View source for 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 covers 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 and the Cx540 N type relay which are often available surplus.

[[File:amp relay.jpg|200px]]

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

[[File:951 relay.JPG|200px]]

==1.3GHz (23cms) and above==

For the microwave bands (1.3Ghz and above) 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. 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. 

Typical SMA relays which are good up to 10GHz are shown below:

[[File:rsma-elay.PNG|200px]]
[[File:sma-relay2.PNG|200px]] 
[[File:sma-relay3.PNG|200px]]

==Coil voltages==

Whilst 12v operation is desirable, most surplus relays are 28v operation.  This can easily be overcome by using a switching regulator from ebay as shown in the picture (search for step up buck regulator) to boost the 12volts on transmit to 24v.

[[File:reg.JPG|300px]]

==Latching relays==

Some relays require driving in to both positions - this can easily be achieved by using another DC changeover relay which is driven by the PTT line and switches the c/o relay DC supply to appropriate contacts.

==Tx / Rx sequencing==

It always pays to use sequenced switching on tx / rx changeover - this ensures you don't try to switch the expensive antennae change over relay when the transmitter is still on. It also protects your pre-amp by switching it off before you transmit. The desired sequence is as follows:

To go to Tx:

1) Turn off receiver pre-amp

2) Switch ant c/o relay to transmit

3) Apply power to PA

To go to rx

a) Turn off Tx

b) Switch ant c/o relay to receive

c) Apply power to pre-amp.


Many circuits and pre-built designs are available including this simple single chip design by Ian Waters G3KKD published in CQ-TV 125.
 
[[File:Tx rx timing.JPG|500px]]

The PIC controlled sequencer at [http://www.shortwaveradio.co.uk/tropic.htm shortwaveradio.co.uk called TROPIC] may well be a good choice. The kit is listed at £11.50.

==Transfer or 4 port relays==

A lot of surplus microwave relays are transfer relays - these can easily be recognised by having 4 connectors or ports.  These are entirely suitable for use as 3 port change over relays and can actually be used to provide additional protection to your pre-amp by putting it in to a 50 ohm load on transmit.

[[File:transfer relay.JPG|400px]]