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As noted in an earlier post I'm stuck with what to do to finish my 70cm Mini-kits amplifier at the point where the instructions end. I put the amp module on a wooden base and fascia then wired it up. There's a relay with signal power from a battery for now. I took it to CAT17 but ran out of workshop time and courage to turn it on! The filters that I made did get attention and thank you to Noel and Clive for setting these up.
It's time to choose a relay arrangement. I'd like to switch between coax bypass and amp TX. I didn't know that coax relays are either expensive or treasured heirlooms. Mini-kits sell SPDT relays that handle 20W or more at up to 3GHz. Does one buy a pair of these to switch between coax centres at the input and at the output? They're about 15 Aussie dollars each.
David Holman M0YDH
At 70cms you need to be reasonably careful, particularly on the output relay - something like the minikits units or this one http://www.ebay.co.uk/itm/Amphenol-318- ... 2568408287 are OK. (BNC / Ntype / SMA generally indicates an OK relay for 70cms - the golden rule is never use PL259s on any frequency above top band!)
On the input I would tend to use this type of thing - http://www.ebay.co.uk/itm/RS-349-686-SP ... 2048751672 although I would never pay £25 for one and you can pick them up at rallys for a couple of quid!
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.
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!
For 70 cm you can use reasonably cheap BNC relays like those above and the Cx540 which are often available surplus. 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 realys (and I don't mean CX520s) tend to be expensive. They do turn up inside equipment, but rarely. 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
One other point is it pays to use sequenced switching on tx / rx changeover 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 recieve
c) Apply power to pre-amp.
This ensures you don't try to switch the expensive relay when the transmit is still on. It also protects your pre-amp by switching it off before you transmit.
A very simple single chip design is shown at the bottom of this page on the wiki : https://wiki.batc.tv/PTT_and_band_switching
EDIT - just realised there is a lot of information here that should be captured on the wiki so we can find it in the future!
I have created a new page at https://wiki.batc.tv/Antennae_changeover_relays
W1GHZ does a sequencer too.
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