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Follow up on South Florida pirate station: filter your harmonics

Pipe organ from Strasbourg (wikipedia).

No need to filter the harmonics coming from this pipe organ from Strasbourg (wikipedia); your pirate transmitter on the other hand . . . 

Thanks to John Anderson and “Spatula” for their comments on why a South Florida pirate radio station was likely interfering with the keyless door entry systems of Lexus vehicles. “Lexus keyless entry operates at ~315 MHz. Harmonic? Blanketing interference? Sounds fishy,” Anderson noted.

Ditto, declared Spatula. “Almost certainly a harmonic around 314.1 MHz.”

As we reported in December, Lexus and Toyota owners in Hollywood, Florida were going a little crazy trying to figure out why their keyless entry devices did not work. Everybody was getting blamed: the cops, other cars, whoever seemed to be around. Then the Federal Communications Commission and local police identified a credible jammer: an unlicensed radio operator broadcasting Caribbean music at 104.7 FM.

It doesn’t appear that the FCC has collared anybody for this particular venture (although lots of Florida unlicensed operations are getting busted). But the relevant gear was found under an air conditioning chiller on the roof of a Hollywood area bank. “You can bet that a transmitting rig hidden inside an air conditioner lacked proper filtering, a properly-tuned antenna, and proper grounding,” Spatula explained in his response to the story.

The takeaway appears to be that if you are going to do the pirate radio thing, your rig needs a filter to filter out the harmonics produced by your transmitter. As Steven Dunnifer’s Pirate Radio Guide at libcom.org explains, harmonics are basically “multiples” of the frequency that you’ve aimed for. The online CSGNetwork Harmonics Calculator runs the harmonics algorithm on 104.7 FM as so:

1st Harmonic: 209.4 MHz
2nd Harmonic: 314.1
3rd Harmonic: 418.8
4th Harmonic: 523.5
5th Harmonic: 628.2
6th Harmonic: 732.9
and so on . . .

Interference with Lexus vehicles at ~315 is the least of your worries. These harmonic frequencies venture into the VHF TV band.

“Failure to reduce this harmonic will cause interference to neighboring TV sets,” Dunnifer’s article continues. “You do not want to generate complaints from folks who engage in the odious habit of watching TV. Noble sentiments, such as telling them to smash their TV if they have a problem will not suffice. Use a filter.”


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One Response to Follow up on South Florida pirate station: filter your harmonics

  1. Eric Weaver January 31, 2013 at 5:59 pm #

    Good advice. Also watch out for intermodulation products, ones created when other signals get into your transmitter and mix with your signal There once was a pirate around here on 104.1, who was putting out intermod products just above 108 – in the aviation band, on a frequency used by an actual air-traffic-control position (NorCal Approach, the San Jose sector, I believe). You better believe THAT got the FCC’s attention fast.

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