Using a Transmit/Receive Switch to Protect an SDR from a Transmit Radio

A question that comes up often is how to combine an RTL-SDR, or any other RX only SDR with a transmit capable amateur radio. It's not possible to connect the RX only SDR together with the TX radio via a standard splitter because the TX radio's power will most likely blow up the SDR with it's powerful output. To solve this problem you need either a manual switch that will switch out the SDR when transmitting which requires absolute discipline to not accidentally transmit in the wrong switch position, or an automatic relay switch.

Over on YouTube channel HamRadioConcepts has given a good overview and demonstration of the MFJ-1708SDR Transmit/Receive automatic relay switch, which is a good product that solves this issue. It is also a fairly budget friendly option, coming in at only US$79.95 over on the MFJ website. HamRadioConcepts notes that the switch automatically grounds out the SDR whenever the PTT on the radio is pressed, and also has a fail safe that will automatically detect a transmission and ground the SDR if PTT is disconnected.

MFJ-1708SDR Transmit/Receive Switch For SDR Receivers

11 comments

  1. Randall Maness

    qrpkits.com has a RF keyed relay kit for $20! But if you are using PTT control which i prefer just get a 12V relay rated for 125V with DP DT. put it in a project box. On the relay run a jumper over from the N.C. for the SDR / RX over to the input of the second contacts. On the N.O. side of those contacts jump it to ground so when you key it grounds the SDR input.

    Is the MFJ box plastic?

    I used a metal project box and double shielded Mil Spec coax for my coax inputs and outputs. Only issue i have is a delay from real time to what i am hearing on my PC. I am not sure if its the dongle, HDSDR or my PC. Ive tweaked all my audio quality settings in my PC and tried different buffers in HDSDR but its about a .75 sec delay! drives me nuts so i just tune around find where i want to TX and flip a switch on my T/R switch putting RX back on my Transceiver for the QSO and when done switch back to HDSDR. I gotta fix this because in tough conditions the SDR wins over my TS-570DG! Amazing what this dongle and HDSDR can do!

  2. David Reed

    The “N” style connector is used for higher power ,5 to 15 Kw and military gear. 25 to 250 Kw is usually Heliax type feed line. 73, ACØYW

  3. Bas - ON5HB

    I crafted a pan-adapter on my TS-590S, works perfectly and costs practically nothing.
    It’s splitted from the RX-TX relay.

    • Dave H

      My guess it’s actually an older T/R switch design that they just re-labeled to say “SDR” for the new century. The SO-239s and the RCA jack for the PTT input are pretty familiar on 1960s-70s gear.

      • John

        The only difference between the old 1708 switch and the new SDR version is that the new one has a jumper to keep the transmit radio connected at all times, so that it’s always receiving. I converted one of mine to the SDR version in about 1 minute (not including heating up the iron).

        UHF and RCA jacks are still in use today on amateur equipment, not just stuff from the 60s-70s.

    • Ebert

      PL-259 and SO-239 are generally rated for one kilowatt which is enough for most people. What do you think would be better ? N type ?

      • KB1YSQ

        My understanding is that PL-259/SO-239 is a bit inconsistent with regards to impedance within the connector, deviating from 50 ohms, perhaps depending on frequency? I think folks prefer the N connector for that reason, and also for waterproof-type-stuff. (Not so much applicable in this situation).

        • Bas - ON5HB

          Nope, in real measurements the connector has hardly any impact on losses. Claims like -3DB are bogus, even on UHF. People pay too much attention to rubbish instead of measure themselves. All too much software simulations that are false and don’t deliver claims.
          Measure yourself people! There is all to much rubbish claims about just everything, still people believe it even proven to be false.

Post a comment

You may use the following HTML:
<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

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