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Flex 5000a - Issues on 20m

I am having a strange issue with my Flex 5000a that I'm hoping someone here can offer some explanations and/or solutions for.

First of all, let me describe my station configuration:

Flex 5000a

MFJ-998 Intellituner

Kenwood TL-992a Legal Limit Amplifier

MFJ-704 Low Pass Filter

MFJ-1701 6 Position Antenna Switch

MFJ-2990 43' Vertical Antenna (160m-6m)

Now, let me describe the Flex setup:

KE9NS PowerSDR Software version

Band is set for 20m

Transmit Drive = 70

AF = 30; AGC-T = 80

Mic = 40; DX (Enabled) = 3; CPDR = 2; VOX = 100; DEXP = -40


Transmit Profile = Heil PR781 Gold EQ

Filters are Low = 0; High = 3000

Finally, here is the issue:

I am able to properly tune the Flex with the Kenwood amp and receive Q5 reports as well as experience full power output 1200 - 1500 watts on all bands, except 20m. The MFJ Intellituner tunes the antenna in every instance to an SWR between 1.1 to 1.3 on all bands. When I transmit on 20m, however, the displayed power output on the MFJ-998 does not exceed 400 watts and I am unable to obtain a 2-way QSO. In every instance the receiving station states that all they hear is an unintelligible noise. Some have said that they believe it is RF on the signal. If I take the amp offline, assuming the 100 watts of the Flex alone is sufficient to be heard, my signal is again Q5. Again, this only happens on 20m. I do not experience this issue on any other band. I have placed clip on torroids on both ends of all coax jumpers, electric chords, audio cables, etc throughout my station. Additionally, I have constructed chokes (6 turns of coax with a 6-7" diameter) just before the connection to the antenna and just before the connection to the MFJ Antenna Switch to assist in eliminating possible RFI.

What else can I do? What am I missing? Why is this happening only on 20m? I appreciate any "constructive" ideas, thoughts, solutions, etc.


  • Bill -VA3WTB
    Bill -VA3WTB Member ✭✭✭
    edited December 2019
    Do you have mix 32 torroids on your Fire wire cable? and what kind of cable are you using?
  • Paul - K6HR
    Paul - K6HR Member ✭✭
    edited December 2019
    Hi Larry,

    I had a similar problem and resolved it by improving my station ground. I now have every piece of equipment tied directly to the station ground. You didn't mention anything about your station ground so maybe this is an opportunity. I also have the mix 31 ferrites on both ends of every cable and they make everything very quiet. I had RF in my audio, and low(er) output on 40m, all other bands were OK, I thought I had tried everything, then invested some time in the grounding and now operate at full power without a hint of RF anywhere in shack. I found the following documents to be very helpful:

    Check these out: http://www.hard-core-dx.com/nordicdx/antenna/ground/index.html 
    and https://www.bwcelectronics.com/articles/WP30A190.pdf  and  http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

    73 for now de K6HR

  • John K2CIB
    John K2CIB Member
    edited December 2019
    I agree with the previous comments - you are on the right track.  Don't forget toroid(s) on your mike cable too - anything that goes in or out of the 5000.  My home station is in the garage and there was no ground anywhere close.  I tried long wires around the garage, but that didn't help.  Finally, I drilled a 1" hole in the garage floor and pounded an 8' ground rod down into it.  This has become my single point ground for the shack.  Make sure you do not daisy-chain the grounds or you may get ground-loops.  I ran a 1" braid from the ground point behind all the equipment and used a short wire and clip from each piece of equipment to the braid.  In my remote station (Flex-6500, Expert 2K-FA, Palstar HF-Auto), I had a ground loop between the equipment and the problem was also subject to the band I was on.  I finally put isolators in the coax between the rig and amplifier, and amplifier and tuner and that removed the ground loop.
    73, John, K2CIB
  • Preston Mclaggan
    edited December 2019
    watch the lenth of the patch cables put ferrite on them ...also nessary to earth the antenna tuner to a short earth try changeing mic and try reduceing power on the offending band De G4mla
  • Luis del MOlino
    Luis del MOlino Member ✭✭
    edited December 2019
    It sounds as if the contacts of a relay on the linear amplfier are arcing. You said that when you put your linear on bypass you are bein received Q5. So look for an RF arcing on some bad ontacts inside the linear or a bad capacitor on the 20 m output.

    Luis EA3OG

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