Home › Forums › temp › The Facts about the EDM electronics FM Transmitter › A Much Easier Way to Measure Field Strength
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Using the FCC’s comments about range is probably not the best way to measure Part 15 compliance; that may be what they expect the range to be, but they are quick to say in their circular that these are not the rules (and in fact, they also state that they expect AM range to be 200 feet, and there are many FCC approved installations that get far, far greater range than that). They also don’t specify the type of radio used to achieve that range.
The actual rules in the U.S. state that an FM transmitter has to have a field strength of 250 uV/m at 3 meters (in Canada it’s 1000 uv/m). Physics dictates what the distance covered will be.
If you have an unobstructed view to your FM antenna, then you can easily calculate what the field strength will be at a given distance; it’s basically the simple inverse of that distance. So the strength will be 125uv/m at 6 meters, 62.5uv/m at 12 meters, etc.
If you have a radio for which you know it’s sensitivity, you can then easily determine whether you are in compliance with the regulations or not. As an example, most car radios (which have antennas about a meter in length) have sensitivities for FM mono in the 3uv range with quieting – for a sensitivity of 3.8uv, that translates to about 192 meters or 630 feet. Of course, any obstructions will dramatically reduce the field strength. The sensitivity of portables is much less (along with the selectivity, which I have found to be the limiting factor; most portables were not designed to stop the bleeding over of strong signals onto weak ones). And if you broadcast in stereo, expect to cut your usable distance by a factor of up to 10 (that same car radio that will quiet with a 3.8uv mono signal probably requires 30uv for stereo with quieting).
You can also expect greater distances if you can tolerate noise (the absolute sensitivity of the radio is generally called the RF sensitivity, which determines the weakest signal you can expect to hear, with a lot of noise – some Marantz home stereos from the 1970s were in the 1uv range).
So the easiest, seat of the pants way to determine if you’re Part 15 compliant – get a clear path to your transmitter, determine the sensitivity of your radio with quieting, find the the maximum distance before your signal gets noisy, and if you use the above example, if it’s greater than 630 feet, you probably are not legal. Not a field strength measurement, but a lot more accurate than the 200 feet rule.