Flex 6700 17M SWR issue

Mark W Scott

I am using a Flex 6700 using v2.5.1 into an 8-element LogPeriodic antenna.  I also use AntennaGenius v3.1.2 to connect the Flex 6700 to the LP antenna.  Recently I started having problems using the ATU on 17M to reduce my SWR on that band.  I have no trouble use the ATU to control my SWR on any of the other bands I use this LP antenna for (10M, 12M, 15M, 20M).  Any suggestions for why I might have a problem on 17M but not the other four bands? 

Santiago Mejia HI8O

This happened to me earlier today as well. I'm using a 6300 with a hexbeam.

Rick - W5FCX

What’s the problem more specifically?

bahillen

I have seen problems in the past with antenna tuners. The tuner is an L configuration. The length of the feedline translates the impedance seen at the antenna to something the automatic tuner can’t resolve. You may be able to add a short length of coax which changes the reactance seen by the tuner. Try 3 feet then 6 feet and test what happens. Check all your bands if it works on 17m to make sure it didn’t affect others. Why it was working then stopped is unknown but could be from changes in the antenna on 18 MHz or any change in feed line. Also try tuning across the 17m band to see if it does tune at one end or the other. Check for snug connectors or other changes made in the feed line. If you have an antenna analyzer it may be interesting to take some measurements and record the data for the future and find how the LP behaves across the spectrum. Things to think about in your search for a solution. 73 Bill W9JJB

Steve Moore

My comments are the same as Bill's, W9JJB.   I have experienced "odd" tuning for no reason.  Come to find out it a Polyphaser was the problem.  The other problem was the feedline length of coax.  I added 6 feet of coax and the problem went away.   This technique is documented in the ARRL antenna book.  

Mark W Scott

When I use the Flex ATU to reduce the SWR it continues to transmit while it tries to find the best settings but never seems to.  The SWR remains high on 17M.  This does not happen on the other four bands, all works as expected.  I've had my rig for almost 18 months without a problem but now I seem to have a problem on 17M.

Rick - W5FCX

What’s most perplexing is that if your log periodic is actually a multiband antenna intended for 17 meter operation, there should be no need to use the ATU as 17 meters is a very narrow band where the antenna should be resonant. Which model LP is it? I agree with using an antenna analyzer to get to the bottom of the issue - instead of potentially masking over a problem with ATU compensation. You want your LP resonant for best performance. Otherwise you have a compromise antenna and what’s the point of the LP? My 2c. Get to the root cause first.