Part 15 FM Compliance

[BobOnTheJob]

Does anyone have a formula or online calculator that will determine how many inches of "antenna" can be used with a 1 watt FM exciter & theoretically still be in compliance with Part 15? I'm using about 2" of wire hanging out of the BNC jack, it serves the average size ranch home adequately & dies on a good car radio at 200-250'. It 'sounds' like it's carrying about as far as a Part 15 device would be expected to but for CYA purposes, I would like to post a calculation that would show that a good faith effort is being made to comply.

 

[rfry]

The link below leads to a NEC-2D study of a 2" monopole centered on, and driven against an 18" x 18" wire frame representing the FM exciter. The radiation pattern is quite well behaved.

But the practical problem with this is that a 2" monopole has very low radiation resistance, and very high capacitive reactance in the FM broadcast band, which means that its input VSWR for a source expecting a 50 ohm load will extremely high, as shown in the plot at the bottom of the page.

So, very little of the output power from the transmitter will be radiated by the monopole. Instead it will be reflected from the antenna back to the transmitter, where it will be largely dissipated by circuit losses.

There is no simple means of calculating the amount of power that will be radiated, as that depends on the characteristics of the output stage of the transmitter with respect to power reflected back into it from its load impedance.

Probably if the range of your setup is about 200 feet, then it is at least close to complying with FCC 15.239.

http://i62.photobucket.com/albums/h85/rfry-100/FMExciter-2inchmonopole.gif

 

[BobOnTheJob]

Thanks R. Appreciate & respect your expertise.

 

[ironbear]

Thanks R. Appreciate & respect your expertise.

Indeed, it's interesting stuff. Would that be 1W TPO, an 80 dB attenuator, and a dipole, approximately? (≅0.01 microwatts)

 

[rfry]

... Would that be 1W TPO, an 80 dB attenuator, and a dipole, approximately? (≅0.01 microwatts)

An EIRP of 18.75 nW (0.000 000 018 750 watts) produces a radiated field just meeting the FCC 15.239 limit, in free space. The Z-matched input power needed by a 1/2-wave, center-fed dipole to radiate that power in its directions of maximum gain is 11.43 nW.

A consideration here is that cabinet radiation from a 1 watt FM exciter/transmitter might be higher than this, even with a perfectly-shielded, 50 +j 0 ohm dummy load directly connected at its output jack.