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Making a Dipole FM Antenna
How to get high quality sound from your iPod or Satellite Radio on your car stereo.

  Long story short: I made a simple FM antenna for my Sirius Satellite radio car kit's FM transmitter, and now the sound quality is excellent.

I subscribe to Sirius Satellite Radio and love the concept, but for a long time I had difficulty getting quality reception. My car only has a tape player/AM/FM radio, and it's not easy to get access to the back panel (why car stereos don't come standard with auxiliary stereo RCA jacks on the front panel is beyond me). I bought one of the car kits for Sirius that uses an FM transmitter, which is a popular, wireless way to make use of your car stereo to listen to portable electronics such as CD players or iPods. This type of system works by taking the audio signal and modulating it onto an FM carrier at 88.1, 88.3, 88.5, or 88.7 MHz (switch selectable). You then tune your car's FM radio to that same frequency, and, voila, you hear the music.

Not so fast, though. The FM signal put out by my receiver wasn't strong enough, so I got a lot of static with it, so much so that I gave up on this solution temporarily. I then turned to a cassette tape adapter. You've probably seen these--a stereo microphone plug, which goes into your iPod or Satellite Radio receiver, connects to a cassette tape which goes into your car stereo. Don't buy one of these. I don't consider myself to be an audiophile, but the sound quality was terrible. The high frequencies get badly attenuated, and s's and th's end up sounding very staticky to the point of being unbearable. So I was back at ground zero unless I wanted to crack open my dash board and get access to the back panel of my car stereo, which I didn't.

Then (yes, I don't know why it took me so long) I noticed that my satellite receiver had a jack called "FM antenna." This jack allows you to connect the receiver directly to your car stereo's FM antenna input, but, as I've already said, I could not easily access the FM antenna input for my car stereo. I reasoned that if I connected a dipole antenna to the jack, though, perhaps that would provide enough signal fidelity to make the FM transmitter work without the static. So I fashioned a simple half-wavelength dipole antenna as pictured below:

A simple FM dipole antenna for an FM transmitter (or receiver) system. It plugs into the FM antenna jack on your transmitter. The wavelength, l, is calculated by dividing the speed of light by the frequency, e.g. 1.18 x 1010 in/s divided by 88.5 x 106 Hz.
I could have made the dipole antenna from individual parts, but I bought a $5 AC/DC adapter from Radio Shack that had the right kind of plug and a cord where I could easily pull apart the two conductors. After cutting the conductors to the right length, I plugged my antenna into the FM antenna jack on my Sirius receiver, and wow, what a difference. The sound quality is now excellent (I'm probably violating some FCC broadcasting limits, but I think I'll take my chances). If you've been struggling with the sound quality from an FM transmitter system for your car stereo, and it has an FM antenna jack like mine, I highly recommend this approach.

Notes: The length of the antenna is not critical. I have listed the optimum length for 88.5 MHz, but your car's external FM antenna uses one length to cover the whole FM band because there is not a large penalty for being off slightly in the length. Similarly, the antenna wires don't have to be kept perfectly straight. I tucked mine into the seam along the top of my dashboard so that it's mostly out of sight.

Here is a good site that talks more about FM and TV antennas.


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