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ATU with low SWR

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Mark Erbaugh
Mark Erbaugh Member ✭✭
edited June 2020 in FLEX-6000 Signature Series
I installed a new Carolina Windom 160 antenna. Since the antenna had changed, I cleared the ATU memories. I engaged the tuner. It cranked for a long time, then finally indicated that it had found a match. When I transmitted , the indicated SWR was about 1.6:1. I disengaged the tuner and the indicated SWR was about 1.3:1. Three questions: if the SWR was low, why did the tuner take so long to find a match, why couldn't it find a better match than it did, and when it realized that the natural SWR of the antenna was better than the match it found, why didn't it automatically disengage? This is on a 6700 running SSDR 1.5

Answers

  • Jim Gilliam
    Jim Gilliam Member ✭✭
    edited June 2020
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    A wonderful investment for any shack is an antenna analyzer which will immediately answer all of  your questions. Multiband antennas can be tricky and there are many variables which affect the antenna impedance looking into the feed line. My guess is you are really putting the tuner through its paces.


    Jim, K6QE

  • Ken - NM9P
    Ken - NM9P Member ✭✭✭
    edited December 2016
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    Agreed. I recently purchased a used AA-54 and it has been an amazing tool. But I have seen cases where currents induced by a nearby high power AM radio transmitter caused weird things to happen on an antenna analyzer with a 160 meter antenna. I don't know about effects on auto-tuners. BTW, I never mess with SWR less than 1.5:1. You will likely lose about as much power through the tuner as you save with a better match.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
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    First off, I can't agree more that the time saved by using an antenna analyzer is worth it's weight in gold.

    Second, multiband antennas can exhibit some very strange reactances.  The ATU may not be able to handle it in some cases.  The fact that the ATU took a long time to find a match is an indication that it was dealing with a very reactive antenna.  Why it found a better match that showed a higher SWR is probably due to variable or high reactance when it was trying to find a match.  The ATU is making a lot of LC changes and testing the result for a short time duration.  It probably found an LC combination that generated a lower SWR at the time of the short duration test.  But when you use TUNE, RF is being sent for a much longer amount of time and due to the stray reactance, that can result in a higher final reading.

    Third, I agree with Ken that if the ATU doesn't make a big change and the original SWR is below 1.5:1, I'd leave the ATU off.  The power loss @ 100W is negligible.
  • Mark Erbaugh
    Mark Erbaugh Member ✭✭
    edited February 2020
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    I will run the antenna analysis this weekend. I know a lot of folks put way to much emphasis on a low SWR. My concern is that I don't want to run a high SWR that would damage the Flex. Is there a maximum recommended "safe" SWR. I did see in another post that the Flex has a high SWR cutoff. Is it safe to run any SWR that doesn't trigger the cutoff, or would that be stressing some components in the Flex?

  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
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    "My concern is that I don't want to run a high SWR that would damage the Flex."

    This is not very likely.  We have internal protection mechanisms to prevent reverse power from damaging the radio.

    "Is there a maximum recommended "safe" SWR"

    I would try and keep it below 3.0:1, but anything below the threshold where it throws a threshold exceeded error is fine.
  • Paul Christensen, W9AC
    Paul Christensen, W9AC Member ✭✭
    edited November 2016
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    Ken,

    I have no experience with the AA-54 but the better VNAs and CIAs hold up well in strong AM broadcast fields.   I have attached an image of an S11 VNA sweep of N4NN's 160m Marconi "T."  I took the measurement with an N2PK VNA.  What's important is that we needed an accurate sweep well into the upper end of the AM broadcast band.  We deliberately resonated the antenna to about 1400 kHz.  There are some advantages in doing so, one being that at 1825 kHz the antenna impedance is 50+j200, requiring only a series capacitor to get a 1:1 SWR on the low end of 160m.

    The measurement was taken approximately five miles to the north of some high power AM broadcast stations, one being 50KW.   When the T antenna is resonated  in this area of the AM band, field intensity is very high and can be several volts at the feed point.    

    Notice the small "blips" on the R scale from the AM stations.  The VNA's output level of +10 dBm and narrow resolution bandwidth minimizes the effect of RF ingress from these stations.  While these blips are seen, the eye can smooth the result.  In fact, the myVNA program can be set to automatically smooth the result.  But the result is impressive even without smoothing.  A measurement at this location could never be taken with a common MFJ SWR analyzer.  

    Paul, W9AC

     

    image
  • Watts - K4QJZ
    Watts - K4QJZ Member ✭✭
    edited February 2020
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    I see the same patterns of operation with my Carolina Windom 160 and my 6700.
    One thing I did find, be sure the feed of the antenna is at least 30' up or you will get even stranger readings. It has been an amazing antenna in operation but it can drive the 6700 tuner crazy if the feed point is not at east 30' above ground. (Installations directions recommend it as well)
      Watts
      K4QJZ

  • Mark Erbaugh
    Mark Erbaugh Member ✭✭
    edited January 2016
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    Watts, That is what I needed to hear, experience from another user in a similar situation. I was concerned that my 6700 ATU was defective and the 6700 needed to go back to Austin. Thanks for the confirmation that it is the antenna and not the radio. By feed point do you mean the matching unit on the top wire, or the line isolator that is 20 feet below that. My line isolator is about 20 feet up, which puts the matching unit at 40 feet. 73, Mark
  • Watts - K4QJZ
    Watts - K4QJZ Member ✭✭
    edited January 2016
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    The feed point where the 20' verticle feed line is connected to the matching unit at the antenna itself. It's the key to the omni directional pattern. Mine is only 30' up. Enjoy it's a great combination!
  • N6WT Kent
    N6WT Kent Member ✭✭
    edited December 2016
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    "We have internal protection mechanisms to prevent reverse power from damaging the radio."

    What is this?? If the SWR is high the radio will protect itself??
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
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    Yes
  • N6WT Kent
    N6WT Kent Member ✭✭
    edited December 2016
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    Thanks Tim

    Does it just reduce power or shut the transmitter off or.......  how? Im just curious.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
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    Stops transmitting and generates a high SWR message.
  • N6WT Kent
    N6WT Kent Member ✭✭
    edited December 2016
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    Hmmm... I have never had the radio stop transmitting or a message pop up. I know a couple of times I transmitted on the wrong antenna and looked up to see the high SWR. What is the threshold for this?
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
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    It is based on reflected power and it is fairly high as the PA transistors are very stout.

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