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R.F. Feedback issue with FLEX-6600 and SPE Expert 2K-FA

Steve Adler
Steve Adler Member
edited May 2020 in New Ideas
Hello Everyone,

I have been battling for months to find the cause of a subtle R.F. feedback issue that manifests itself as slightly raspy transmit audio, when using my FLEX 6600 and my SPE 2K-FA linear amplifier combination on the air.

Other Amateurs make me aware of this distortion, as it only happens when transmitting into an antenna.

All HF bands suffer to some degree however; 40 metres is by far the worst.  All my antennas are resonant and exhibit a return loss of greater than 18dB. I have also installed a good station RF Earth system.  Very frustrating to say the least!

Ultimately, the solution was to fit a 10 turn RG58C/U coaxial common mode choke, wound on a 61mm diameter Type 43 toroidal core, between the output of the FLEX-6600 and the input of the SPE 2K-FA amplifier.

An unconditional fix for all HF bands! 

I hope this information helps others who might be experiencing this issue whilst operating a similar setup to me.

Best 73 Steve VK5SFA


  • iw2clm
    iw2clm Member ✭✭
    edited May 2020
    Thanks Steve,
    I'm in similar situation and I get some result (not 100%) using a short Cellflex 1/4" connection from Flex6600 to Expert 2K-FA, but Cellflex is like a water pipe and not manageble, sigh!.
    I'll check your solution, it is interesting ...
    My audio chain to input in Flex6600 is not very succesful for me anyway (BAL in or LINE in no much difference), compared to IC-7700 and IC-7300 where it works very fine also with full power with no RFI.

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited May 2020
    Hi Steve

    Thanks for sharing.   RFI and SWR are often not related.  In fact, a good SWR and a good return loss reading my actually be the opposite as it is likely your antenna is radiating as you want it to.  It usually means more RF out.  

    About 10 years ago, I got into using Chokes.  Mix 43 and Mix 31.  Lots of them.  I have them at the feedpoint of the antenna and at the radio end as well.  Having them between the Amp and the Radio is not a surprise and is in fact recommended.

    In fact, this is the document that got me going on it http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

    And, the more chokes I added, the better I was hearing things.  My noise floor got lower which tells me the coax was picking up local noise too.  

    Welcome to Common Mode Currents and this is the way to resolve them.

    73, Mike va3mw

  • on5po
    on5po Member ✭✭
    edited May 2020
    Hello Steve,
    For my part, I slightly improved the problem, by moving the flex 6700 away from the expert 2k at 1.5m, instead of 20cm.
    there is also, that I am under 4 meters from the antenna, there, I can not change anything.
    I would like to receive more specific information on your system, if you can, my email address see qrz.com.
    thanks in advance
    73 on5po, Janny

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