TX/RX Switching and the MFJ-1708SDR

Discussion in 'Software Defined Radio (SDR)' started by WA7F, Mar 18, 2019.

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  1. WA7F

    WA7F Premium Subscriber QRZ Page

    A forum user pointed me towards the MFJ-1708SDR for project that requires sharing antennas with the transceiver and SDR. (Thank you, K6CLS.) I commented that I wasn’t that fond of MFJ products but, a few days later I figured what the heck and looked it up. After reading MFJ’s product description I was actually thinking it might work.

    K6CLS had also pointed me to recent QST review but, that spoiled the party for me. The RX insertion loss seems quite high; 14 MHz/3.6 dB, 50 MHz/3.94 dB, 144 MHz/7.4 dB and 440 MHz/19.6 dB. The transceiver isolation was also not that impressive with 14 MHz @ 17.4dB and 440 MHz @ 0.8dB. I’m assuming the isolation is referring to receiver isolation or a lot of people would be letting the smoke out of their SDRs.

    So I could probably live with the isolation but, the RX insertion loss seems unacceptable. I do want to work SSB from HF to 70cm. My initial RX/TX switch concept was just a little microwave coaxial relay and a driver circuit to trigger it off the amp keying line. The SDR (RSP2) is a better receiver than my 857D so the plan is to just switch the transceiver entirely out of the antenna circuit during RX.

    Am I missing something here? What are you guys doing for RX/TX switching?
  2. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    SDR Play Speaker Problem Solved
    We are using a MFJ-1708BSDR, to solve the problem of feed back thru the speakers
    Using the AUX connection on the MFJ-1708BSDR to control a small 2 pole 12v relay to open the audio and speaker ground on transmit works great. You have to break both speaker and ground, breaking one does not seem to work. We are using RF Sense to control the MFJ-1708BSDR, you have to adjust the delay so you do not get slight audio at the end of the conversion or if you are a slow talker and it switches back to receive in the middle of talking.

    * Not sure how much current the AUX connection can handle or the 12v relay requirement so I used a small relay.
    * Getting the relay 12v from power supply not from the radio
  3. WA7F

    WA7F Premium Subscriber QRZ Page

    KF5NAJ - I agree that the 1708 has a nice feature set but, do you find the SDR to be a little deaf due to the insertion losss?
  4. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    It seems about the same with the edge going to my Icom 756 Pro 2
  5. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks. I haven’t had a 756 P2 but, I had a 756 P3 for awhile. Nice rig.
    I would expect that a SDR would outperform a stock 756. What SDR do you have and have you compared your it with and without the 1708 in line?
  6. K6CLS

    K6CLS Ham Member QRZ Page

    If it were my setup, that's what I would do. I have a pile of CX800 SPDT coax switches. 12vDC, takes more from the battery, only 40dB isolation, more to carry around and set up and break, doesn't allow QSK, yadda yadda.

    I got the MFJ1708 from an estate sale. Word is the SK didn't like the insertion loss either, his ELAD couldn't quite make it up. And running a pre-amp on HF is ... mmm ... noisy, no better signal.

    tell us what you end up with.
  7. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    The difference between receiving thru the MFJ-1708BSDR and not is hardly noticeable.
    I also made a setup to use the SDRPlay or Flex1500 as a pan adapter/receiver with a 50yr old Dow-Key T/R antenna switch that works, plus it had better isolation than the MFJ-1708BSDR. The SDRPlay can handle 0 dbm it looks pretty wild on the display screen but at -50 dbm well within the spec of the SDRPlay
    The problem I had with all the different combinations was sound feeding thru the speaker while transmitting, the solution was to break both the audio and ground to the speaker. Nice feature some Dow-Key switches have are (2) sets of no/nc external contacts that I used to break the audio.
    The MFJ-1708BSDR is probably the easiest way, once you solve the audio problem but any T/R switch that can auto switch on transmit can be made to work with a little thought
    You do not need to buy a new $$$ radio with a tiny waterfall display to have the fun of SDR, plus it can be added to any radio even one that is 20+ years old. It does not need to be a SDRPlay I'm sure other SDR's could be used
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    gee-whiz how many threads can you start on the same subject ?

    Please post the link to other threads so we can keep up.

    You are killing me smalls.
  9. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Don you need to get a life and pay attention, I did not start this thread.
    I did not see were there is a limit on how many posts you can post or answer. If it is a problem for you to keep up, start an Excel spreadsheet
    Since you have nothing to add to solve the problem you should stick to the HAM RADIO DISCUSSION form
    You are killing me also

    Good to hear from you anyway
  10. WA7F

    WA7F Premium Subscriber QRZ Page

    Hey Guys, thanks for the replies. It’s all good info; ELAD didn’t get along with 1708, speaker noise. I also have some relays that will do the job and I think I’m going in that direction over the 1708. They will be mounted in the rack so they are no extra hassle once installed. I’ll buy or build a front end protector and BCB filter for a little extra insurance for both the rig and RSP2. That will add some loss but still far less than the 1708.

    Having a SDR waterfall is great but, my primary goal is to achieve better receive performance then the 857D will offer. I have a 7300 and two Flex 5000 v/u rigs in the shack but, this set-up is all for a Go Box. Think field day, camping, contests, etc. I thought about using the 7300 but, it takes up to much room in my little 3u Gator case and I want VHF/UHF capability for satellite and SSB. You never know, if I find that I’m not utilizing the V/U bands the 7300 might find it way into the go box in the end.

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