ATU loading 20-6 metter Logperiodic with low SWR, but Collins wattmeter disagrees

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Antenna Tuner Anomalies: Flex 6700 running 1.0, Win 7: While listening on 40 meters just now I decided to try the ATU with my Tennadyne T-6 six element log periodic designed for 20 through 10 that also works with a clover leaf pattern on 6. Much to my surprise the ATU clicked a few times and reported "success." I was "dumbfounded". I increased power to about 30 or 40 watts and the Flex SWR meter continued to show very low SWR. Not trusting this at all I looked at my Collins directional wattmeter that revealed a pegged needle and 200 watts in both forward and reverse -- with maybe 40 watts actual power, a sign something is really amiss. My guess is the Flex is loading the coax somehow and the difference in position of the Flex compared to the Collins (there are a few feet of coax between the two) accounts for the this. In any case my gut tells me not to try actual operation. My concern is there may be instances where the ATU is misleading possibly with damaging results. So, experts, am I way over cautious and the 6700 ATU really can load up bedsprings or am I right not to try actually using the log periodic on 40?
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Jim Bryce W5HFS

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Posted 5 years ago

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Ken - NM9P, Elmer

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An external meter won't reflect anything the rig's tuner has done. It will still be showing the mismatch of the antenna before the tuner did it's stuff. If the Collins meter is located between the antenna and the 6700 it is worthless to seeing what the tuner is doing, because the tuner is internal to the rig and all the matching is between the rig's Antenna port and the rig's finals.
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Dudley - WA5QPZ, Official Rep

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Jim,

Ken is right, in that your external meter is on the output side or antenna side of the tuner.. Also as the impedance moves away from 50 ohms, as with miss matches, the readings on the external meter will be much less accurate, and location along the feed line will show varying readings.. I have 5 meters in line and all read different , as the impedance changes.
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K1UO - Larry

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Dudley, You got to remove some of those meters . That will help to not add additional impedance bumps and might even gain you some signal at the receiver by reducing all those extra connections.... just kidding. I understood your point and you are correct. :-)
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Jim Bryce W5HFS

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I'm still laughing at myself about this. The Collins wattmeter is in line for use when my antenna is switched to a dipole fed with open wire line through a Johnson Matchbox. In that configuration I set the Flex 6700 to bypass the ATU and, as I understand it, provide a 50 ohm connection to the Matchbox that I adjust for lowest SWR on the Matchbox and Collins meters. This works great. I'm so used to distinct boxes I overlooked the internal nature of the Flex ATU. Of course even if the ATU matches the T-6 I'm probably warming the LMR-400 coax since it is has much more loss than open wire with high SWR.

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