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flex-6300 dummy load SWR to high

Mike VA3MDMike VA3MD Member
I have a dipole with a SWR on 7.150 of 1.5 : 1, using a external Bird Wattmeter and a MFJ-259B that both agree. My 6300 tell me that the SWR is 1.75 : 1, this start a long test of using a precision  Dummy Load and other Transceivers. I then hooked up the dummy load straight to the back of the Flex-6300 and still the SWR was 1.25 : 1. My other radio gave me a 1 : 1 , I have bypassed the ATU in the 6300  with the same result. How come this 6300 has a constant higher SWR of .25, I know this SWR is not a big deal but it should read correctly.


  • Jim GilliamJim Gilliam Member
    edited October 2017

    Have you tried using your tuner on the dummy load to see if it will make the SWR appear lower?

    Jim, K6QE

  • Mike VA3MDMike VA3MD Member
    edited October 2017
    ATU will not engage as the SWR is to low with a dummy load.
  • Bill W2PKYBill W2PKY Member ✭✭
    edited October 2017
    This came up recently; Right Click on the ATU button and clear memories.
    Hope this helps.

  • George Molnar, KF2TGeorge Molnar, KF2T Member ✭✭✭
    edited February 2019
    Keep transmission lines as short as possible, and measure at the lowest frequency you can. That will minimize errors induced by slight imperfections transformed in the line.

  • Mike VA3MDMike VA3MD Member
    edited October 2017
    Thank you Bill it worked, I cleared all memories and disconnected the 13.8V to the radio. After restart the SWR is 1:1 on a Dummy load . It is nice to see that this forum works because of Amateurs like Jim Bill and George. 73 Mike, VA3MD
  • Bill W2PKYBill W2PKY Member ✭✭
    edited October 2017
    I think this is something new and was discovered a few weeks ago. My 6700 developed this on 40M.
  • Mike va3mwMike va3mw Member ✭✭
    edited February 2018
    The best way to test is with 3 dummy loads. 25 and 100ohms and that will give you a 2:1 SWR to test to. 50ohms will give you 1:1 of course.
  • edited April 2019

    There is something occurring with the SWR detection circuit in the Flex 6000 series.  A purely resistive test (as Mike suggests) would probably not exhibit this issue.

    I have seen numerous 6300 and 6500 radios display higher than actual SWR at various times, especially on 40m, and into tube amplifier input stages (due to flywheel effect).  Some of you may recall the fiasco when "swr protection" functionality was added into SmartSDR a few releases back, then quickly disabled in the next release, due to all the issues it caused.  

    I can use the 6300 and 6500 and see a high SWR condition (approximately 2-2.5:1) with the radio SWR meter, yet my LP-100A meters (one on the input of the amp, and one on the output) and amplifier SWR meter (whether the amp is in operate or standby) all show a low SWR (provided that is the case with the actual antenna system components at time of measurement).  I can then simply rotate my radio selection coax switch to a different radio, such as an Icom or Kenwood radio, and the newly selected radio shows an SWR more in line with the external meters. 

    I am guessing this is hardware design flaw in the 6000 series.  For what it is worth, the Apache Labs 100/200 series radios also suffer from this same condition.  At least with the ANAN radios, you can download the PA board schematics and draw your own conclusion.   A schematic of the 6000 series would surely help us better understand what is going on with the detection circuit. Questions as to how and why this occurs were never fully answered in the previous threads (when the SWR protection feature was added.

    Has the FRS engineering team redesigned the swr detection circuit, or fully tested the new 6400/6600 radios on complex impedance loads, hopefully at the prototype stage?  

    I will be curious to see if this same issue occurs with the new radios. Maybe someone who has experienced this with a 63/65/6700 and has upgraded to a 64/6600 can report back in due time.


  • Mike va3mwMike va3mw Member ✭✭
    edited February 2018
    One thing about SWR is that is variable off the resonance point if I remember my college days in RF school.

