fixed to a pole or tree and
the end closest to the transmitter secured by a length of rope to a wall
or chimney of a house with the end dropping down to the transmitter.
This results in high voltages or currents close to the house and the
strong radiation can upset TV’s and other electronic equipment. If this
antenna is your only choice it is better to feed it at the far end and bury
the feeder.
The Long Wire
Antennas (again)
• A dipole is always a half wavelength long
• A Yagi is directional
• A long wire can have high voltages or currents close to
the feed point
• A ground plane is omni directional
• 5/8th wave is better at directing signals to the horizon
• Remember:- The antenna system must be suitable for
the frequency of the transmitted signal. If it isn’t it will not
match the transmitter and will not work effectively
Polarisation
Depending upon how the antenna is orientated physically
determines it's polarisation. An antenna erected vertically is said to
be "vertically polarised" while an antenna erected horizontally is
said (not so surprising) to be "horizontally polarised". Other
specialised antennas exist with "cross polarisation", having both
vertical and horizontal components and we can have "circular
polarisation".
Note: When a signal is transmitted at one polarisation but received
at a different polarisation there exists a great many decibels of
loss. This is quite significant and is often taken advantage of when
TV channels and other services are allocated. If there is a chance
of co-channel interference then you can try a different polarisation.
Have you ever noticed vertical and horizontal TV antennas in
some areas. Now you know why.
Polarisation of a radio wave
A vertical dipole or Yagi will radiate a vertical radio wave and it
will also best receive vertical waves.
It does not matter which polarisation you choose as long as both
the transmit and receive antennas are the same. However,
groundplane and 5/8 antennas are always vertical. The
mobile antennas mounted to vehicles are vertical for
practical reasons.
There is an amateur convention at VHF and UHF that
when using FM the antenna is vertical as most mobile
operation uses FM and SSB operation uses Horizontal
polarisation.
Effective Radiated Power (ERP)
You should also be aware that in radio communications, effective radiated
power or equivalent radiated power (ERP) is a standardized theoretical
measurement of radio frequency (RF) energy using the unit watts, and is
determined by subtracting system losses and adding system gains. ERP
takes into consideration transmitter power output (TPO), transmission line
attenuation (electrical resistance and RF radiation), RF connector insertion
losses and antenna directivity, but not the height above average terrain or
HAAT. ERP is typically applied to antenna systems.
Effective radiated power or ERP is the product of power and antenna gain.
For example, 50 Watts to a Yagi with gain of 4 is 200 Watts ERP in the
direction the Yagi is pointing. This is equivalent to 200 Watts into an
antenna with no gain (which radiates equally in all directions)
Matching Antennas
Antennas & ATU’s
If the size of the antenna is correct, ½ wavelength long at the wanted
frequency if it is a dipole for example, then the antenna will match the
transmitter and feeder. This means that the power from the transmitter
will be properly radiated by the antenna.
If the antenna is used on the wrong frequency (not the correct size)
some of the power will reflected back down the feeder instead of being
radiated by the antenna.
• If the antenna has not been designed for the particular frequency at
HF an Antenna Tuning Unit or ATU makes it possible for the
antenna to accept power from the transmitter.
• It can allow a single antenna to operate on several bands
An antenna tuner, transmatch or antenna tuning unit (ATU) is a
device connected between a radio transmitter or receiver and its
antenna to improve the efficiency of the power transfer between them
by matching the impedance of the equipment to the antenna. An
antenna tuner matches a transceiver with a fixed impedance (typically
50 ohms for modern transceivers) to a load (feed line and antenna)
impedance which is unknown, complex or otherwise does not match.
An ATU allows the use of one antenna for a broad range of
frequencies. An antenna plus matcher is never as efficient as a
naturally resonant antenna due to additional induced losses on the
feed line due to the SWR (multiple reflections), and losses in the ATU
itself, although issues of pattern and capture area may outweigh this in
practice. An ATU is actually an antenna matching unit, as it is unable to
change the resonant frequency of the aerial. Note that similar
matching networks are used in other types of
equipment, such as linear amplifiers to
transform impedances.
