Transcript Antenna Types

Antenna Types
WB5CXC
Resonant Antennas
• These are antennas that are resonant on
the operating frequency.
• The most common is the dipole and it’s
derivatives.
50 ohm coax
1/2 Wave Dipole
Length in feet = 468/ F (Mhz)
Lower Current / Higher Voltage
Higher Current / Lower Voltage
Current on a Dipole
Voltage / Current on Antennas
• The center point of the Dipole is low
impedance (50 ohm).
• The ends are High impedance.
• The center of the Dipole will have 70 volts
with 100 w.
• The end will have an impedance of 10K +
ohm which give it a voltage of 1,000+ volts.
• Have good insulators at the end of the
antenna.
Inverted Vee Antenna
Inverted ‘L’
Dipole Antennas
• Most Dipole antenna are single band or
two bands. (They can work on Odd
harmonics – 3rd.)
• They are a good match to 50 ohm coax.
• They should be at least ¼ wave above the
ground or higher. (On 80 and 40 this is
usually not the case.)
Dipoles Cont’d
• Back in the 60s we made Dipoles with 3
wires.
• The first would be for 80 meters, the
second would be for 40 meters, and then
the last would be on 10 meters.
• 80 meters would use the 3rd harmonic for
20 meter.
• 40 meters would use the 3rd harmonic for
15 meters.
3 Wire Multi-Band Dipole
Dipoles Cont’d
• You can make a multi-band antenna by
using traps. The highest frequency is the
first portion and the lower frequency uses
the entire antenna.
• The traps will also shorten the antenna
somewhat.
20 M
10 M
10M Trap
2 Band Trap Dipole
Traps Cont’d
• The Trap is a parallel resonant circuit. At
resonance it presents a very high
impedance.
• This high impedance acts like an ‘Open’
circuit and disconnects the rest of the
antenna.
• Off resonance the impedance is low and
acts like a ‘Short’ thus connecting the two
parts of the antenna.
Trap
1
2
=2
fC
( XC )
f
fL
( XL)
If you reduce the frequency , then the Trap becomes Capacitive !!
20 M
10 M
10M Trap
XC = XL
You cut the longest part of the antenna to work on 20M.
f r = XC = X L
With the Trap in the circuit and you lower the frequency it becomes Capacitive and
thus reduces the resonant frequency (which effectively makes the antenna shorter).
(The antenna was cut for 14.2 Mhz, but now it tunes to 13.9 Mhz [too Long now]– you
have to cut the antenna to make it tune to 14.2 Mhz.)
Verticals
• Verticals antenna are usually a variation of
the ½ wave dipole. The vertical is only ¼
wave and the ground reflection provides
the other half of the dipole.
• Optimum vertical operation depends on a
good ground. This is usually made with
radials buried in the ground.
¼ Wave Vertical
Verticals Cont’d
• Traps can be added to verticals to provide
multi-band operation.
• There are several advantages of verticals,
the main advantage is that it requires less
real estate and height.
• The vertical are a true Omni directional
antenna and they have a low angle of
radiation.
Verticals Cont’d
• Trap vertical have a height of 10 – 30 feet.
• One of the newer popular verticals is the
53 foot. This uses no traps and works
most bands (best on 80 – 20M). It
requires an antenna turner at the base
(similar turner as a long wire turner).
Vertical Dipoles
• Vertical Dipole antennas is a new concept to
amateur radio (within the last 5 years).
• It looks like an ‘H’ turn on it’s side or some
call it a double ‘T’.
• It uses capacitance hats to shorten the
elements.
• They require less real estate and also have
approximately the same gain as a dipole.
20" X 5/8" Tubing
20" X 5/8" Tubing
Sigma 6M Vertical Dipole
60" X 1" Tubing
20" X 5/8" Tubing
20" X 5/8" Tubing
36"
Vertical Dipoles Cont’d
•
•
•
•
They mount low to the ground.
The coax has to come off at ~ 45 degrees.
They have a low angle of radiation.
