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Some of the better car radios have sensitivity specs of 1.5uv with quieting (generally about 30-40 db). But it takes a greater field strength – on the order of 2.5 times that – to induce that kind of signal into the radio (that’s an estimate from previous calculations here). That translates into a required field strength of 3.75uv/m.
If you do the math, starting with a field strength of 250uv/m at 3 meters, you’ll get that approximate level of field strength at 200 meters, or about 650 feet. And that’s line of sight to the antenna, no obstructions.
However, you could expect to get a little more range with less quieting with that kind of radio – the signal will be noisier, but still listenable, at least for a while. And a mono signal will eke out a bit more distance. That’s where in a previous post I came up with the 600-800 feet estimate.
However, since most consumer radios have sensitivities probably on the order of 25 to 50uv, you’ll get much less range on them (and that’s ignoring their lack of adjacent signal rejection, which can reduce range for your puny Part 15 signal even more). If we’re charitable, and assume 25uv, then you need a field strength on the order of 62.5uv/m. Doing the math, you’ll get 125uv/m at 6 meters, and 62.5 at 12 meters or under 50 feet. That 200 foot estimate is pretty generous for ordinary radios.