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

Several recent posts in this thread have mentioned the "horn" microwave antennas used by AT&T Long Lines. As David Lesher noted in TD 28:127:

Those horns often carried six circuits: 4 Ghz horz > polarization, 4 Ghz vertical, 6 Ghz h & v, 11 Ghz h & v. They > delivered a jaw busting 48dB of gain at 11 Ghz, with a beam > width of about 0.75 degrees. But then they had 36 ft^2 of > throat, were 14 ft+ tall and weighed several thousand > pounds... despite being aluminum...

Narrow bandwidth not only provided substantial main lobe gain, but it reduced side lobe gain. This, in turn, reduced interference to or from other antennas, including satellite antennas.

Cable TV companies sometimes used TVROs based on the same design in order to block interference from point-to-point microwave links. Two examples:

Lyndhurst, New Jersey, 1982:

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Lamar, Colorado, 2007:
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Of course, in some situations, there was an easier way to block interference: put the antennas behind a hill or a berm. Two examples:

West Haven, Utah, 2001:

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Warwood, West Virginia, 2008:
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Neal McLain

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Neal McLain
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Greatly reduced side lobe and especially back lobe gain was _the_ crucial feature of these antenna for the Penzias Wilson Nobel Prize experiment. When you're trying to see 3 K microwave background radiation in the main lobe point pointing vertically out into space, having the side and back lobes picking up 300 K radiation from the ground all around you can be a real problem.

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

Being a ham operator, you'd think I would know, but I'm ignorant of the trade-offs and benefits of using the various types of microwave antenna. If the readers can supply an "executive summary" that telecom managers could use when considering microwave alternatives to leased lines, I'd be happy to publish it.

Bill Horne Temporary Moderator

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AES

The Bell System horn-reflector antenna could transmit both vertically and horizontally polarized radio waves. In the antenna's original design, the azimuth radiation pattern for vertical polarization had a plateau of sidelobes which were not present in the pattern for horizontal polarization. I forget the exact numbers, but the problem occurred from about 30 to 50 degrees off axis, and the sidelobes were about 20 dB higher than with horizontal polarization. The higher sidelobes limited the angles at which routes could meet at a junction station.

I was assigned to find the cause. Experimenting on the antenna range at the Whippany, NJ lab location, and found that the cause was the method of attaching the weather cover. At the bottom of the weather cover, bolts went through the cover into captive nuts mounted inside the throat. Each captive nut was held in place with a small bracket just big enough to cover the nut and riveted to the antenna skin. When I removed these bolts on the test antenna, the sidelobes reduced. The sidelobes were caused by scattering from these nuts.

We came up with an alternative arrangement consisting of a bar of metal on the inside of the throat with studs sticking out, with nuts fastened to the studs on the outside of the antenna. To retrofit antennas already in service, our mechanical engineers came up with a device which would securely grab each nut as it was removed, preventing any nuts or pieces of rivets falling down the antenna into the waveguide.

We held a meeting at our labs location with Long Lines radio engineers from all over the country. The weather cover mounting modification was one of several topics. One afternoon, it became my turn to tell them about modifying the antennas. I described the problem, the cause, and the solution. We officially had named the tool a

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Richard

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