Standing
Waves
&
SWR Meters
Standing Wave Ratio
SWR is used as an efficiency measure for transmission lines,
electrical cables that conduct radio frequency signals, used for
purposes such as connecting radio transmitters and receivers
with their antennas. A problem with transmission lines is that
impedance mismatches in the cable tend to reflect the radio
waves back toward the source end of the cable, preventing all
the power from reaching the destination end. SWR measures the
relative size of these reflections. An ideal transmission line would
have an SWR of 1:1, with all the power reaching the destination
and no reflected power. An infinite SWR represents complete
reflection, with all the power reflected back down the cable. The
SWR of a transmission line can be measured with an instrument
called an SWR meter and checking the SWR is a standard part
of installing and maintaining transmission lines.
The SWR is usually defined as a voltage ratio called the
voltage standing wave ratio or VSWR.
SWR Meter
The SWR meter or VSWR meter measures the standing wave ratio in a
transmission line. The meter can be used to indicate the degree of mismatch
between a transmission line and its load (usually a radio antenna), or
evaluate the effectiveness of impedance matching efforts.
Note: An SWR meter does not measure the actual impedance of a load (i.e.,
the resistance and reactance), but only the mismatch ratio. To measure the
actual impedance, an antenna analyzer or other similar RF measuring
device is required.
An SWR meter can measure the power flowing back down a feeder
Allowing the operator to adjust the ATU until the antenna system is
Matched and the reflected power to the transmitter is minimised.
If the reading on the meter has unexpectedly increased it means
That more signal is being reflected back from the antenna. The most
likely cause is damage to the antenna or moisture in the connectors.
A dipole is a good match only when cut to the correct length. At other
Lengths the match will be poorer depending on how far from the
Ideal length it is.
So we now know that • The SWR meter shows whether an antenna presents the
correct match to the transmitter. If it does, then it will reflect no
power back to the transmitter.
• If some power is reflected back there will be a “standing wave”
on the feeder.
• Tolerable levels of SWR are up to about 2:1
• High SWR’s measured at the transmitter end mean a fault in the
antenna or feeder not the transmitter.
Baluns
A balun is a device that joins a balanced line (one that has two
conductors, with equal currents in opposite directions, such as a
twisted pair cable) to an unbalanced line (one that has just one
conductor and a ground, such as a coaxial cable).
A balun is a type of transformer: it's used to convert an
unbalanced signal to a balanced one or vice versa.
Baluns isolate a transmission line and provide a balanced output.
A typical use for a balun is in a television antenna. The term is
derived by combining balanced and unbalanced.
In a balun, one pair of terminals is balanced, that is, the currents
are equal in magnitude and opposite in phase. The other pair of
terminals is unbalanced; one side is connected to electrical
ground and the other carries the signal.
A dipole is symmetrical with two halves the same. It is called a
‘balanced’ antenna and requires two signals, one for each half of
the dipole. The signals are balanced because when the wave on
one side is going up its going down on the other.
Coaxial cable is not electrically symmetrical and is called unbalanced.
It has one centre live conductor and an earthed screen and is not
suitable for connecting directly to a dipole. A balun takes the signal
from the coaxial cable and converts it to two signals suitable for
feeding to the dipole.
If the coaxial cable is connected directly to the dipole, RF current will
flow back down the screened of the cable. This current will radiate and
the screening properties of cable will be upset. Since this cable runs
back into your operating position radiation will take place inside the
room and may cause interference to electrical equipment
or even your neighbours.
Antennas - balanced and unbalanced
•
If the antenna is symmetrical (e.g.
dipole where both halves are the
same) it is called “balanced” and
needs symmetrical or “balanced”
feeder.
•
Co-axial cable is unbalanced but
can be used via a “balun”
(balanced-unbalanced
transformer).
Dummy Loads
Dummy load is not actually an antenna, it dissipates all transmitted
power in a form of heat. So what's the use of it? Well, it is presents
an ideal match for an output of your transmitter (usually 50ohms).
