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Post by Deleted on Dec 27, 2015 19:52:49 GMT
Using crippled antennas as a way of meeting 15.239 results in poor signal distribution within the allowed field of coverage.
Crippled antennas include shortened telescopic, rubber duck or wire stubs intended to confine field strength to the small physical space achievable under 15.239.
Crippling the antenna results in antenna mismatch which increases the incidence of multi-path and canceling reflections within the legal range.
To experience maximum performance from a Part 15 FM transmitter the optimum signal is achieved with a well matched and tuned antenna well placed in the environment. The means available to achieve 15.239 compliance is to reduce the RF power output accordingly.
A well matched and tuned FM antenna will provide a solid, positively receivable signal everywhere in range.
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Post by thelegacy on Dec 27, 2015 20:28:38 GMT
The whole House FM Transmitter 3.0 seems to have a decent stick antenna with less multipath at part 15 setting. The SainSonic AX-05B had far more issues with multipath at its low power setting. Its supplied antenna was bigger. Why was this the case?
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Post by Deleted on Dec 27, 2015 21:16:39 GMT
Some Thoughts on FM Antennas
One serious "crippling" of Part 15 transmitters that all of the certified models seem to do is not having any contact for the negative side of the FM output path, thus preventing the user from having an antenna counter-poise or ground-plane.
An FM antenna that only has the positive antenna, be it telescopic, rubber duck or wire, is lacking the ground plane elements or lower half of a dipole. This throws the antenna out of balance.
Of course cutting an antenna to the proper 1/4, 1/2, 5/8 or full wavelength helps get closer to a match, but the antennas that come with these transmitters are too short for that.
Placement or location of the antenna is far more important than we have talked about, and indoors is a lousy place for a transmitting antenna because of all the reflections and cancelations caused by walls, furniture, bodies and pipes. Center of the room is best, but that's usually not a convenient spot to place a transmitter/antenna.
Of course placement by a window has benefits for getting outside, but all the physical objects around the window cause massive reflections.
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Post by bluebucketradio on Dec 30, 2015 1:13:02 GMT
Dummy Loads
Some of these little transmitters also have some sort of dummy load inline with the rf out circuitry making the short antenna even less effective. Usually some type of resistor or network of resistors.
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