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

Connectors BNCPL259 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 Wire DipoleYagi 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) 3x 2 6x 4 9x 8 10x 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? essexham.co.uk/train