Since all power (virtually 100%) is transverted into heat there won't
be any interference to your neighbours while you do tuning and
testing. This is what dummy load is usually used for; testing and
tuning transmitters. If you don’t have dummy load, you can build
one easily from a BNC or other RF connector and the proper
wattage/value of CARBON resistor(s). DO NOT USE WIREWOUND
OR METAL FILM RESISTORS! A useful one can be constructed
with 4 -220 Ohm 1/4 watt resistors in parallel (220/4 = 55 Ohms)
with center conductor to outershell (ground) of an RF connector.
That is pretty close to 50 Ohms and if you use 1/4 watt resistors you
get a nifty 2 Watt Dummy Load for testing your equipment without
an antenna. Commercial Dummy loads are available.
Dummy Load
A dummy load is a screened resistor, capable of absorbing all the
power from the transmitter and presenting a good match i.e. no
power reflected. It must be well screened to minimise unwanted
radiation and is is connected instead of an antenna to allow the
transmitter to be set up and tested without radiating a signal.
Propagation
Schematic diagram showing the propagation of high-frequency
(shortwave) radio waves by reflection off the ionosphere
Specific ionization conditions vary greatly between day (left) and night
(right), causing radio waves to reflect off different layers of the ionosphere
or transmit through them, depending upon their frequency and their angle
of transmission. Under certain conditions of location, ionization, frequency,
and angle, multiple “skips,” or reflections between ionosphere and Earth,
are possible. At night, with no intervening layers of the ionosphere
present, reflection off the F layer can yield extremely long transmission
ranges.
Spreading Out
Propagation is the technical term for how radio waves behave once
they have left the antenna. Radio waves travel in a straight line unless
they are reflected off a suitable surface or are refracted rather light
going through a prism.
Radio waves also spread out from an antenna. Close to the antenna
they are concentrated so a receiving antenna will pick up a strong
signal. Further away the signal will be weaker. Too far away and the
signal will be too weak to receive.
Radio waves,
spreading from the
centre of a dipole
Buildings
Radio waves can penetrate buildings like an xray can penetrate skin but
bones will leave a shadow. Some of the energy is lost in penetrating the
building. In a basement or middle of a large building where there are
several walls to pass though the signal may be too weak to be of use.
The penetrating ability of radio waves depends on their frequency. For
lower frequencies in the MW and HF bands the wavelengths are large
and the buildings ‘appear’ fairly small in comparison. Such waves
penetrate the building quite easily but have difficulty with obstacles like
mountains. At higher frequencies like VHF/UHF the wavelengths are
much shorter and the buildings comparatively much bigger causing more
of a problem to the VHF and UHF waves.
There is a small advantage to higher frequencies. If the wavelength is
smaller than a window, the window appears as a big hole for it to get
through, then there will be a reasonable signal with a window on the side
facing the transmitter.
The best range with VHF and UHF radio services is
achieved when the transmit antenna is mounted
high up and clear of obstructions like trees and
buildings. It also helps if the receiving antenna is
sited in a similar way.
Broadcast transmitters have very tall masts
supporting the antenna for that very reason. It also
helps in getting the signal into dips behind hills and
into valleys. Such places would otherwise be in
shadow of a transmitting antenna and maybe not
receive the signal.
Range
The range you can achieve with a radio signal depends on a number
of factors. A more powerful transmitter will have a greater range though
this is not always as noticeable. Consider a torch beam. At double the
distance the circle of light is twice as wide and twice as high. It has 4
times the area to cover. Each part of a wall its shining on only gets a
quarter of the light. To get the same strength you started with you need
four times the power!
In our terms it is much more effective to use a Yagi antenna to focus all
our transmitted power in the right direction than getting a bigger
transmitter. Also the Yagis ‘gain’ is effective on receive so picks up
weaker signals better.
Frequency can effect the range too. The higher the frequency
the bigger trees and buildings appear to the wave and more
wave is lost penetrating them. Hills cause shadows and the
curvature of the earth also has an effect making the hills in the
middle of the path appear taller. At VHF and even more so at
UHF the range is not normally further than ‘line of sight’ and
depending on the terrain that may be from 10 or 20 km up to 60
or 80 km in open country from a hill top. Handheld radios down
amongst buildings even 1 km may be difficult.
•
•
•
•
•
•
•
•
•
•
•
Radio waves travel in straight lines unless reflected or “diffracted”.
