Antennae changeover relays

Introduction

On the common frequencies used for ATV (146 MHz above) the antennae changeover relay is a critical component and must be low loss so not to adversely affect both transmit and receive performance.

146 and 70cms

A good paper to read is Ian White's DG8 pre-amp article - http://www.ifwtech.co.uk/g3sek/vhfdx/dg8-preamp-v7.pdf - it's designed around 145 MHz but cover a lot of good practical engineering practice for pre-amps etc and does talk a little bit about the need for good relays.

For 70 cm you can use reasonably cheap BNC relays like those above and the Cx540 which are often available surplus. SMA relays are much more common for about £10 and up at a rally or microwave round table. It helps to know what you are looking for, RLC, Radiall, Teledyne etc. Don't be put off if it has a 28V coil or 4 ports (usually a transfer relay). Ratings can be estimated here http://www.rlcelectronics.com/index.php ... id=5&id=61

On the input I would tend to use something like the RS951 and you can pick them up at rallys for a couple of quid!

23cms and above

For microwaves 23cm and down you need microwave relays. These come in many types but the most common are TNC, N or SMA. Good N-type relays (not the CX520s) tend to be expensive. They do turn up inside equipment, but rarely.

The problem only gets worse as you go up in frequency and at 5.6GHz the relay (which will need to be sma or N type) will probably cost more than the FPV kit!