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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.
Further to your advice, I found that Transfer relays with SMA connectors to be better value on Ebay and struck lucky with the pair pictured below which I purchased in person from a trader in Wolverhampton. I went to meet Stew who turns out to be a broadcast radio and communications engineer with a house full of surplus gear that he has to sell. [The 1.5kW dummy load was as big as a labrador dog!]. N.B. he has some super inter-digital filter that he plans to re-align for 437MHz on his VNA
The 5 port 4 way was described thus:-
DC- 12.4GHz 5 Port 4 way 24V SMA Coaxial relay 850W - 85W (@100MHz -12.4GHz)
This is a used but good condition 4 way changeover relay, 24v coils, plus revertive contacts to indicate the made position if switch-over confirmation is needed.
The specs are at http://www.rlcelectronics.com/products/ ... eries.html
with a power graph at http://www.rlcelectronics.com/index.php ... id=5&id=61
Would you be able to add notes on revertive contacts and their application to ensuring safe switching on the Wiki ?
The other relay is a "SR-TC relay, plus an explanation of the numbering system is at;
http://www.rlcelectronics.com/products/ ... eries.html "
I think I'll only need one aerial on 5.6GHz when I build that rig at some point next year!
- Anyone want this circuit?
- 20170930_Relay Controller in L Band Switcher.jpg (220.87 KiB) Viewed 2305 times
- 2 RLC coax relays bought from billybob90210
- 20170930_Coaxial Transfer Relays bought today.jpg (208.18 KiB) Viewed 2305 times
The 3, 4 and 6 way relays are relatively common and not too expensive because people don't realise what they are. However, they are not usually transfer relays, they connect the centre port to one of the outer ports. You need to be careful using them for TX/RX switching as there isn't really anything to prevent you from activating more than one port at the same time, which means you could connect the TX to the RS. Some sort of fail safe is advisable but these are best suited to switching between multiple antennas or signal switching.
The transfer relay is ideal for what you are trying to do. It does the job of two SPST relays (take care the isolation is sufficient). Be careful with power ratings, I have seen these melted on 10GHz in key down modes. Stick to less than 10W and it ought to be OK though.
I don't know if your relay is a latching one. If it is W6PQL has a PCB to drive it.