(Diffraction occurs when a radio wave grazes an obstacle and it results in
radio signals being picked up away from a straight line)
Radio waves get weaker as they spread out. (Same amount of power has
to cover a greater area)
At VHF and UHF hills cause shadows.
Radio waves get weaker penetrating buildings - but windows are more
transparent to radio waves.
The range of a signal at VHF depends on the antenna height, a clear path
and the transmitter power.
Higher antennas are better than higher power - they improve reception as
well.
Outdoor antennas will perform better than indoor antennas.
At VHF/UHF, range goes down as frequency goes up.
Line of sight at VHF/UHF is a little further than the horizon because of very
slight bending in the atmosphere (like a mirage). Hills and buildings cause
path loss.
VHF/UHF range is generally little
more than line of sight.
The Ionosphere
The ionosphere is a part of the upper who's layers of partially conductive gas
occur from about 70 km to 400 km in altitude. These layers are formed by the
action of ultra violet light from the sun interacting with air molecules in the
upper atmosphere. At this time you only need know that the strength and level
of the ionisation varies with the time of day – The amount of sunlight. It also
varies with the seasons from summer to winter.
The ionosphere can refract or bend radio waves in the same way as a lens
bends light some thousands of miles away from the transmitter. The key point
is that the signal can be heard far over the horizon and well beyond the range
of what would be achieved by a wave travelling direct to the receiver.
The ionosphere is important for radio wave
(AM only) propagation....ionosphere is
composed of the D, E, and F layers the D
layer is good at absorbing AM radio waves D
layer dissapears at night.... the E and F
layers bounce the waves back to the earth
this explains why radio stations adjust their
power output at sunset and sunrise
Frequency & Time of day
When the ionosphere is strong or highly ionised it can bend radio frequencies
back to earth than when it is weak. During the day frequencies as high as 30MHz
or more may be returned whereas during the night this may be as low as 3MHz.
In the summer the highs and lows are more modest and often the highs are
higher and the lows lower.
The highest frequency that will return to earth is the Maximum Usable Frequency
or MUF and this depends on the time of day and the season.
Any one band may only offer communications for a few hours each day. When
this happens the band is said to be ‘open’. As the morning progresses it may be
necessary to move up one or two bands until early afternoon when you may have
to drop down again to maintain communications. Further moves down will be
needed as the evening and night progresses.
World-wide propagation is possible by ionospheric or ‘sky wave’ paths and a
single ‘hop’ can be up to 4000km. The radio wave can bounce off the earths
surface allowing multiple hops and world-wide coverage.
• On HF (< 30MHz) almost all There are layers of “conductive gas” at
heights between 70 and 400 kms up.
• These layers reflect radio waves (below about 30MHz back to the
ground). Higher frequencies normally pass through (so the
Klingons can watch our UHF TV).
• Effective communication relies on these reflected waves.
• The earth can then reflect the wave back up for another go.
• So - HF radio waves can bounce around the world.
• But it depends on the time of day, the frequency and the time of the
year.
• Each HF band will only support propagation to a particular place at
certain times when it is said to be “open”.
Electromagnetic
Compatibility
What is Electro Magnetic Compatibility or
EMC
• EMC is the avoidance of interference
between different pieces of electronic
equipment.
• Transmitters can cause interference to
nearby electronic and radio equipment.
• Radio receivers can also suffer from
interference.
How is interference caused?
• Interference can occur through local radio transmissions being
conveyed to other equipment through the house wiring, or TV
antenna down-leads, telephone wires, etc. and particularly at
VHF/UHF by direct pick-up in the internal circuits.
• There are so many different scenarios that it is impossible to
say in advance just what the effect may be
• But problems can be minimised by putting antennas as far away
as possible (including high up) and by using balanced antennas
at HF.
• Even though your TV is not affected others further away could
still be affected.
• At HF, horizontal dipoles are less likely to cause problems than
other types.
• The more power a station runs, the more likely it is to cause
interference.
What can be done to minimise interference?
Good practice starts in the radio shack and simple precautions will minimise
the chances of problems occurring and also of your receiver suffering from
interference caused by domestic appliances.
