T/R switching and Solid State or SDR receivers

Discussion in 'Amplitude Modulation' started by KC2ZFA, Jan 14, 2021.

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  1. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    will my Icom R75, or my old sdrplay1, survive the pictured t/r switch ? power levels 20 to 250 watts carrier.


    ps. the R75 when muted cuts both audio and lowers the sensitivity of the front end.

    pps. I have a dowkey and the scope pic shows what I saw single-sweep on the receiver side on key-down...brrrr

    Attached Files:

    Last edited: Jan 14, 2021
  2. K0OKS

    K0OKS Ham Member QRZ Page

    I use a similar setup with my Valiant.

    You can check your isolation by starting a low power and checking the receive signal at the receiver. Then turn it up slowly making sure you don’t exceed the Max input for the receiver.
  3. N1BCG

    N1BCG Ham Member QRZ Page

    Make sure your T/R relay fully actuates before the transmitter keys. In my case, the transmitter is keyed by the antenna switch/muting circuit to ensure this.

    It’s important that your receiver input is shunted to ground on transmit, and with the transmitter connected at the lower left on your circuit, that looks to be the case.

    You’ll also want to add a relay to interrupt the SDR audio output as well or at least attenuate it enough to prevent feedback.

    Be seeing you on the air...
    KC2ZFA likes this.
  4. KE0ZU

    KE0ZU Ham Member QRZ Page

    I would slightly bend the contact grounding your receiver's antenna input so it will ground same before the transmitter is turned on, and release after the transmitter has turned off.

    A better solution woud be to consider building a sequencer, similar to whats below. This will guarantee everything will happen in the proper sequence and that your SS receiver, regardless of type, won't suffer a blow out in the front end.

    Another option is to use an electronic T/R switch. There were a couple makers of T/R switches, and the one I've used for years is an E.F. Johnson 250-39.


    Regardless which electronic switch you use, you'll need to add a couple of back to back 1N914 diodes from J4 to ground.
    KC2ZFA likes this.
  5. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Not just with SDR; I do all the above with my 75A-4, with the aid of a sequencer. The antenna change-over relay is paralleled by a second milsurplus coaxial relay, kind of a miniature version of the Dow-Key with BNC connectors, to shunt the antenna input to the receiver to ground. The receiver runs all the time, muted by disabling the audio to speaker/headphones, with a by-pass switch to leave the audio on if I want to monitor the signal. Enough RF still feeds through to register about 20 dB over s9 on the S-meter.

    I don't completely disable the receiver when using the SE-3 sync detector. The transmitter signal is what keeps the sync detector locked on to frequency during transmit. If the receiver is switched to stand-by mode while transmitting, the SE-3 loses lock and eventually drifts completely off frequency.

    Which explains why I don't always hear a station that answers my CQ off frequency, or who tries to break in to the QSO off frequency when I stand by.

    All T/R functions use mechanical relays operating off 24 vdc.
  6. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    yes, I've built a simplified version of Don's ER sequencer (see schematic, from AMfone), need to wire up the SPST switches that can override parts of the sequence when in receive. I made R1 a pot that adjust the release time of relay A.

    Attached Files:

  7. WB2GCR

    WB2GCR Ham Member QRZ Page

    According to what W8JI posted on his website, the EFJ electronic TR produces a fairly large
    pulse for a few milliseconds when transmit is initiated. Large is relative, of course. Likely no
    effect on "boat anchor" front ends. Might be a problem on solid state front ends.

    The Dow Key people make one too - that is afaik untested WRT this issue.

    Two solutions come to mind. Use clamping diodes between the electronic TR and the rig - make
    sure they are fast and able to handle ~100v RF into a 50ohm load (whatever power that works out
    to - for a millisecond, of course). Ok, that's one.

    For the stock TR DowKey relay, one solution is to key the relay and let the relay contacts key the xmitr
    after the relay has moved to the xmit position... but again, this makes the unkey sequence a little problematic as
    the key OFF will cause the DowKey to go off the xmit connector before the relay unkeys the rig.

    I think short of the Electronic TR, or a PIN DIODE method, the sequencer is safe and effective.

  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The EF Johnson TR switch is basically a two-stage tube amplifier stage used to isolate the transmission line from the receiver while keeping the antenna connected. I think they were primarily for QSK cw operation. They do nothing to mute the receiver. I always thought this was a bad idea, since adding an extra active stage between the antenna and front end of a receiver inevitably raises the noise floor plus exhibits intermodulation distortion, therefore degrading receiver performance.
    W1BR likes this.
  9. WB2GCR

    WB2GCR Ham Member QRZ Page

    Well it is a question of degree...

    How much noise over the existing noise floor?

    When I used to use the DK electronic key, back in the 60s, on 10M and 15M, it was an advantage
    as it provides a few dB of gain.
    I suppose in a nearly ideal receive environment, very low background noise, and a modern receiver
    or an optimized R-390A with a very low self-noise it could start to be an issue.
    As far as IM, my only question is in a real world use, how would the IM show up on the receive side?

    And yes, it does not mute the receiver, but you may find it interesting that I do not mute my
    receiver(s) when transmitting! I place the speaker at a distance and position where I do not get
    feedback to the microphone, and monitor my own voice that way. Been doing this for decades, ever since
    I set up the station without an easy way to mute the receiver, was lazy, and ran it anyway. I decided
    that I liked it!

    I suppose that one might be able to trigger a mute relay off an RF sensing circuit.
    Or run a line from the PTT circuit of the transmitter, or the mic keying line to a small relay
    to mute the receiver??

    These tube type electronic keys are preamps when in receive and set up to cut off when RF
    tickles their inputs... the antenna runs straight through from the transmitter, the electronic key
    circuit "hangs" off that bit of straight wire...

  10. K9UR

    K9UR Subscriber QRZ Page

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