AMATEUR RADIO TRAINING

Feeders & Antennas

v1.101 © essexham.co.uk

FEEDERS & ANTENNAS

• What is a Feeder?• Antenna connectors• Types of antennas• Polarisation• Matching / SWR

Feeders

Coaxial Cable (coax) Ladder-line

• Unbalanced• Centre 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

A feeder connects the antenna to the transmitter

ConnectorsBNC PL259

Bayonet locking

We use 50Ω (not the 75Ω version which has a different size pin)

Screw-thread locking

Larger, more common

Antenna Types

¼ wave

Long WireDipoleYagi

5/8 wave

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

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

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

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

Antennas: Yagi• Directional

• Focuses signal mainly in one direction

• Dipole with a reflector and several directors

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

ERP

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

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

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’

• There is a practical on matching a dipole.

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

Feeders & Antennas Summary• ATU to match antenna to band

• Polarisation: Horizonal vs Vertical

• SWR: Standing Wave Ratio

• Gain: 3db = x2 ; 6dB = x4 ; 9dB = x8 ; 10dB = x10

• 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)

FEEDERS & ANTENNAS

Any questions?

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