Taking Off The Training Wheels

Its interesting when you actually start designing for yourself and not just follow someone elses potential blunders, how many small things you have to overcome once you start prototyping your designs that you never considered would be an issue at all.

In my latest project i figured I would use a relay for the Tx Rx switching along with a micro controller and a push button that might one day become rf sensing. It works a treat does exactly what its meant to do and I am happy. And then i got to thinking, how much isolation is there between the 2 output poles of the relay. Its not an RF relay just the cheep 5v relay using in arduino projects, so i dont expect it to be great.

So i injected a 27dbm signal into the common of the relay, that about 0.5 watts and measured what was being seen on the isolated pole, 2dbm or about 1.5mw. So i have about 25dbm of isolation and enough power floating on the isolated port to burn your retinas out if you are a double balanced mixer, let alone the 25mw of Rf that’s going to be there when 5W is put thought it, assuming the isolation remains constant with power and im not sure it does, I do not have 5w to stick in there to see atm.

So i looked at the specs of a number of RF Relays, and they are only sporting 40 to 50dbm of isolation, much better but still soul destroying if you are the receiver DBM. So either way I have to find a way to protect the mixer and so im thinking either bidirectional amplifiers or just muting the rf preamp by cutting the power to the transistor. Hooray for me, I now have more thinking to do.

Thanks for reading an hombrewers lament 🙂


3 thoughts on “Taking Off The Training Wheels

  1. Why can’t you use a pair of back to back diodes? That’s what I do at the antenna terminal of my receivers. I have a lot of 1N4004 so I use them.

    1. Hi Stan,

      Thanks for the suggestion, a number of people pointed out that using 1n4007 diodes as PIN switches would work fine as you suggest (unless of course you mean as a clipper)and I will likely do that in future projects, its what Elecraft used in a bunch of their rigs and minikits.com.au used in his M1 Tranceiver. What i have decided i would do, is put a shunt diode switch on the receiver input somewhere to take care of any bleed over. Probably right before the DBM input.

      Thanks for your comment,


  2. Thank you for taking the time to show your thoughts and the problems you come up against.
    Lane Cox N6NLB

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.