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

I'm surprised at the difference in loss of waveguide vs. coax:

> all the cell sites I've ever seen appear to use coax, so either > they're using the flexible type [of waveguide] or the cellular > engineers are employing the coax loss to contribute a large part > of their loss budget for the antenna arrays being used.

I think what you're seeing on cell towers is flexible waveguide ("Heliax"):

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Back in the good ol' days (before this fiber optic stuff came along), many of us older cable guys used flexible waveguide for 12- and 18-GHz microwave transmission systems. We could transmit the entire CATV spectrum over a distance of about 25 miles. Back then, of course, the "entire CATV spectrum" only extended up to about 400 MHz, or channel 53.

These systems could be used to distribute signals throughout a city, or to distribute signals to distant communities in rural areas. In the

1970s and early 80s, hundreds of these systems were in use.

These systems used the same off-the-shelf RF transmission components -- antennas, radomes, waveguide, connectors -- that manufacturers were making for other industries. At the time, Andrew was the biggest manufacturer in the business. You could see those big Andrew microwave antennas (easily identified by the red "lightning flash" logo on the radome) hanging on water towers in small towns all across America.

Most of the radio equipment was manufactured by Hughes Aircraft Company. When I first got into the cable business, I was surprised to learn that a big defense contractor like Hughes was making stuff for the cable industry. But in retrospect, it makes sense: the stuff Hughes was building for the cable industry wasn't much different from the stuff they were building for other purposes. The basic components were essentially the same.

Of course, once fiber came along, all this microwave stuff suddenly became obsolete. Most of the old microwave systems have been replaced with fiber, and the equipment has been removed. Some of it has been sold to cable companies in South America, but most of it has been recycled or junked.

Neal McLain Retired Cable Guy

Reply to
Neal McLain
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It does indeed contribute a lot of loss, typically 3db. Wave guide would be alot more efficient but for the frequencies used for cellular (850-900 Mhz), or even the PCS bands (~2000 MHz) it would be impractical due to size.

"Heliax" (Andrews trade name) as used on cell towers is actually a hard line coax, not wave guide. There is a w/g product thats very hard to tell from the hardline from more than a few feet away but the w/g generally has an oval cross section where the hardline is round. The lowest frequency I've seen the w/g used for is 6 GHz.


Reply to
Howard Eisenhauer

On Sat, 02 May 2009 09:49:39 -0400, Neal McLain wrote: ........

....... I thought the big "lightning flash" was a generic warning logo for any microwave transmitter? (having seen them on the Telco towers here in Australia).

Reply to
David Clayton

[Moderator snip]

I've dealth with hardline for amateur radio repeaters. Tough to work with but fairly low loss on 70cm.

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Everything is relative, compared to Radio Shack RG58 it certainly is low loss :).

Not all hardline is created equal, the 3dB I spec'ed is a typical "aim point' for the cellular industry, when you get to the point feedline losses exceed 3dB then you up to the next larger size line, with a correspondingly lower loss. If you can get by with a smaller line (less $$$) & not exceed the 3 db loss figure then you go with the smaller line. If you've maxed out on size then you move up to an air-dielectric cable which is even lower loss than the typical foam core cable.


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

Compared to Radio Shack RG-58, Dixie cups and string are low loss!

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Reply to
Howard Eisenhauer

LOL on the RG-58 comment. We used Belden 9913 for most coaxial applications. But even that has a nominal loss of 2.6db per 100 feet at

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