FM Carrier Current?

The topic of FM carrier current is being discussed in another thread where the legal considerations are presented but there are technical factors also, so in order not to pull that thread off topic, this thread addresses only the technical part of the subject.

Many years ago I was given a device which connected to the antenna terminals on the TV and plugged into the AC main. This was promoted as a device which "turns your entire house wiring into a giant TV antenna." It worked no better than rabbit ears by my experience and by that of others as reported in magazine reviews. One critical comment I remember reading said essentially "Everyone knows that TV antennas are not the size of a house so what's the advantage?" This made sense when the wavelengths of TV signals were considered though the low channels had fairly long wavelengths.

I opened the device and inside was a single capacitor which connected one side of the twin lead to one side of the power line (unpolarized plug) with nothing else...no balun, resistor,...nothing. I gave it no more thought until now.

Does using the power line to distribute a FM signal make sense? Just as a thought experiment the answer is "maybe yes, maybe no." If the signal is coupled differentially to two conductors of the power outlet then these act as a transmission line so it is likely that the signal will propagate along the power conductors.

Transmission lines are characterized by three parameters: the inductance per unit length, the capacitance per unit length, and the attenuation per unit length (Nepers per meter...one of my favorite units of measurement). The inductance per unit length and the capacitance per unit length determine the characteristic impedance. Assuming that this impedance can be matched via balun or other well known technique this can be disregarded as a limiting factor other than to say that junctions will cause reflections throughout the power system which can severely disrupt the signal and the match.

What remains then is the attenuation per unit length (a/ul), "leakage from the lines", and anything along the line(s) which will shunt and thus attenuate the signal. Since the cables used for power distribution are not intended to carry RF they are not characterized for loss at FM frequencies but such losses are likely high due to the dielectric used as insulation. This would mean that the signal would be lost at some distance from the transmitter. Without knowing the actual a/ul a prediction of the signal strength at a distance is not possible and would need some lab work to determine.

Carrier current would imply that coupling to the receiver would be either through the receiver power leads and/or by radiation from the power lines to the receiver antenna. An ideal twin lead transmission line will not radiate a signal unless there is some unbalance in the currents in the conductors and such an unbalance is likely to be caused by junctions and devices connected along the line which are not predictable other than to say there could be radiation due to this. Many electronic devices have noise filtering at their power line inputs which will shunt the RF signal from line to line and line to ground so this works against signal propagation.

Unbalance could be achieved by feeding one side of the coupler differential output to a power conductor and the other side to another physically separate cable or ground lead but doing so will not yield a predictable load impedance, though this could be addressed by adjusting the coupler for proper loading.

Back to the "maybe yes and maybe no" answer, considering the theory it is worth experimenting with the idea of FM carrier current.

This raises the question of the 250 uV/m at 3 meters limit in regards to what is the 3 meter distance measured from but I'll leave that to another thread.

Neil