This topic originated in the "EDM" thread, but seems to need separation from it, as it applies to all FM tx systems.
This topic originated in the "EDM" thread, but seems to need separation from it, as it applies to all FM tx systems.
ArtisanRadio wrote: 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.
There is a bit more to consider here, though.
First, the receiver sensitivity spec refers to a voltage (or power) applied to its input terminals, not to a field strength.
Second, an antenna 1 meter long immersed in a field of "X" µV/m does not necessarily produce "X" µV at its output terminals. What it does produce is a function of its "antenna factor," and that is related to the frequency, and the design of the receiving antenna system (as Neil mentioned).
For example, suppose an FM receiver has a sensitivity spec of 3 µV (21 dBfW), and its input impedance is 75 ohms. We connect it to a 75 ohm antenna system having a gain of -2 dBi, and 2 dB of cable and connector loss. Doing the math for this shows that a field strength of about 9.5 µV/m would be needed to produce 3 µV across the 75 ohm input resistance of the receiver.
So if a judgment was made that the field strength was 3 µV/m at that location, the field strength 3 meters from the transmit antenna in that direction would be about 3.2 times higher than it is assumed to be, over a free space path.
Other variables here are knowing what value of quieting you are hearing equates to the rated quieting of the receiver at its rated sensitivity, the fact that the azimuth pattern of the receiving antenna probably is not uniform (especially if you are using the FM receiver in your car), and the fact that multipath (reflections) can cause the received signal to vary from 6 dB above to more than 10 dB below its free space value.
So all in all, this technique probably is not very suitable for determining compliance to Part 15 FM.
Note that this post is made only to provide accurate technical information in this area. Sorry if it is discouraging.
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Rich,
Thanks for the writeup on the problems with field strength measurements.
One big problem could be solved if we had a known FS reference to calibrate our equipment, such as from a broadcast station signal at a known distance. I am sure there are many problems with this.
Another problem is attaching a meter to the receiver AGC circuit in an attempt to get an objective measurement. Due to limiting in the IF stages, this is not a straighforward proposal.
Do you have any suggestions regarding what the average hobbyist could do? The best I can come up with is limiting the FM range to 200 feet on a car radio, but maybe a valid approach would be to purchase a certified transmitter and compare my kit transmitter signal reception to that. Of course, if I had a certified transmitter, then there would be no point to compare this to a non certified kit. I would just use the certified transmitter and forget about the rest.
Neil
Neil wrote: One big problem could be solved if we had a known FS reference to calibrate our equipment, such as from a broadcast station signal at a known distance. I am sure there are many problems with this.
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Regrettably, and as you predicted, there are problems with this approach too. Even if the specific power radiated by an FM broadcast station toward a given point at some distance along the ground from the transmit site is accurately known (which knowledge is highly unlikely even by the engineer for that broadcast station), this still is insufficient to accurately predict the field strength that will exist at that distant point.
Such field strength depends on the characteristics of that radio link, which include (mainly) the terrain profile from the tx to the rx site, the presence and the location along/near that propagation path of man-made and natural obstructions and reflecting surfaces, atmospheric conditions, the true radiated power, and the height of the transmit and receive antennas.
No doubt this is why the FCC defines a legal field for Part 15 FM as that field existing 3 meters in all directions from the transmit antenna. At that distance, the modifying effects on the radiated field as noted in the paragraph above are rather negligible.
A Part 15 FM system that is FCC-certified necessarily would need to include the antenna in that certification. It is not just the transmitter that needs to be certified, it is the performance of the complete system, including the transmitter and its antenna.
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A Part 15 FM system that is FCC-certified necessarily would need to include the antenna in that certification. It is not just the transmitter that needs to be certified, it is the performance of the complete system, including the transmitter and its antenna.
And remember, citations are written based on the FS measured by the field agent some distance away. Nobody gets their XMTR measured, the FCC just stands outside and takes a click or two, and that's total system performance. Some units, like the PanAxis ACC 100 are really specific about using the included ANT exclusively or Certification is not guaranteed. Ernie does use total system performance, so your only wiggle room is getting it high and in the clear.
It's especially sad to see the developing intensity of enforcement. Not a time to be silly right now, IMHO...
Experimental broadcasting for a better tomorrow!