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Mismatch between ATU and SSDR SWR readings

Here is my setup: Flex 6400, MFJ993b auto tuner, 18" RG213 coax between Flex and ATU, 18" RG213 coax connecting the dummy load to the flex

The following test setups for SWR readings:

(1) Flex connected to ATU connected to dummy load. ATU shows 1:1 swr, SSDR shows 1:1.7 swr

(2) Flex direct into dummy load. SSDR shows 1:1 swr

Per these results I suspect the coax between the Flex and the ATU. There is another discussion thread on this topic-- https://community.flexradio.com/discussion/comment/19995809#Comment_19995809 where the solution was the coax cable between the Flex and ATU was lengthened.

My questions:

(1) Am I correct to suspect the coax between the flex and the ATU?

(2) Are there additional things I can/should check?

(3) if trying to increase the length of the coax is something to try, how long of a cable should I try next?

Thank You in advance for reading this and your suggestions.


  • Al K0VM
    Al K0VM Retired Member ✭✭✭

    I would start by replacing the ATU with a UHF female to female.. If its not 1:1 its a coax problem . Otherwise its an ATU problem.

    Does the ATU have a bypass mode ? What do you get in bypass? A defective ATU could provide a false match to a 50 load.

    Is it the same on all bands ?

    AL, K0VM

  • W7NGA
    W7NGA Member ✭✭✭


    Per your description it would appear that your ATU after tuning is not 50-ohms and presents a slight mismatch to the Flex. Not to worry ... this might change with frequency and might also change slightly after each auto retune. Changing the length of coax isn't necessary as it is frequency dependent and this slight mismatch is essentially inconsequential.


  • The external ATU is introducing a small mismatch, Your dummy load is 50 Ohms so the ATU sees that as a 1:1 match. Your Flex also sees the dummy load as a 1:1 when connected directly. However the INPUT of the ATU does not appear as 50 Ohms. It may be non reactive or not but it is most likely not 50 Ohms. Now, is the ATU manual or automatic? If it is manual, you may need to give it a small adjustment.

    Does it have a bypass switch? Most do. Try bypassing the ATU with its own switch. See any difference? 73, Jim

  • Dom wa2got
    Dom wa2got Member ✭✭

    Thanks for all the input. Makes sense the MFJ 993b auto tuner is not presenting 50 ohms to the transmitter. Based on the responses I did the following.

    (1) I connected the Flex to 18" coax - connected to UHF-UHF female connector - connected to 18" coax (2nd of 2 jumpers I have) connected to dummy load. The Flex SWR meter reads SWR of 1:1. I think this rules out a problem with the 2 coax jumpers I am using.

    (2) I ran tests with the following connected; Flex connected to ATU (mfj 993b autotuner) connected to dummy load. I checked the Flex SWR meter readings with the ATU in bypass and not in bypass ("inline") on several bands and got these results. BTW my Flex does not have an internal tuner.


    15m - 21.325 ATU in bypass swr 1.1, ATU inline swr 1.4

    20m - 14.211 ATU in bypass swr 1.1, ATU inline swr 1.3

    30m - 10.125 ATU in bypass swr 1.1, ATU inline swr 1.2

    40m - 7.209 ATU in bypass swr 1.1, ATU inline swr 1.1

    80m - 3.800 ATU in bypass swr 1.1, ATU inline swr 1.1

    It does make sense to me the tuner is not presenting 50 ohms to the transmitter at least on bands 15,20 and 30. Could this be a defect with the tuner? Or perhaps a calibration item which I can adjust myself, how? For my antenna, currently I only have one. A GR5V which has a design point for the middle on the 20m band. But it is on the 20m band when using the G5RV I'm getting the highest SWR readings from the Flex SWR meter (1:1.8 to 1:2.0

  • David Decoons, wo2x
    David Decoons, wo2x Member, Super Elmer Moderator

    Sounds like a “defect” with the tuner. It could be the design of the tuner does not present the radio with a 50 Ohm impedance (resistive and reactive) when on the higher bands.

    You can try a longer jumper (6 foot if you have one) but it may only mask the problem.

    try a call or email to MFJ with your findings. Is the tuner new? If used may be internal damage or there could be updates to design that were done.

    Dave wo2x

  • You need not use a tuner if the antenna is closely matched. Where the SWR is rather low, just keep the tuner in bypass mode. The mismatch in the tuner will lose more power than the slight mismatch in the wire. 73, Jim

  • Dom wa2got
    Dom wa2got Member ✭✭

    @James Del Principe Thanks Jim, I'm going to see if I can borrow an antenna analyzer to check the antenna resonance. Being that it is a G5RV antenna with a design point in the 20m range, I'm not optimistic I'll have much usable bandwidth where the tuner can be bypassed. But perhaps at least I can use 20m without the tuner. I may need to revise my tuner situation so I can have a tuner which is usable for remote operation. I really have no control of it when I operate remotely. Basically I can put it into 'tuner when RF present' mode or 'tune when a button is pushed' on the tuner front panel. K9DUR has a utility called SWRPlotter that will do the analysis. Have you heard of it? Used it? I'm a bit nervous subjecting the Flex to 100 swr tests per plotted frequency, when the SWR could be through the roof on some frequencies. SWRPlotter user guide describes since the fwd and reflected power are not measured at the same time there could be some error introduced. To compensate, an average of 100 readings per frequency tested is calculated.

