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Performance of the DAX decoder - Flexradio 6500

Wolf Member ✭✭
edited February 2022 in DAX

Hello All,

I had a hard time with my 70 cm EME rig and the performance of the Rx path. Sensitivity of the system is not what is expected. I changed all components (PreAmp, Transverter, Flexradio, cabeling, . . .). I replaced Flexradio and Transverter with an ICOM 705 - no change in performance. Compared to other EME stations my Rx sensitivity did never fit.

A good friend recommended to use an external 24 Bit Soundcard at the Audio Output . . . and luckily I did.

With this change my Rx has changed significantly. It decodes with WSJT-X now signals down to -30 dB (on the WSJT-X scale). Before I could reach hardly -17 / -18 dB.

So - my question: What is wrong with the DAX decoding ? Did I use a wrong configuration or is the sensitivity of the decoder limited due to less than 24 Bit ?

73 Wolf - DF7KB


  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited February 2022


    DAX also runs at 24 bits and DAX IQ is at 32 bits so the performance should be the same.

    Can you run the same test using the Transverter feeding the IC-705 and see if you get the same result? If you can source a signal generator, I would do a SINAD test on both radios setups and make sure you have similar numbers. That way you aren't guessing about RF performance.

    Based on your test so far, it might be related to the transverter.

    Also, regardless of what you are seeing in the WSJTx numbers, are you hearing stations you haven't heard before?

    Let us know.

  • Wolf
    Wolf Member ✭✭

    Hi Mike,

    Just found your answer - a finger tip to late. I send my question 5 seconds before directly to you . . .

    Anyway - I did before what you recommended.

    (1) I used the IC705 connected via WLAN as 70 cm rig - replacing transverter and Flexradio.

    (2) I used IC705 as 10m receiver behind the 70 cm / 10 m transverter - replacing the Flexradio only.

    Both showed similar results.

    Stations I haven't heard . . . : I extensively tested it. Last ARRL EME contest ( Nov + Dec) I was heard by about 25 stations in JA , VK, India, US,... with strong signals. I could not decode them. Probably I lost more than half of the contacts. Only the "big guns" I was able to confirm. But I received a variety of requests via e-mail . . .

    Again and again I tried with EME operators in NL, ON HB0, . . . same effect.

    I replaced really all components of the Rx path: PreAmp, cables, Coax relay, transverter, shortwafe radio, even with diplexers tried to reduce possible impact of strong 70 cm signals - no result at all . . .

    With the external soundcard I contacted some of them meanwhile and we had no issue anymore to complete the calls.

    SINAD Testing I couldn't do so far.

    Is it a wrong configuration of the Flexradio or is it broken . . . ?


  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    Hi Wolf

    It could be the sensitivity on the 6500 could be the issue. The SINAD test will give you the answer.

    I would open a support case.


  • Duane  N9DG
    Duane N9DG Member ✭✭

    Are you running with any preamp gain enabled? I have found with my 6500 as the transverter IF that I need have the "RF Gain" set to +10dB for the system to fully realize the sensitivity of the DownEast Microwave L144-28 transverter.

  • Wolf
    Wolf Member ✭✭

    Thank you Duane.

    I run the Flex6500 with RF Gain set to + 20 dB.

  • Wolf
    Wolf Member ✭✭

    Mike, I did all the testing meanwhile. It looks like my Flex Rx is broken . . .

    The attachment shows the details.

    What do you think ?

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    Time to open a support ticket.


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