1. AMATEUR RADIO TRAINING
Feeders & Antennas
Release: v1.31
hamtrain.co.uk

2. Feeders & Antennas What is a Feeder? Antenna connectors
Types of antennas
Polarisation
Matching / SWR

3. Feeders Feeder connects the antenna to the transmitter / receiver
Coaxial Cable (coax)
Ladder-line
Unbalanced
Inner conductor carries the signal
Outer braided (screen) to keep the signal within the cable
Amateurs use 50Ω impedance - do not confuse with 75Ω TV coax
Balanced
No screen
Constant separation between the two wires

4. Connectors BNC PL259 Bayonet locking
We use 50Ω (not the 75Ω version which has a different type of pin)
Screw-thread locking
Larger, more common

5. Antennas Convert electrical signals into radio waves (and vice-versa)
Different sizes for different bands
In general, lower frequencies need larger antennas
Several different types used by amateurs

6. Antennas: Dipole The dipole is a basic antenna
Half a wavelength long, e.g: 10 metres long to work 14MHz (the 20 metre band)
Half a wavelength λ/2

7. Antennas: ¼ Wave Ground Plane
Vertical antenna
Quarter of a wavelength long. e.g: 50cm long to work 144MHz (2 metre band)
Four horizontal wires, called radials, form a ‘groundplane’, which act as a mirror for the radio signals
λ/4
Coax

8. Antennas: 5/8 Wave Ground Plane
Vertical antenna
5/8 of a wavelength long
Better signals towards the horizon
Coil at the base for coax ‘matching’
Often used for VHF / UHF mobile
5/8 λ
Coax

9. Antennas: End-fed
Basic long-wire antenna Unlikely to be correct length, so needs to be ‘matched’ More likely to cause interference (EMC) than other types

10. Antennas: Yagi Directional Focuses signal mainly in one direction
Dipole with a reflector and several directors

11. Antenna Gain
The Yagi antenna focuses signals in one direction, and has “gain”
Antennas have a measurement of gain in Decibels (dB)
Gain (dB)
Gain (times) 3
x 2 6
x 4 9
x 8 10
x 10

12. ERP ERP (watts) = gain x transmitted power
Stands for “Effective Radiated Power”
The antenna gain multiplied by the transmitted power
ERP (watts) = gain x transmitted power
Example: 10 watts x 3dB = 20 watts

13. Polarisation Horizontal or vertical
Receive and transmit antennas should have the same polarisation
Yagis can be vertically or horizontally polarised
Most VHF/UHF is vertical
Much of HF operation is horizontal

14. Antenna Types
5/8 wave
¼ wave
Long wire
Yagi
Dipole

15. Dipole antenna (balanced)
Balun
When connecting an unbalanced feeder to a balanced antenna, you need a BALUN (BALanced to Unbalanced)
Dipole antenna (balanced)
Balun
Co-ax feeder (unbalanced)
Example: A BALUN is required to connect unbalanced coax to a balanced dipole antenna

16. Matching Each band requires a different length antenna
If the antenna is the correct length for the band you want to work, there is a “match”
If not, you need to use an ATU (Antenna Tuning Unit) – Commonly used when working ‘multi-band’
One of the Foundation practical assessments involves matching a dipole.

17. SWR
Use an SWR (Standing Wave Ratio) meter to measure the flow of power back from the antenna. Ratio of 1:1 is ideal
Example: SWR of 2:1 means that 10% of your power is reflected back
Poor SWR can damage the transmitter
Measure Forward & Reflected power with an SWR meter
A dummy load can be helpful for station testing

18. Feeders & Antennas Summary
ATU to match antenna to band
Polarisation: Horizonal vs Vertical
SWR: Standing Wave Ratio
ERP (watts) : TX Power x Antenna Gain
Antenna types: Dipole, ¼ wave, 5/8 wave, Yagi, End-fed
Balun: To match BALanced antenna to UNbalanced feeder
Connectors: BNC (bayonet) and PL259 (screw-thread)
Feeder types (unbalanced coax ; balanced ladder line)

19. Feeders & Antennas
Any questions?
HamTrain.co.uk
© EssexHam.co.uk