Download 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
Antennas & Feeders
1
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
Antennas & Feeders
2
Feeders
•
Feeder types: Coaxial, Twin Conductors

Inner Conductor is shrouded by dielectric,
with outer (braided) screen.
For Radio 50 Coax is used (TV is 75)
Two conductors kept at
constant separation by
insulation - no screen
Balanced Feeder is
available in 75-300 
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9
Antennas & Feeders
3
Feeder Impedance
A
B
B
• Feeder Impedance is a form of AC Resistance
• Impedance is based on the Ratio of A and B
A
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9
Antennas & Feeders
4
Balanced/Unbalanced
• 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 and unwanted radiation
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9
Antennas & Feeders
5
Decibels
• Gains and Losses are expressed in dB’s
• 3 dB is half steps and 6dB is quarter steps
• 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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9
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 is both in Transmit and Receive modes.
• 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
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 course value of 50 ohms.
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9
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
Antennas & Feeders
9
Electromagnetic Waves
x
Electric
Field, E
y
Direction of
Propagation
z
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
Antennas & Feeders
10
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
Antennas & Feeders
11
Dipole Radiation Pattern
Radiation Pattern for a Vertical Dipole:-
Chelmsford Amateur Radio Society
Intermediate Licence Course
Carl Thomson G3PEM
Slide Set 9
Antennas & Feeders
12
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
Antennas & Feeders
13