Saturday, February 4, 2012

Proper FM antenna tuning.. the simple brain dead way

Every wonder how to TUNE that antenna for your pirate radio station to 89.5 or 103.9 (or whatever frequency you can find that's open)?  Here's a simple and highly effective formula I've used many times.  Works great.

Using a properly tuned antenna is essential for micropower broadcasting on the FM band. An antenna that is not properly tuned will not pass along your transmitter's power as efficiently as it could - and this leads to a general degradation of signal coverage.

Fortunately, calculating the precise length of your antenna is pretty easy to do if you follow these three steps. Get a calculator to help with the math:

1. To determine the wavelength of your signal in inches, divide 11811 by your transmitter's frequency in megahertz (MHz). 

2. Multiply the answer from #1 by the fraction of wavelength of your antenna's design (most antennas are 1/2 (.5) or 1/4 (.25)  wave; the popular Comet is a 5/8 (.625) wave antenna). 

3. The answer from #2 is the length of your antenna in inches. 

Try to fine-tune your antenna using a properly-calibrated SWR meter for maximum antenna efficiency.  A perfect SWR match is 1:1; in the real world, you should be satisfied with any SWR of 1.5:1 or less.  Radio shack has SWR meters, with instructions.

You can do it without the SWR meter though.  I have, many times, and the formula above works beautifully.

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