Download Technical Basics

East Kent Radio Society EKRS
1
Intermediate Course
(5) Antennas and Feeders
Karl Davies
System
2
50 Ohms
Output
Transmitter
Standing
Wave
Ratio
Meter
Antenna
Matching
Unit
Antenna
Feeder
Receiver
Feeders
3
 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 
Feeder Impedance
4
A
B
B
• Feeder Impedance is a form of AC Resistance
• Impedance is based on the Ratio of A and B
A
Balanced/Unbalanced
5
 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
Decibels
6
 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
Feeder Losses
7
 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
Antenna’s
8
 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.
Voltage Standing Wave Ratio
9
 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
2
0
1/4 WAVELENGTH
1/4 WAVELENGTH
0
1/4 WAVELENGTH
1/4 WAVELENGTH
Electromagnetic Waves
10
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.
Polarisation
11
 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.
Dipole Radiation Pattern
12
Radiation Pattern for a Vertical Dipole:-
Yagis
13
Direction of
Radiation
Boom
Feeder
Reflector
Driven
Element
Directors
Unwanted
Sidelobes
Radiation Pattern