Compliance lab testing for FCC certification under §15.239 produces one set of measurements for one physical configuration of the transmit system for one set of propagation conditions. If those measurements are accepted by the FCC then a compliance certificate is granted to the equipment manufacturer.
However the fields that such certified equipment will produce after its installation by the end user in a somewhat different, or even the exact same configuration as in the test lab likely will not be the same as measured there, because the propagation environment and the distance and height of the receive antenna can vary from that used in the test lab.
The graphic below illustrates this. Note the wide variation in the fields at 3 meters with respect to their height above the earth.
Even if the field intensity meter used was perfectly calibrated, the field it would measure at an h-distance of 3 meters and an elevation of, say, 2.5 meters is quite a bit less than 250 µV/m, due to cancellation by reflections from the ground plane (the earth). This occurs even though that transmit system would produce 250 µV/m at a distance of 3 meters in a free space environment (no obstructions, no reflections).
This might lead the end user to increase the transmitter output power until the FIM read 250 µV/m, or slightly less.
But by doing so, the fields at higher elevations above the earth will be considerably greater than 250 µV/m.
The distance and height of the FCC's FIM receive antenna, the height of the transmit antenna, reflections along and near the propagation path, and earth conductivity all will have a significant affect on the field intensity the FCC will measure, wherever they measure it.
It is not generally known how, or even if the FCC accommodates this set of realities before deciding to issue a Part 15 FM NOUO.
This is the dilemma.
Rich said:However the fields that such certified equipment will produce after its installation by the end user in a somewhat different, or even the exact same configuration as in the test lab likely will not be the same as measured there, because the propagation environment and the distance and height of the receive antenna can vary from that used in the test lab.
I said: Not true with regards to test labs. That is why the FCC requires a Site Attenuation Test for all sites submitting data to the FCC. If the site tracks well with the theoretical prediction then the data from one site to another is repeatable within typically 2 dB.
wdcx wrote: ... If the site tracks well with the theoretical prediction then the data from one site to another is repeatable within typically 2 dB.
Apparently you are comparing the measurements made by two test labs, both of which had acceptable Site Attenuation tests.
But the site attenuations for the various random physical locations where end users install their Part 15 FM systems, and those effects on measured fields there would be unknown, wouldn't it?
A difference of 2dB would allow a power difference of 1.58. That could result in a field strength of 315uV/m@3m. If the FCC issued a NOUO for 641uV/m@3m, they could probably issue one for 315uV/m@3m. For part 15 FM transmitters operating at the limit of what is legal for unlicensed use, 2dB is a huge uncertainty.
Field strength is easy to get wrong, especially in the direction that makes someone who thought their transmitter was 15.239 compliant technically into a pirate. A piece of metal near the transmitter could directionalize the radiate field. The field strength measurement could be taken at a height where reflections sum in phase. A co channel signal could cause the field strength meter to read high even though the transmitter is fully compliant. And so on. There is nothing wrong with transmitting in 88-108MHz, but testing compliance is not easy for the typical person to do.
A reasonably prices part 15 transmitter is not even close to a perfect isotropic radiator. If an FCC inspector wanted to find noncompliance for a transmitter that was set to be just at the limit, given enough time the inspector could probably find some location where the combination of reflections, transmitting antenna pattern, distortions of the field from nearby objects, and propagation path from the receiver would lead to a no compliant reading.
The dilemma with part 15 FM is while it is legal to transmit because of the provisions of 15.239 checking compliance at every height along every azimuth at every distance is not practical so there is no assurance a setup is NOUO proof.
(On the other hand, a tape measure and a multi meter can demonstrate compliance with 15.219 on AM for a transmitter that has a place to measure the voltage and current into the final stage.)
It strikes me as odd that accurate FM field intensity meters cost around $22,000, while certified FM transmitters can be had for the low low price of around $50.
Quite some disequalibrium.