Direct pick up - Field strength, the strength of the radio wave, may
be too high for the effected equipment. More of a problem on
VHF/UHF. Reduce this by moving the transmitting
antenna further away
RF conducted in mains cables - RF signals may leak out of the transmitter
along its power supply leads. Fit filters on the power leads to the transmitter
RF from the Antenna picked up and conducted into different devices - Move
antenna away from the house and fit filters to the effected device.
RF fed back into mains earth wires – Sort out earthing arrangements in the
shack
Some examples of RF filters and their use
AKG RF filter BB1
switch this into your TV's antenna lead
if you suffer from RF breakthrough.
keep your neighbours happy
An ‘inline’ mains filter
A ‘clip-on’ Ferrite RFI / EMI / TVI Filter
used for Noise Suppression of electrical
interference. Can be clipped on to power or
coaxial cables and is available in various
sizes.
Many mains filters employ Ferrite materials.
These have some useful properties and can
be employed in various ways. It consists of
a block of ferrite material that can be
clamped around a mains cable and
surround a section of the cable. This means
it is exposed to the electromagnetic fields
around the cable.
Clip on ferrite rings - Alternatives
Clip on’ ferrite blocks are easy to buy and use. However they tend to work best at
high frequencies unless you have a large chunk of ferrite extending along the cable.
There are however some alternative arrangements which can give high Common
Mode series impedance (the interference voltages on the two wires are identical,
and they produce the same amount and direction of current on each wire). These
are based on using rings or loops of ferrite materials. In
fact these filters are basically similar to the clip on block.
Earthing
In the radio shack there are two separate reasons for earthing
equipment, especially transmitters.
1.
As with all equipments that are not especially made to be ‘double insulated’
an earth is needed for safety reasons. This earth MUST NOT be removed.
Most amateur equipment is not double insulated.
2.
Current always flows round a closed circuit or loop. Many antennas have
only a coaxial feed – the centre of the coaxial cable. There must be a return
path for the current and this is normally the earth. If the transmitter is earthed
to the mains, then this RF signal will flow in the mains earth and via the
house wiring into other electrical appliances in the house and/or that of your
neighbours.
RF in the mains can be avoided or minimised by providing an RF earth in
addition to the normal mains safety earth. This is done by knocking a metal
spike into the ground close to the point where the feeders enter the house.
This is connected by heavy gauge wire directly to the transmitter or, if fitted,
the wall socket terminating the feeder to the antenna.
To limit RF flowing into the mains all three mains leads, live, neutral and
earth, need to be filtered. This is best done using a ferrite ring. Ideally 20
turns so possibly using 4 rings – 5 turns on each.
Choice of Antenna
At HF an effective option to reduce interference is to use a balanced
antenna such as the dipole. A balun will be needed if coaxial feeder is
used and the RF on the earthed outer of the feeder is minimised if the
feeder drops symmetrically away from the dipole. That is, it drops at right
angles and not alongside one of the dipole halves.
End fed HF antennas always need an effective earth path. Even so, the
fed-end has a high current or voltage and is prone to EMC problems such
as direct pickup and pickup by mains and telephone wiring. If an end-fed
must be used it is best to run the feeder down the garden and fee the
antenna from the far end away from the house. A good earth on the feeder
braid at this point is important.
Power
&
Modes of transmission
The more power a station runs, the more likely problems will occur. At
power levels used by the amateur foundation licence (10w) the likelihood
of problems is not too high.
The different modes AM, FM, SSB and data tend to cause different
levels of interference. The modes which have a constant output are less
likely to be a problem.
FM
This is the most benign since there are no changes in level at all
SSB
This is the worst. The level varies continuously in time with the
transmitted voice and the interference caused sounds like a
distorted voice which is subsequently annoying.
Data
Many of the data modes have a fairly constant power level and
often cause problems
Morse CW if well keyed with smooth changes from ‘on’ to ‘off’ can be
(CW) can be reasonable. Much depends on the quality of designs
of the transmitter and the keying circuits.
Immunity
• “Immunity” is the ability of equipment to function correctly in the
presence of strong RF.
