Having trouble with the Flex 6500 antenna tuner/memory system on 40 m band

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Measured SWR on 40 meter end fed dipole (with 9:1 unun at fed end and 1:1 unun at ground level) ranges from 7.5 to 8.3.  Sometimes the tuner does not reach "Success" and just shows Mem but tuned swr is 2-2.5.  When I return to a successfully tuned frequency, sometimes the swr is above 3 (pegged on flex SWR meter) and I have to retune.  My coax is grounded at a grounding rod before passing through a conduit to the basement shack.  Length is about 18 ft from fed end 9:1 unun to 1:1 unun.  1:1 unun is to curb rf on coax shield since it is the counterpoise for the antenna.  Also about 18 ft from the 1:1 unun and ground rod to the radio.  Cannot reduce the last 18 ft any more.  Any thoughts on cause/cure?  System tunes up well on other bands which have lower SWR for this end fed antenna.  Coax switch serves as ground point for station. Thanks for any comments!
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Keith - KE4TH

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Posted 3 years ago

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WW1SS - Steve

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Try removing that 9:1 unun. Also try adding coax to the feed line. I had one that wouldn't tune properly once and just adding 7 feet did the trick. Also I never use a coax switch for a ground point and I never have grounded any coax before coming into shack.
(Edited)
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Bill W2PKY

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Hello Keith- I recently broke down and bought an AA-54 antenna analyzer to help me with issues that you are describing. I have found that reducing the SWR below 5:1 is helpful in achieving success with the auto tuner.
The Antenna Analyzer is so helpful when making changes like baluns, wire, and coax lengths.
The 9:1 balun has favorable lenghts specified to achieve low SWR on most bands. Have you seen the chart and is your antenna the correct length? Hope this helps.
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Keith - KE4TH

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Thanks for the comments Steve and Bill.  I think the impedance would be too high for an end fed antenna without the 9:1 unun.  I reduced my coax length from 26 ft to 18 ft last week with no change.  Due to local thunderstorms I have the coax grounded at a lightning arrester mounted on a gound rod.  I just added a 12 ft counterpoise to the 9:1 balun and did get modest reduction in SWR on the 40 m band.  I think inspite of the 1:1 unun and grounding the coax braid at the lightning arrester/ground rod, I must still have too much common mode rf current on the outside of the coax.  I will buy a few more ferrite chokes to add to my existing 3, and see if that helps.  I also ordered a new insulator today and will try adjusting antenna lenght to one of the preferred lenghts as posted by Balun Designs.  I'm using their ununs for the 9:1 and 1:1.
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Bill W2PKY

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Hi Keith- I started out with the 9:1 balun and thin wire because of the HOA no antenna restriction. More counterpose, even if it's on the ground, may help. Your feedline is a bit short for 40M as well. Perhaps lengthening the feedline would help too. The feedline needs to be 1/2 wave multiples to reflect the SWR at the antenna. If you have a low SWR at the antenna then that value is reflected every 1/2 wavelengths. Coiling up total of 66 feet of coax might help as well. There is a similar thread: https://community.flexradio.com/flexr...
See if anything in this thread can help.
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WW1SS - Steve

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You can get rid of that common mode current by winding 21 turns of coax on a 3" piece of pvc. I ve done that and it works well. No Need to buy anything. Why an end fed antenna?? I have  a bazoka and it tunes and operates great. Don't need any 9:1 and 1:1  ununs.
(Edited)
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Keith - KE4TH

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Steve - the antenna is end fed for minimal visual impact in our new neighborhood.  The fed end is on the eaves of the house.  Only a single wire is visible from most angles.  On the coax RF choke, the coax braid from the lightning arrestor/ground rod to the shack is the station RF ground.  So I'm trying to keep the length down.  I looked up the bazooka design and like the broad band aspect.  Looks like a good solution for a monoband antenna.
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Keith - KE4TH

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Update - I shortened the antenna by 7 ft and the 40 m SWR has dropped to less than 6. I also plugged the radio power supply into the same surge protector/electrical outlet as the PC and its accessories. The tuning is more stable now - no sudden increases in SWR. I still have what is almost certainly RFI on the PC so I will try 21 turns of coax to make a choke. This will also increase the coax length to the radio. I will also try unplugging the PC speaker cable and choking the CAT 5 cable to see if they are the antennas for the RFI to reach the PC.
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Bill -VA3WTB

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Keith, should do each one at a time, I would check out the ugly ballun for your choke, if not done correctly the choke will not help. 
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WW1SS - Steve

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I misquoted. The balun is NOT 21 turns. It is 18 to 21 feet of coax.

Check it out here  http://www.hamuniverse.com/balun.html
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Keith - KE4TH

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Official Response
Update - I was checking resistances and found a 100+ ohm resistance between the 1:1 unun and the lightning arrestor/ground rod on the coax braid side. This same jumper was 0.2 ohm previously. Found the coax ring was about two turns loose on the bottom of the unun! This loose connection may account for the sudden swings in SWR. All symptoms are better now, so I will open another thread if RF on the PC or tuning issues crop up again. Thanks for the interest and comments. Should have checked all the connectors first thing!
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Tim - W4TME, Customer Experience Manager

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Good engineering troubleshooting!