Total posts : 45366
That’s probably because it’s not legal. I don’t think that particular transmitter is FCC certified (and even if it is, as you yourself have pointed out in previous posts, that doesn’t really mean much – it’s up to the user to ensure that the installation is legal). If you do the math, you’ll find the range estimates I gave are pretty accurate for a legal Part 15 FM field strength. Unless, of course, you have a field strength meter and can verify that your installation is compliant (not many of us have that piece of expensive hardware available).
When I was broadcasting in FM up here in Canada (where we are allowed 4 times the field strength of the U.S.), my range topped out at about 1 km, and that was with an Industry Canada certified transmitter in a weatherproof box, mounted high above a roof, and located on a hillside with my listening area down the hill (so antennas were pretty much at line of sight, with no obstructions). Typical range was 1/2 km, maybe a bit more. And that range is what the math indicates.
[Allowable U.S. field strength is 250uv into a 1 meter antenna at 3 meters. That translates to a field strength of:
25uv at 30 meters
5uv at 150 meters
2.5uv at 300 meters
Now, a field strength of 2.5uv doesn’t mean that you will induce a 2.5uv signal into an antenna. There’s some complicated math that will calculate that with a typical car antenna (found in other posts), but I usually rather simplistically estimate a signal level of 50% of field strength, so that means a 1.25uv signal at 300 meters. That’s at the extreme lower end of detectable sensitivity for a mono signal – very important – with an extremely sensitive car receiver. Recent Kenwood car radios will receive a 2 to 2.5uv mono signal with 40db quieting, most others top off at 3uv or worse, so expect plenty of noise (most likely nothing) at the 1.25uv level. A field strength of 4uv/signal strength of 2uv would be produced at approximately 180 meters with no obstructions, or close to 600 feet; that would generate a good signal on that Kenwood car receiver with quieting. Of course, if you just happen to have a gain antenna, such as a Yagi (as has been pointed out), attached to your car, you could extend the distance somewhat.]