• Immunity of most types of equipment can be improved by fitting
external chokes or filters in mains leads, loudspeaker cables, and
TV antenna leads etc. These must go as close to the affected
device as possible.
• Anything fitted to the mains wiring must be properly made for the
purpose (but ferrite rings OK). Don’t mess with mains wiring and
DO NOT fit home made items to the mains
• Information about getting and fitting chokes and filters is readily
available from the RSGB.
• RF earth connection is to provide a path to ground for radio
signals and prevent them entering mains wiring.
The Neighbours
The subject of interference is all the more important when it involves
Your neighbours.
If neighbours have a problem with your transmissions you have got
to deal with it and a cooperative approach is best. As to see what
the problem is and carry out tests to see how the problem can be
resolved. Maintaining good will is essential if a satisfactory solution
is to be found. It is likely you will need to stop transmitting until you
can fine the cause of the interference. Often though it is usually
inadequate immunity of the effected device that is the problem. If
that’s the case it may be better to get independent assistance.
Remember, if you do have an interference problem outside your
own home •
EMC can upset your neighbours!!!
•
Ask to see what the problem is. Carry out tests to see if it can be
resolved.
•
Be diplomatic if there is a problem and maintaining good will is essential
and possibly get independent assistance.
•
Often it is their equipment that has inadequate immunity.
•
Get advice from other amateurs, Radio Clubs, RA publications, the
Internet etc.
•
The RSGB EMC committee maybe able to offer advice.
Your local office of the RA may call if neighbours
complain and your station can be inspected and
are willing to risk a £50 charge.
Safety
Considerations
High Voltage
Remember –
• Mains voltages carry a risk of electrocution and are potentially lethal.
As little as 80 volts can be dangerous.
• High currents can cause fire by heating the wires carrying them.
• Mains powered equipment must have a safety earth - so that if any
fault causes a live wire to touch the case, then the case will stay at
earth potential and the fuse will blow.
• If adjustments or replacements are needed inside equipment, they
must be switched off and unplugged before work begins.
High Current
High Current supplies also carry a risk, even if only low voltages are
involved.
A short circuit can result in currents high enough to overheat wires
and start a fire or cause a burn. Many batteries especially rechargeable
batteries can give surprisingly high currents which will cause wires to
become red hot.
Rings and metallic watches should be removed when dealing with sources
Of high current.
Mains Plugs & Earths
UK Mains plug
• Remember how to wire a plug!
– Brown is live
– Blue is neutral
– Yellow/green is earth
– Make sure cable clamp in plug is clamping the outer insulation
and not the individual wires.
– Have no “whiskers” protruding inside plug.
• Only work inside mains equipment if it is disconnected from the
mains (don’t rely on switches)
Earths
The mains earth is a safety earth designed to protect you if faults
develop which would otherwise result in exposed metalwork becoming
live. This protection also relies on the correct fuse being fitted so that the
fuse will ‘blow’ before anything else becomes too hot. It is not acceptable
to just fit a 13A fuse in every plug. A thin mains flex will overheat at well
below 13A. Fit the correct fuse and do not use higher values even
temporarily.
At this CIS level assume that it is inappropriate to wire mains plugs and
work inside mains powered items. You should still be able to recognise if
a plug is safe.
Protective
Multiple Earthing
PME
Protective Multiple Earthing is a particular method of electrical
supply to the home which effects the manner in which devices are
earthed via the mains supply. Your electricity supplies will be able to
tell you if you have a PME supply. If you have, you MUST consult
an electrician before fitting an RF earth.
The reasons are beyond this CIS level but a leaflet is available on
the CIS website and from the RSGB EMC committee
www.rsgb.org/emc
Note: If your house has “Protective Multiple Earthing” then you need
to get a leaflet from your electricity company and may need to take
special extra measures e.g. bond your RF earth to the house earth.
Accidents & Emergencies
In the unlikely event of an accident in your radio room, there is always
the possibility that the person has suffered an electric shock and may
still be in contact with live mains.
If you then touch the casualty then you could also suffer a shock and
become a casualty yourself.
Your first action should be to isolate the mains supply. This is best
achieved if all your sockets in the room can be isolated by one
single off switch that is clearly marked and everyone knows.
