Antennas-and-Feeders

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Transcript Antennas-and-Feeders

Chelmsford Amateur Radio Society
Intermediate Course
(5) Antennas and Feeders
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
1
Basic System
50 Ohms
Output
Transmitter
Standing
Wave
Ratio
Meter
Antenna
Matching
Unit
Antenna
Feeder
Receiver
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
2
Feeders
• Feeder types: Coaxial, Twin Conductors

Coax:
Inner Conductor is shrouded by dielectric,
with outer (braided) screen.
For Amateur Radio 50 Coax is used
(whereas Video/TV uses 75)
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Twin Feeder:
Two conductors kept at
constant separation by
insulation - no screen
Balanced Feeder is
available in 75-600 
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
3
Feeder Impedance
A
B
• All Feeders have a Characteristic Impedance, Z0
(eg 50 or 75 Ohm Coax, 300 Ohm twin feeder etc)
B
A
Chelmsford Amateur Radio Society
Intermediate Licence Course
• Z0 is based on the Ratio of A and B
(and the nature of the spacing dielectric)
• If correctly terminated by a resistive load then
Z0 determines the ratio of Vrms / Irms in the feeder
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
4
Balanced/Unbalanced/Match
• Coax is unbalanced - Inner has signal, Outer is at ground
• Twin Feeder is balanced - conductors have equal and opposite
voltages/currents/fields
• Mounting Twin Feeder near to conducting objects will cause an
imbalance in the conductors, a change in its Z0 and unwanted
radiation (or loss of immunity from external interference)
• Similarly, severe bends or crushing coax changes its Z0 and
causes internal mismatches
• Any mismatch within a feeder, or its termination, will result in its
input impedance no longer being its characteristic impedance
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
5
Decibels
• Gains and Losses may be expressed in dB’s
• 3 dB is half steps, 6dB is quarter steps, 10dB is tenth
• You will need to remember this table for exam:
3dB
x2 or a half
6dB
x4 or a quarter
9dB
x8 or an eighth
10dB
x10 or a tenth
Example: 3dB Gain doubles power, whilst 3dB Loss halves it
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
6
Feeder Losses
• ALL feeders have loss - the longer the feeder the greater the loss.
Twin Feeder has a lower loss than Coaxial cable
• This loss affects both the Transmit and Receive paths
• For some standard cables the loss is:
Per 100m
RG58
RG213
10 MHz
4.8 dB
2.0 dB
30 MHz
8.2 dB
3.2 dB
144 MHz
21 dB
8.6 dB
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
7
Antenna’s
• All Antennas have a feed point impedance.
• This is determined by the dimensions which will relate to the
wavelength of the applied signal and the height of antenna
• Dipoles are a half wave length long and are a resistive match at
only one frequency.
• If you replace the antenna by a resistor of the same value as
the feed point impedance the transmitter will not be able to tell
the difference
• Dipoles in theory are 73 ohms but in practice approx 65 ohms
so close enough to the nominal course value of 50 ohms
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
8
Voltage Standing Wave Ratio
• If the feed point impedance is incorrect then it will not match the
impedance of the feeder and some energy will be reflected back
down the feeder.
• When this reflected energy is returned to the Transmitter it is again
reflected back to the antenna and is radiated.
• The combined energy is known as the forward and reflected power
and gives rise to the Standing Waves on the feeder.
I
2
V
0
1/4 WAVELENGTH
Chelmsford Amateur Radio Society
Intermediate Licence Course
1/4 WAVELENGTH
1/4 WAVELENGTH
Carl Thomson G3PEM
1/4 WAVELENGTH
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
9
VSWR & Impedance
• If we have a mismatched antenna, energy is reflected back to the
Transmitter where it is again reflected back to the antenna and is
eventually radiated - Energy is not lost
• The combined forward and reflected power and gives rise to the
Standing Waves on the feeder.
• The reflected signal will change the input impedance of the feeder
so that it is no longer the characteristic impedance and the feeder
no longer presents the correct impedance to the transmitter.
• Antenna Matching Units transform this effective impedance back to
nominal so that the radio operates correctly
- even though the antenna remains imperfect
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
10
Electromagnetic Waves
x
Electric
Field, E
y
z
Direction of
Propagation
Magnetic
Field, H
• Electromagnetic radiation comprises both an Electric and a Magnetic Field.
• The two fields are at right-angles to each other and the direction of
propagation is at right-angles to both fields.
• The Plane of the Electric Field defines the Polarisation of the wave.
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
11
Polarisation
• Polarisation is the plane of the antennas radiating electric field.
• Common polarisations are Horizontal and Vertical.
• Transmitter and receiving antenna polarisation need to match for
optimum signal strength, especially at VHF/UHF
• Verticals (/4, 5/8) give vertical polarisation.
• Yagi and Dipoles antenna’s may be either horizontal or vertical
depending on their mounting.
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
12
Dipole Radiation Pattern
Radiation Pattern for a Vertical Dipole:-
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
13
Yagis
Direction of
Radiation
Boom
Feeder
Reflector
Driven
Element
Radiation Pattern
Directors
Unwanted
Sidelobes
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9: v1.2, 31-May-2009
Antennas & Feeders
14