    @David Decoons, wo2x Thanks. I've sent off an email to MFJ to see what their advice may be. I hope they have a short backlog of emails :-)

    Dom, wa2got 73

  • Dom, I am very familiar with SWR plotter and it is a great program. I have charted all my antennas on all HF bands with it. The actual power used is very low and will cause no harm. I then made an Excel spread sheet of tuner settings because I use a manual tuner. It takes very little time to tune to a given frequency. I usually use an EFHW for 80 to 10 meters but also have an OCFD 80 to 10 and a 160 M EFHW. Each is charted with SWR Plotter. G5RV antennas seem popular but people seem to either love them or hate them. I have no personal experience so I really cannot comment. Also, I don't remember 100 tests per frequency. Let us know your results. I am interested. Best 73, Jim

  • John KB4DU
    John KB4DU Member ✭✭✭

    The little mismatch between the flex and the tuner is insignificant, especially with the short jumper. The mismatch is also not enough to cause power fold back in the radio. The g5rv will most likely need the tuner, so just go ahead with ops as it is set up.

  • Dom wa2got
    Dom wa2got Member ✭✭

    @David Decoons, wo2x sent off an email to MFJ with the following data. They came back asking I do a factory reset which I did, but got the same results. Will see what they suggest next. These tests were all performed into a dummy load.

    10m - 28.500MHz - 993b in autotune mode reads 1.0 SWR, flex indicates 1.6 SWR. 993b in bypass, flex reads 1.1

    15m - 21.325MHz - 993b in autotune mode reads 1.0 SWR, flex indicates 1.4 SWR. 993b in bypass, flex reads 1.1

    20m - 14.211MHz - 993b in autotune mode reads 1.0 SWR, flex indicates 1.2 SWR. 993b in bypass, flex reads 1.1

    30m - 10.125MHz - 993b in autotune mode reads 1.0 SWR, flex indicates 1.1 SWR. 993b in bypass, flex reads 1.1

    40m - 7.209MHz - 993b in autotune mode reads 1.0 SWR, flex indicates 1.1 SWR. 993b in bypass, flex reads 1.1

    80m - 3.800MHz - 993b in autotune mode reads 1.0 SWR, flex indicates 1.1 SWR. 993b in bypass, flex reads 1.1

    @James Del Principe - I attached the SWR plotter results for 20 & 40m. I don't think they help to show the interesting difference between the MFJ and FLex SWR readings. But does show I need to tune the antenna better. I'll have to think about it a bit as to should I shorten or lengthen. Have to think through the formula. The blue line is the antenna directly into the Flex.

    @John KB4DU I agree John it is a small difference and I do continue to use my setup. Curious as to what could explain the difference in SWR readings between that of the tuner and Flex.

  • John KB4DU
    John KB4DU Member ✭✭✭

    Yes, curiosity sends me on a lot of side trips, radio and otherwise.

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

    The SWR Bridge on the radio is a calibrated MiniCircuits SWR Bridge. It measures the SWR as the Power Amp as close as you can get to it.

    A quick bench test to see if it is accurate is to run a test with a 25 Ohm load (2 - 50 ohm dummy loads in parallel) and an 100 ohm load (2 - 50 ohm dummy loads in series) . That should give you a 2:1 SWR in each case.

    SWR readings are not so black and white as you can see. From your measurements, you will see that when your Tuner is in line, it presents a higher SWR to the radio. This sounds like an alignment issue in the ATU. As you can see, when you put the ATU into bypass the Flex reads the correct value. (if I read what you wrote correctly)

    SWR readings almost never match on the same feedline. This ARRL article explains why and does a great job of explaining it.

    The G5RV is a swiss army knife of antennas. While it does a lot of things (bands) none of them really great, except 20M, the band it was designed to work with. :)

    Note that the G5RV is a variation of a dipole which is 102 feet long and matches best on 20M—the 102 foot length is three half-wavelengths at 14 MHz. To make it usable on other bands, Varney added a 34 foot (half wavelength at 14 MHz) tuned feeder constructed of 450 ohm ladder line that behaved as a matching device.

    Originally, the G5RV was used with tube rigs that would match a wide range of impedances using the plate/load tuning controls. Tubes were tolerant of mismatches, unlike today’s solid-state transceivers that balk at anything over 2:1. And the only bands used at the time were harmonically related, theoretically making matching chores a little easier. None of the WARC bands were yet available, which complicates matching today’s G5RV.

    Sometimes you have to work with what you have, so I also understand your position. I hope that helps. Personally, I don't worry about SWR until I see over 2.5:1 and that is even with the PGXL blasting at full power.


    tldr; Don't worry about it. Get on the air and make contacts!!

  • Robby
    Robby Member ✭✭

    I agree with Mike. I have worked G5RV in the distant past and never forgot our QSO. He was retired and using the call CX5RV. We talked about the G5RV antenna. I have never used one but he told me the G5RV was an experimental antenna so no measurements you use are necessarily wrong and if you build one, toss out your SWR meter. That's from the horses mouth.


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