RE: Waveguide (was "size a major consideration...") [Telecom]

I was amazed to hear of the cable co. stringing miles of > waveguide. At the cost, I'd assumed they used microwave > links or coax.

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

formatting link
down to "A Pole Supporting a Microwave Antenna."

Neal McLain

Reply to
Neal McLain
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That makes far more sense. But it is true that at one point, AT&T Long Lines was experimenting with buried waveguide for long haul transmission. One such station was in Ballard MO.

I've known of CATV companies that did use some coax backhauls to the head end; now fiber rules the nest.

Reply to
David Lesher

No. To transmit the video of one TV signal, AT&T Long Lines used FM modulation of one 20 MHz channel at 4 GHz or 11 GHz or one 29.xx MHz channel at 6 GHz. The audio was transmitted separately on other facilities using wide-band audio channels which were intermixed with regular 4 KHz-wide analog voice circuits. The microwave systems had names like TD, TH, TJ, and TL; the "T" meant televison because they originaly were designed to capture the television transmission market. The wider-band systems at 6 GHz were developed in the late 1950's in case high-def TV for theaters was developed.

When not carrying television, these systems carried 1200 multiplexed SSB voice channels (1860 channels for the 6 GHz system).

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