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

CATV systems commonly used coax to backhaul signals from points within the distribution system (e.g., municipal buildings, schools, the CATV company's office) back to the headed for redistribution. But these signals were usually carried over the same coax network that carried downstream distribution signals. They were carried in the "subband" (5-20 MHz) below Channel 2. A hypothetical example:

In this example, a signal is carried upstream to the headend on a subband channel, and converted at the headend to Channel 13 for distribution to subscribers.

Thousands of CATV systems also employed coax for "forwardhaul" to carry signals from the headend to the distribution network. Most of the early systems were built in mountainous areas where coax was needed to get signals from the headend on the mountain to the residents in the valley. In places with relatively flat topography, CATV companies built tall towers, often outside of town, and used coax to carry signals to the distribution network.

Example: Hillsdale, Michigan:

I wrote:

Did "T" always indicate television? I thought TD could refer to a group of 1200 multiplexed SSB voice channels.

In any case, the comparison I was trying to make is that AT&T used SSB suppressed carrier modulation to combine voice signals into groups. I understand that AT&T used FM for the microwave link whether it was a TD group or one video signal.

Frequencies within the CARS band can be used for FM or SSB AM. There are several frequency plans; the frequency plan I described in my previous post is Group C. Other Groups (A, B, and K) can be used for FM. The allocation plan, as defined in the FCC rules at 47 CFR 78, is at

Many CATV companies used FM transmission in Groups A, B, and K to carry individual TV channels from remote points back to headends.

Two examples:

- Benton KY to Paducah KY. In this case, an FM CARS link was used to carry a Nashville TV station, received off-the-air in Benton, to the CATV system in Paducah.

- North Bergen NJ to Lyndhurst NJ. In this case, an FM CARS link was used to carry a Philadelphia TV station, received off-the-air in North Bergen, to the CATV system in Lundhurst. Even though Lyndhurst is actually closer to Philadelphia than North Bergen, the difference in elevation between North Bergen (210 feet) and Lyndhurst (20 feet) made the link necessary.

The transmitter at North Bergen was located on the top floor of a multistory apartment building, inside an apartment that the CATV company rented. The equipment racks were in the living room, and the antennas were installed on the balcony. The whole thing came to an abrupt end in 1982 when a fire broke out in the apartment. It must have been a big fire: when I visited the site a few days later, everything in the apartment was black, the glass balcony doors were broken, and the aluminum antenna reflectors looked like Salvadore Dali's pocket watches.

Neal McLain

***** Moderator's Note *****

I didn't do a lot of work on microwave, but I _was_ trained in "L" carrier. What little I remember was that one L carrier Mastergroup needed one coaxial cable, and that a single TV channel _also_ required a coaxial cable to itself.

The coax had noticeable transit delays when used for TV, which were long enough that Huntley and Brinkley had to use a carefully-crafted set of "handoff" cues to compensate, so that one had to start talking a couple of seconds before the other one stopped: I don't know if Microwave eliminated the problem.

I think TV signals that traveled on coax included multiplexed audio in later years, although I'm not familiar with the modulation scheme that was used.

Bill Horne

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Bill Horne Temporary Moderator