rf everywhere

RF

hardware

Have you ever had to row a boat out to a tower after a heavy rain & lightning storm to replace the AC lines for the tower lights?

--
Politicians should only get paid if the budget is balanced, and there is 
enough left over to pay them.

In a coaxial cable the EM field propagates in the dielectric, not in/on the inner and/or outer conductor. The outer surface of the inner conductor and the inner surface of the outer conductor will just confine the EM field into the dielectric as does the inner surface of a waveguide.

For this reason, the quality of the dielectric is critical, dry air is nearly as good as vacuum at lower frequencies (below a few hundred GHz).

In addition, the metallic surfaces should have infinite conductivity, but of course, skin effect and oxidation will degrade the performance.

The 15 cm coax is not very usable at frequencies well above 1 GHz, since the EM field starts to propagate in some strange waveguide mode above these frequencies.

For lower frequencies, coaxal cables work OK even with much larger dimensions, such as 50 cm coaxial "cables" at a 500 kW short wave station.

What is the advantage (if any) to using co-ax v. open-wire feed for above HFBC example?

Coax has the fields contained within the cable, open-wire does not. Structures, people, and critters in proximity to coax don't matter. It's a different matter with open-wire. Open wire usually requires a BalUn, as well.

When you have several (sub)megawatt HF transmitters and a few directional antennas aimed on different continents, you really need a big switchyard. with coaxial relays. I do not understand, how to do this with open-wire systems.

Apparently...

Why not?

\

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com

No one home.

Oops, nix the trailing backslash.

Tim

-- Deep Friar: a very philosophical monk. Website:

The WWII, British, CH radar (circa 50MHz), used open wire feeders to the transmitting antennas. The transmitters were a Marconi-EMI television design, of a few hundred kW, modified for pulse modulation, one of the reasons for killing the early British TV service.

The guy I knew who'd worked on it described how the routing at one site, around the edge of the field, with several corners, to conceal the feeders from the air, couldn't be made to work, and they had to go direct, making a beautiful sighting line to the camouflaged transmitter.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Good grief, I didn't say open-wire was never used, just that it is more difficult.

Perhaps Coax wasn't available? Seems like a perfect opportunity for a ruse. ;-)

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Our site in New Orleans used 9-3/16" air-dielectric coax. (That's right - NINE inches!!). I believe it was made by an RFS subsidiary. New Orleans h ad 3 Class-C FM's, with room for 6. I didn't design it.

In St. Louis, actually a much taller tower, we used 2 runs of 4", one fed t he top half of an 8-bay Harris cavity backed radiator, and the other run fe d the bottom 4 bays of the array. That antenna had 8 Class-C's. Nice setu p. After I left they tried to add a C2 (or whatever?) and burned up the co mbiner. Not too bright.

12-inch continuous supposedly exists, but I've never seen or worked with it . The turning radius must be an absolute bitch!! :)

Link:

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Um.., I don't believe that all oxidation "chemistries" affect performance. For example, silver sulfide is highly conductive. Though, I guess you cou ld argue that tarnish is not "oxidation" in the strict sense. Still, I'd b e curious to review the data behind your statement. Any good references?

This was 1942 Britain.

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Duh, really?

NINE inches!!). I believe it was made by an RFS subsidiary. New Orleans had 3 Class-C FM's, with room for 6. I didn't design it.

top half of an 8-bay Harris cavity backed radiator, and the other run fed the bottom 4 bays of the array. That antenna had 8 Class-C's. Nice setup. After I left they tried to add a C2 (or whatever?) and burned up the combiner. Not too bright.

The Ch55 tower had an 8-bay at 1200' with five stations but that wasn't any of my work. They had a 'engineer' who lived on site to repair the old junk they had moved from other sites, and no spare transmitter. Five different models of various ages and parentage. Each station paid 20% of his wages since the site was up to an hour's drive from their Orlando studios at rush hour.

The turning radius must be an absolute bitch!! :)

Can you imagine trying to put a piece of that inside the Ch55 tower in Orange City? Lowering a 1700' piece of pipe into the tower? Maybe there is a logical limit on length that they don't tell you till you try to order and that any bends are separate sections?

--
Politicians should only get paid if the budget is balanced, and there is 
enough left over to pay them.

That station is long gone, and the site it sat on is now a golf course. I got the nickel tour of that station in '70. SOme of the switching was done in underground vaults at that time. They were in the process of a total upgrade of that plant at the time. 10 new 50 KW custom National .5 to 30 MHz transmitters. A complete new control room, and they were replacing the wood poles with small towers. The only thing that wasn't replaced was the Curtain antenna that was aimed at Europe & Russia.

--

Politicians should only get paid if the budget is balanced, and there is 
enough left over to pay them. 

   Sometimes Friday is just the fifth Monday of the week. :(

Well, a lot of it is. THAT antenna array Is still there, however.

No, it isn't. it isn't even a Frisbee golf course.

That particular array is one of the only remaining historical elements of the site. It is now a learning center for radio broadcasting.

And your alzheimer's set in in the mid '90s.

And some was done by hand on that array in the picture.

3 transmitters were replaced.

Original footprint.

Never happened.

VOA BBC still exists.

The BBC used motorized balanced feeder switches that were about two stories tall. They actually convey a lost sense of whimsy that once made pre-CAD technology interesting.