CARRIER CURRENT

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This is the way that you can get into trouble, learn a bunch, listen to traffic reports, control your lights, or ........

BACKGROUND

I first became introduced to "carrier current" when I was in college at the University of Maryland about 1972. We had a campus radio station called WMUC that was student run and funded by the school. Several of my friends in the television department worked at the station. This included Jerry Cesak, Dan Mayer, and Richard Friedel.

Basically, WMUC operated out of a bungalow. There were a couple of audio production studios and a audio distribution amplifier ( DA ). The DA fed telephone circuits that ran all over the campus to various dormitories. In these "dorms", there was a small 5 or 10 watt AM ( Amplitude Modulated ) radio transmitter. Some of these transmitters were home built and some were from a company called LPB for Low Power Broadcasting. I am not sure if LBP still exists, I have looked on the WEB for it but found nothing. I imagine they do still exist, though they probably don't specialize so much in AM transmitters any longer.

Anyway, these transmitters, numbering in the dozens, had outputs coupled into the main AC ( Alternating Current ) power systems that feed each dorm. The theory is that the RF ( radio frequency ) carrier from the AM transmitter could be carried to any radio plugged into that power source.

Basically the system works. Inherent in the system are two things. Hum and Leakage. The Hum we hated, the leakage we liked.

So, you have this big network of transmitters, college students playing music and feeding the transmitters, and then a handful of students listening to the station in their dorms. NOT BAD !!!

SOME TECHNICAL STUFF

Here's how it works.

A 60 cycle AC line has a RF carrier introduced onto it. In the case of WMUC and WOCR, we used 650 Khz. This because not only were the frequencies available in the area, but they were low on the band. We found, with experience, that the lower frequencies were easier to couple and radiated further. The 650 khz carrier was then amplitude modulated with the audio. YOU HAVE IT RADIO !!

Coupling was usually done with a complicated network of inductors and capacitors. This was a difficult task when you think about it. A typical AC line at 60 hz was basically zero ohms or ground. At 650 khz, it was just high impedance enough that we could modulate it with low power transmitters. The problem was tuning the coupling. Since the power load changed all day as people turned off and on equipment, it was impossible to use really good inductive coupling. Therefore, a simple capacitive coupler was easier and worked just as well. You take a ten watt transmitter and hook it to the power line with a .1 1kv capicitor and that was it!

Because of the close coupling, there was always a little 60 cycle hum in the background, but most people didn't hear it or care.

The FCC ( Federal Communications Commission ) has implemented rules for use of carrier current radio under part 15 subsection C. These rules apply to the leakage allowed such systems. Basically, a formula is indicated that determines the maximum field strength reading allowed at a certain distance from the antenna, which in this case would be the power system. In the case of carrier current radio at 650 khz, we find that a usable 15 microvolts field strength is allowed at about 250 feet from the source.

Most AM radios can pick up a usable signal in the 15 to 20 microvolt range. Since the field strength falls off by the inverse square of the distance, after the approximately 250 feet, the signal falls off so much that there is nothing. Of course, issues like water, and power line impedance change the readings continually. Practically, it is impossible to monitor the signal all the time, so periodically monitored and averaged readings are usually considered acceptable by the FCC field engineers.

The final result is that if you cannot get more than a couple of hundred feet form a radiating source, the power line, you can legally have any level of signal you want with no FCC license. The only restriction is that you cannot interfere with another licensed radio station.

This became the essence of the idea that Dan Mayer had for the creation of WOCR. Basically, there was no way to get over 200 feet from a power line in the whole city without going into the Atlantic Ocean! And, who cared about what happened out in the ocean?

Subsequently, the FCC under some political edict from bureaucrats, decided to interpret the rules to allow interpolation of the readings. In otherwords, using the inverse square law, since they could not go out in the ocean to take readings, they determined that they could go closer than the 250 feet specified in the rules to take readings. Our little radio station WOCR was then "over power". No way, could a couple of college students fight the FCC on this. We didn't have enough money to pay a lawyer to do this kind of endless task. WOCR had to go off the air.

DO YOU KNOW SOMETHING ABOUT CARRIER CURRENT ?

This page, by no manner, is designed to be the end all to information about carrier current. Carrier current is now used by the X-10 family of household controllers and many other devices. Low power regulations are used by many organizations, private and public, to transmit information about road conditions, parking and touring information for parks and other places of interest. Special cables have been designed to allow limited and precitcable radiation over long distances. These cables have been installed along side highways, in subway tunnels and in other similar places.

I would enjoy including other descriptions of low power and carrier current applications in these pages. Maybe you were involved in a carrier current station in college and have stories to tell. If so, just write me here and I will include your information gladly.

Greg McMurry