DO NOT TOUCH THE CASUALTY UNTIL YOU KNOW THAT THE
POWER IS OFF
Help should be summoned. Anyone who has suffered
an electric shock should receive medical attention.
Antennas & Feeders
Antennas should always be mounted so
they cannot be walked into
and out of reach of being touched
RF Burns
RF burns are electric shocks from feeders and antenna elements carrying RF
power. The burn may not be particularly painful but can be quite deep. The full
extent of the injury may not be known for sometime. Antennas must not be touched
when you are transmitting and insulated but unscreened antennas and wires can
cause almost as much damage as touching bare wires. Antennas and feeders
must be securely mounted and well away from overhead power lines. The wind
loading on an antenna can be quite high during a gale which may bring it down
causing immediate injury or causing a trap to be walked into later. It may blow or
fall against overhead power lines resulting in all the exposed metalwork in the
shack becoming live. You may not notice this risk from inside the shack. High
antennas may have a ‘higher risk’ of lightening strikes. Planning permission may
be needed before a large mast is installed. Erecting the antenna can also be
hazardous. Working at any height carries with it the risk of falling so always have a
second person around to summon help if required. Not all risky activities are
obvious so an adult must be present when anyone is up a ladder. Those on the
ground should have head protection in case tools are dropped.
Remember:• Elevated wires need to be out of the way of people, lorries, etc.
• Think about what might happen if they sag.
• Make sure they are strongly mounted to withstand birds, winds, etc.
• Never put anything near overhead power cables.
• Antenna erection is hazardous. Things can break and fall down and
always have one adult present and always have a second person to
summon help.
• Antennas should not normally be touched when transmitting (except
low powered hand-held equipment).
• High power can cause arcing and burns - also don’t leave them
touching trees!
Car Batteries
Risks
The main risk from car batteries is the high current available. However
charging them causes the battery to give off hydrogen which can be
explosive in confined spaces so they should not be charged indoors.
Safer, sealed batteries should be seriously considered. If the battery
is tipped over they may leak fluid which is a highly corrosive acid and
any spills will need to be mopped up immediately.
Splashes
Splashes on the skin require immersion in running water for several
minutes (15 minutes is recommended) and splashes to the eye need
immediate and constant irrigation, then medical attention
Headphones
Wearing headphones carries two risks:Firstly, it is very easy to cause hearing damage. Particularly laud noises
can cause damage and pain quickly. Changing channels after listening
to a very quiet station may cause momentary pain or discomfort which,
if repeated often, will lead to damage or hearing loss in the future.
Secondly, but of more concern is listening too loud. It may take a year
or more for this to add up to a loss of hearing but by then it will be too
late.Try turning the volume down until it is too quiet and then turn it up a
little. Some try the opposite way and turn the volume up until it is too
loud. This should be avoided but may take a deliberate effort on your
part to achieve.
Another risk, when servicing equipment, is that the headphones may
complete a the electrical circuit and enhance the effects of an electric
shock.
DO NOT wear headphones unless you are seated
and operating your equipment normally.
Safety Recap
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In case of accidents/emergencies turn power off first.
NEVER touch a casualty until the power is turned off
Danger of trailing wires across floor - trips, drags equipment off table, frays cable.
Antenna erection is hazardous. Things can break and fall down. Always have one
adult present and always have a second person to summon help.
Antennas should not normally be touched when transmitting (except low powered
hand-held equipment). High power can cause arcing and burns - also do not leave
them touching trees!
Wearing headphones carries two risks
1. It is easy to cause hearing damage e.g. loud noises when tuning or high volume for
prolonged periods
2. When servicing equipment the headphones may complete the electrical circuit
Other Hazards
Other hazards
The shack may contain a number of potential
hazards apart from the risk of electric shock.
Tools have sharp edges causing cuts.
Soldering irons get hot and can cause serious burns.
Wires trailing across floors can trip people or drag equipment on the
floor or expose live parts.
Overhanging wires are liable to snag people and other items.
Wires, especially mains wires running under carpets will become
frayed over time and the damage may not be noticed until it is too
late and a fire or electric shock is occurs.