Separate Transmit and Receive Antennas Produce Audio Distortion (possible RF feedback?)

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  • Updated 5 years ago
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Separate Transmit and Receive Antennas -- Distortion: Beta v1.0.5; Flex 6700; Win 7. In the last day or so since I installed Beta v1.0.5 I've received several reports that my audio is distorted, and I've received several that it's beautiful. Just now I discovered the what I believe to be the variable. When I am using my Tennadyne T-6 to both receive and transmit on ANT1 I get great audio reports. When I use my Tennadyne to transmit on ANT1 and use my Pixel RF PRO-1B magnetic loop antenna to receive on RXA I get terrible audio reports. I can switch back and forth between the two states to demonstrate this. I had not experienced this before, but I am not sure that it is a function of the new software or was there all along, and I was not often using the loop. It may be a problem in the new software. It may be necessary to use the loop amplifier's ability to be switched off during transmit. I am not using a power amplifier; my impression has been the switching off would only be needed, if at all, with power amplifier and then only to protect the loop's amplifier and, possibly, the receiver's frontend.
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Jim Bryce W5HFS

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Posted 5 years ago

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Dave-N9CHM

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I've been using 2 PRO-1B loops with my 6500 (1.0.5 software) for the last several days with no audio distortion problems. I do use an amplifier, running 1000-1200W on 40 & 75M and I use the relay switch for both loops as suggested to protect the LNA in the loops. I would try running an RCA phone patch cable between the radio and the relay in the loop to ground the loop/LNA on transmit. I suspect that is the problem.
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Jim Bryce W5HFS

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Dave's answer confirms my suspicion of the solution. The Pixel's optional switch off provides a fix for this environment. I suggest additional testing by Flex, because many other configurations may lack such an option. I've noticed that when I have the Pixel connected through RXA on other bands I can continue to observe those bands during transmission. I do not at this time know if that type of operation is also potentially a source of distortion.
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Dave-N9CHM

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I haven't been able to make the observations you're had since I ground the loops on TX and I only have a single SCU. However, even before I had the FLEX, I always ran my low band RX antennas through a front end saver, which also grounds the RX antennas on TX. As a matter of fact, I run the RX out from the loops into my FES before going into the 6500. So I have 2X the protection on the FLEX's front end! It only seems prudent with the cost of the radio, and I'd hate to fry the FLEX's front end!
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Jim Bryce W5HFS

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Dave: Thanks for the ideas. What are the particulars of your "Front End Saver"? I
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Dave-N9CHM

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I've been using the FES made by KD9SV for years. The only place I know to order one is http://www.radio-ware.com/ You can read the details there.
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K1UO - Larry

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Array Solutions also has a good FES. No connection..just passing along the info. It is highly recommended that one ,of some type , be used on the RX only ports before feeding external RX antennas directly into the Flex.