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Drake PRN1000 Antenna Issues

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mfn002

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I recently acquired a Drake PRN1000 that is in excellent condition, to replace my ICR-71A that was far too complicated for the type of SWL I do. I noticed that the Drake, when connected to an antenna made of speaker wire, tends to pick up some sort of interference from some local FM stations. The interference was incredibly strong and washed out the entire 7 MHz band, and parts of the 9 MHz band as well. In my playing around with the antenna, I noticed that if I disconnected the antenna from the ground terminal most of the interference went away, although some still persists in the 9440-9550 kHz area. Also, I've been noticing that I'm only picking up stations from Asia, although that problem began long before I switched radios. What's going on here?
 

ka3jjz

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Sounds very much like you've got a FM broadcast station which is nearby to you overloading the receiver - the fact that you still hear it even with no antenna (although at reduced levels) suggests this strongly.

At a minimum I would try using one of the PAR electronics filters - see if they have a band stop filter for the FM broadcast band (88-108 Mhz) Mike

[edit] I would also shorten the length of the AC cord up simply by wrapping it into a nice tight loop held in place by wire ties, as close to the receiver as you can get. The shorter the better. There is a possibility here that the AC cord is being coupled as a FM antenna (the more so if the length happens to be close to a 1/2 or 1/4 wave at FM frequencies). If that further reduces the interference (particualrly if the antenna is not connecte), then some additional AC line filtering is called for.
 
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mfn002

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There is a station about 2 or 3 miles from my house, and the type of music I was hearing sounded like the type played on one of those stations. To clarify things a little, the problem got better once I hooked up my antenna to only the positive terminal of the antenna screw terminals. When I split the speaker wire and connected both the positive and negative ends to both of the antenna screw terminals, the interference was horrible. I can't really use any of those filters because of the type of antenna connector that the radio uses.

The whole phenomena you mentioned about how the power cable can act as an FM antenna reminds me of an incident that happened a few weeks ago before I finished my internship as an engineer at a local radio station. The local newspaper had requested that we set up a small studio in their building so they could run a daily sports show on a local AM sports radio station without having to make the ten-minute journey to our studios. So, we went over to the paper's offices and began setting up this studio. The paper wanted to use headsets instead of standard microphones. Doing this required us to fabricate what are known as "cough boxes", which have a volume control and a momentary switch that, when pressed, cuts the microphone so the user can cough. Each of these cough boxes needs a cord to connect it to the audio board. After we (me and two other techs) were finished making them, we tested them. When we tested the first one, one of the techs said he heard music in the background. Perplexed, I decided to listen to it. Sure enough, there was music there. Upon listening further, I realized that it was another local radio station whose studios were across the street. Apparently, the cable had been acting as an antenna, something the two other techs had never experienced before.
 
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