Melted coax (was: ANI vs. Caller ID) [Telecom]

Since you've had a lot of experience testing microwave equipment in > the field, I'd appreciate your help to debunk some _very_ tall > tales. I have heard hundreds of apocryphal stories about microwave > failures due to utterly unbelievable causes. The list includes: > > 1. Melted coax due to concentrated solar energy, on one > particular day of the year.

I think this is a reference to "solar outage" phenomenon that occurs in satellite receiving antennas.

Every fixed satellite antenna is aimed at a fixed spot in the geostationary orbit. Twice each year, in the spring and fall, the earth's revolution about the sun causes the geostationary orbit to pass through the ecliptic (the sun's apparent path with respect to a point on earth). This transition can require several days.

During this period, once each day the sun moves directly behind the satellite (or, more precisely, the rotation of the earth causes the satellite to transit the sun). The white-noise RF energy emitted by the sun far exceeds the satellite signal, and simply swamps the satellite signal. A received television signal will slowly fade to noise, then slowly come back. The worst-case situation occurs when the satellite transits the center of the sun; in this case, the outage can last as long as ten minutes. For several days before and after the worst-case day, shorter and less obtrusive outages occur. The entire sequence of outages can last as long as a week.

These outages occur on different days, and at different times during the day, depending on the longitude of the satellite and the geographic location of the ground antenna. Several websites provide "solar fade" calculators; here's an example:

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As you can probably guess, this phenomenon is a big headache for cable TV companies. As soon as the first noticeable outage occurs, the phone starts ringing.

A related problem is solar heating. During a solar outage, infrared light reflected into the feedhorn can damage the feedhorn, and in an extreme case, it could damage coaxial cable connected to it. This is probably the origin of the "melted coax" phenomenon that Bill mentions.

This, of course, depends on the reflectivity of the reflector. For this reason, the reflector has a "matte" finish that will (hopefully) scatter the long-wavelength infrared light but accurately reflect the shorter wavelengths of the satellite signal into the feedhorn. Flat exterior latex paint works well for this purpose.

But, as a certain unfortunate cable TV engineer once learned, high-gloss silver paint does not.

Neal McLain

Reply to
Neal McLain
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satellite receiving antennas.

Putting coax in front of a jet blast deflector on an aircraft carrier WILL produce this effect. The dialectric melts and the center conductor shorts to the shield.

Mark L. Smith

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

Mark,

I'm sure the putting coax cable in the exhaust path of _any_ jet engine would produce that effect, but I was seeking information about damage caused by the sun's rays, not General Electric's.

Bill

P.S. It's nice to see a website that supports folk music, but the dancing ponies _are_ a bit much, you know? ;-)

Bill Horne Temporary Moderator

Reply to
Mark Smith

Actually, it's the other way around. Infrared light has a _shorter_ wavelength (750 nanometers to 100 micrometers) than satellite television signals (1 centimeter or longer). The longer wavelength TV signal is oblivious to the matte pattern.

Reply to
Richard

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