Taking Off The Training Wheels

Discussion in 'Homebrew and Kit Projects' started by VK4FFAB, Apr 18, 2017.

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  1. VK4FFAB

    VK4FFAB Subscriber QRZ Page

    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 thats 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 :)
  2. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    The PIN diode is your friend.
  3. VK4FFAB

    VK4FFAB Subscriber QRZ Page

    Yeah but the ones i have found that wont break the bank and will deal with 10w, are 3mm 12 pin quad flat packs. Not unless you have some parts numbers that are easy to find that i could not find.
  4. DK7OB

    DK7OB Ham Member QRZ Page

    The standard 1N4007 diode seems to be a good replacement for a PIN diode. I have not done tests myself, but there was an article in a recent German ham radio magazine where it was used in a 600W capable T/R switch. I'll try to find out if that article is available online when I'm at home.
  5. VK4FFAB

    VK4FFAB Subscriber QRZ Page

    Hi Wolf, I just got an email from Mark at minikits http://www.minikits.com.au/ and he said to do the same thing. Its what he used in the M1 Transceiver kit and what Elecraft use in a number of their radios. If popcorn will do, I say get out the butter. Any tips or tricks on biasing from the article you mention would be great.
  6. N1UV

    N1UV Subscriber QRZ Page

    VK4FFAB likes this.
  7. DK7OB

    DK7OB Ham Member QRZ Page

    VK4FFAB likes this.
  8. N2EY

    N2EY Ham Member QRZ Page

  9. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Your orginal idea will work provided you use a shunt diode switch on the input of your DBM.
    Reverse bias the diode in RX, with enough reverse bias you shouldn't have any problems with IND caused by the diode. In TX dump a fair amount of current in the diode in the forward bias. That will drag any residual RF to ground during TX.
    VK4FFAB likes this.
  10. N2EY

    N2EY Ham Member QRZ Page

    The 1N4007 is actually a PIN diode. However, the lower-voltage siblings in the 1N400x family are not - use ONLY 1N4007.

    73 de Jim, N2EY

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