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Sunday, March 13, 2011

Podcast QRM Culprit Found: WBIG-FM 100.3 MHz

Listeners noticed that recent SolderSmoke podcasts had some other audio playing faintly in the background. At first I though this must have been AM broadcast band breakthrough, so I built a little crystal radio, only to discover that the powerful AM stations here were not playing the ZZ-Top style classic rock that I was hearing in background noise. Bert, WF7I, suggested that I listen carefully to the QRM for a call sign. I tried that this morning. I couldn't get a clear call sign, but I picked up the unmistakable rhythm of Peter Frampton's "Do You Feel Like We Do?" Fortunately that song is 1) infrequently played and 2) quite long; I had enough time to take a spin around the FM radio dial. Sure enough, I caught the tail end of the Frampton song on WBIG-FM 100.3 A quick check of the web shows that this is a 50,000 watt station that has its transmitter within about 1 kilometer of my house. Bingo.
The mic cord is just about 1/2 wavelength. That would put the high voltage nodes at the ends, right? I notice that the interference drops noticeably if I wind up the cord, and increases a lot if I stretch out the cord.
I can hear some other stations in there also (one country station). It may be that there are multiple FM stations broadcasting from that antenna site. I can see the tower's red lights blinking from my front lawn.
Thank you Peter Frampton!

3 comments:

  1. I guess Frampton is almost as good as a callsign, hehe.

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  2. But how is it being demodulated?

    73.......Steve Smith WB6TNL

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  3. Aha! Yes, I was one of those who reported the signal. At first when I heard it, I thought perhaps you had some radio playing in the background when you were recording the podcast. Then I thought, "Really?! I don't think that would be thing for an astute radio-knowledgeable person to do."

    Well, glad you found the source. When I recorded the birth of my firstborn with an audiocassette recorder in a nearby room, I had to quickly solder a .001 uF ceramic disc across the plug going into the recorder to prevent a nearby 1300 kHz AM country station from interfering. But FM...now THAT is a station of a different color.

    I did note in our EE lab only a block or two away from the then transmitting antenna location of KUT-FM at UT in Austin, we had a tiny, tiny crawl of 90.3 MHz (freq back then) running around on the really tiny signals we were trying to take Polaroid photos of for that assignment. But we didn't HEAR them.

    Do you think your switch to the D-104 microphone and its high impedance might have provided some way to make the signal live on in your podcast? Try a lower-impedance mic and see if it goes away.

    73, K5FNI

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