    We did a test where I had to make about 1 dozen jumper cables all of various designed lengths.  This was to show that when inserted into a 2:1 SWR load, depending where you inserted the SWR meter, the reading was different.  Yet, we knew the resistive load was 2:1.  And, that was in a controlled environment.

    In the case of a mismatched condition, something interesting happens along the transmission line. Before, with the matched antenna, the same voltage existed anywhere along the line. Now as you move along the distance of the line, the voltage will change. It now has peaks and valleys

    The above quote is from https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf.  Well worth reading so that you understand SWR and how it is 'not that simple'.

    Others on this forum may have better or remember better than I did, but I believe this to be true.

    Also, RF flowing back on a feedline may affect SWR readings if there is RF flowing on it.  As an idea, you may want to choke the jumpers to be sure.  

    Short story, in my whole bunch of years of operating, it was not uncommon to see different SWR readings that my bird watt meter that was different from what was on the radio.  There is no doubt in my mind that the radios all displayed the SWR they were reading at that point of the feedline.  

    My 2 cents.

    mike va3mw
  • Gerald-K5SDRGerald-K5SDR FlexRadio Employee ✭✭
    edited October 2017

    We use a Stockton Bridge, which has the benefit of not requiring adjustment and is flat over a very wide frequency range.  There is no balance adjustment required in this type of circuit.  

    From the bridge output we drive forward and reverse samples to a dual demodulating logarithmic amplifier that is good from 1 MHz to 10 GHz.  The -26 dBc forward and reverse samples go through pi network resistive attenuators to bring the signals into the appropriate levels for the chip.  This provides a pure resistive load to the stockton bridge, which is required for proper operation. 

    The chip has >60 dB of dynamic range with simultaneous logarithmic forward, reverse and return loss computation outputs.  Return loss is computed as log(A/B) = log(A) − log(B).  The only calibration is the slope and intercept for the log detector, which we perform according the the chip manufacturer's specifications.   RL accuracy is independent of the phase angle since we directly measure the vector amplitude inside the log detector.  Return loss is return loss.  We mathematically convert RL in dB into SWR in the software. 

    This type of circuit is much more expensive than what you will find in most other radios but provides a corresponding improvement in accuracy across that is flat across the entire frequency range.

  • Bill -VA3WTBBill -VA3WTB Member ✭✭✭
    edited October 2017
    I don't believe the swr protection was disabled, it had a fold back issue that kicked in to early, as some reported on it. Flex made software changes to smoothen it it out.
  • Mike LukasikMike Lukasik Member
    edited October 2017
       When I was a young Ham back in the 1970's I had a high SWR on 20 meters using a Heathkit Amp. No matter what I did the SWR would not come down, if I bypassed the amp it was in normal range. I then asked for help from a local Ham Helmut VE3DBO, he reached in his box of coax's, handed me a random length coax and told me replace the one that goes from the transceiver to the amp. As I know everything in those days and thought this old guy was nuts but I did what he said anyway.

            To my amazement all was well with the world, like a good mentor Helmut would not give the answer but told me that all would be revealed in the ARRL handbook.  When doing test with multiple Antenna  or TXers it is best to take the PL259 and transfer to the another Radio. If you need to use a coax switch keep all the coax's the same length, same type to back of each transmitter.   

           We all have coax switching in our Ham Shacks but little regard for the type and quality of coax used in the switching process. Clean up your coax connection between equipment and you may find that some of the interference on some bands is in all that switching mess.

    73 .. Mike VA3MD 

  • Stan VA7NFStan VA7NF President Surrey Amateur Radio Communications Member ✭✭
    edited October 2017
    However Mike, when a coax is terminated in its matching impedance, reverse power will not be created and the SWR should be a constant 1:1.  
  • Mike va3mwMike va3mw Member ✭✭
    edited October 2017
    correct.  it is only 1:1 at 50ohms

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