Sigma (Texas Antennas) and Transworld
Antennas are two manufacturers of
vertical dipoles.
• They also have multi-band vertical dipoles.
Non Resonant Antennas
• There are also Non Resonant antennas.
• The Long Wire antenna is a Non Resonant
antenna.
• It is multiple wave lengths long. When we
think of a Long Wire antenna we think of
an wire that is 200+ feet long.
• Tuner is under the antenna and takes a
different kind of turner.
200' +
Tuner
Long Wire Antenna
G5RV Antenna
• G5RV antennas are multi-band antennas.
• They are not a resonant antenna (except
maybe on 20M).
• They will work most bands 80 – 10 (not
useful on 30, 17, 10M [by some comments])
• They look like a dipole except that on the
center insulator it will have 450 ohm ladder
line.
G5RV Cont’d
• The ladder line has to hang vertical and
can not be near anything.
• You need an antenna tuner for this
antenna.
• Normal size is 102’ top and 30’ of 450
ladder line – need ~70’ of coax for antenna
to perform properly.
G5RV Cont’d
• G5RV Jr. is 51’ top and 17’ of 450 ladder
line – need 35’ of coax for antenna to
perform properly. This antenna is good for
40M and up.
• The ladder line is part of the antenna and
radiates.
450 ohm ladder line
G5RV Antenna
G5RV Cont’d
• There is a lot of FOLK Lore about the
G5RV.
• Some swear by it -- others say it is
worthless.
• You need to be aware of what it will do and
the pitfalls.
• Most of the poor operation is do to bad
installations.
G5RV Cont’d
• The antenna should be higher than 34’.
• The top of the antenna needs to be inline.
• The ladder line has to hang straight down
and not touch anything or be near anything.
• Should have long lengths of coax for it to
work properly.
Gain Antennas
• Gain antenna use power radiated in
unwanted areas and puts it into a more
useable pattern.
• On a Dipole a lot of power is goes up in the
air, most of us are not talking to planes so
this is wasted power. We also are not
talking to stations behind us.
• We direct this wasted energy into a more
useable direction (in front / and lower).
Gain Antennas Cont’d
• The most common Gain antenna is the
Yagi.
• Other Gain antennas include the Cubic
Quad, Hex Beam, Moxon.
• In the HF bands these antenna can
become quite large and you need a rotor to
turn them.
Yagi Antennas
• A Yagi antennas has 2 or more elements.
• The simplest is a 2 element Yagi –
Reflector & Driven Element (Driver)
• The Reflector is the longest element it is ~
10% longer than the Driver.
• The Driver is where you feed the antenna.
• On multi-element Yagis you added
Directors (shorter than the Driven element).
Director
Reflector
Direction
of
Radiation
Driven
Element
3 Element Yagi Antenna
Yagi’s Cont’d
• Yagi come in single band or multi-band.
• Probably the most popular is the Tri-Band
Beam with traps – 10, 15, 20. (Reduced
size)
• Some of the multi-band beams have
multiple element and become quite large
(and expensive – need big towers and
rotators).
Moxon Antenna
• A Moxon antenna is a 2 element antenna
with bent elements.
• It is smaller than a normal 2 element Yagi.
• It can be built using normal items from a
hardware store and can use wire for the
elements.
• Spacing and dimensions are critical.
• Use Moxongen.exe for the design.
Driven Element
Reflector
Moxon Beam
10M Moxon – PVC & Wire
Hex Beam
• Hex Beams are another reduced sized
beam.
• It used bent elements (wires) and a nonconductive frame.
• Frame is usually fiberglass rods.
• The frame is made with 60 degree angles.
• The name comes from beam configuration
– 8 sided (Hex).
Hex Beam Cont’d
• Hex Beams are usually multi-band and
can have up to 5 bands on one antenna.
• They bend the rods and string the wiring.
• It looks like an upside down umbrella.
• There is a lot of information on the Internet
to build your own.
• Commercially manufactured by Traffie
Technology and others.
Driven Element
Non-Conductive
Frame
Reflector
Hex Beam
Hex Beam