Huh? Where did you hear that?
If you "heard" it in my post in this thread in Volume 28 Issue 121, I guess I'd better clarify things.
CATV companies used microwave relays to transmit signals up to 25 miles in the "CARS" band (11.7-12.2 GHz). This band was open to franchised CATV operators and non-profit co-ops comprised of two or more franchised CATV operators. It was shared on a co-equal basis with other fixed microwave services assigned to other industries.
Back in the early days of the cable industry, "CATV" stood for "Community Antenna Television" because that's all it was: an antenna (often on a nearby hill) that picked up broadcast signals and distributed them to customers. So the FCC called the microwave relay service "CARS" for "Community Antenna Relay Service."
As the industry grew and started carrying non-broadcast signals, the term "CATV" evolved to mean "cable television." The FCC renamed the CARS service "cable television relay service" ("CTRS"), but cable guys liked "CARS" better, so the term persists to this day.
What makes the CARS band unique is the allocation of frequencies within the band: it precisely matches the CATV RF band (54 MHz and up) shifted up to the 11.7-21.2 band.
The modulation scheme is single-sideband suppressed-carrier amplitude modulation operating at a carrier frequency of 12.6465 GHz. Thus, for example, the channel 2 visual carrier is upconverted as follows:
54.25 MHz + 12646.5 MHz = 12700.75 MHz.Nothing unique about that, of course: it's the same modulation scheme that AT&T Long Lines had been using for years for their intercity relays.
As I noted in the previous post, these systems used radio equipment (transmitters and receivers) manufactured by Hughes Aircraft Company. Hughes called the equipment "Amplitude Modulated Link," or "AML."
Waveguide was used only at the transmit and receive ends to connect the antennas to the radio equipment. For short runs, we used elliptical heliax; for long runs (for example, a vertical run on a tower) we used round rigid waveguide with short pieces of elliptical for the last few feet at each end.
Transmit antennas were mounted on towers or building roofs. Some transmit sites had as many as a dozen antennas transmitting in different directions.
Receive antennas were mounted on whatever structures were available: buildings, water towers, radio towers, or even wood poles. Receivers were usually installed outdoors, at the base of a tower or on a building roof near the antenna. Hughes designed the receivers for outdoor installation in locations without environmental protection or AC power. The receiver had only two ports:
- INPUT: RF at CARS band from the antenna.
- OUTPUT: RF at CATV distribution frequencies, for direct connection to the distribution trunk. The output port also served as the power input port, designed to accept a 60-volt square wave, the standard industry voltage used to power line amplifiers.
My "Utility Poles" website has an example of a receive antenna and a receiver mounted on a wood pole. Accompanying drawings show a block diagram of the receiver and the frequency allocation for cable channels
2-24.Neal McLain