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

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

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

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

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

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 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

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

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

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

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.

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 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? HamTrain.co.uk © EssexHam.co.uk