Leaky Coax

The normal minimum size of a rhombic is four wavelengths on a side, with a 60 degree angle at the apex and 120 degree at the sides. For

80 meters (3.75 Mhz), that covers 1800 by 525 feet. And yes, a line that big is normally copper-clad steel for strength.

Depends. Receiving rhombics usually use a common resistor (carbon comp is OK at this freq, but a metal film would be better) in a minimalist weather protection device - I've seen everything from a porcelain tube formerly used for knob-and-tube electrical wiring up to a NEMA enclosure mounted on the far mast. All of the transmitting rhombics I've seen use a chunk of transmission line, usually running above ground on supports under the center axis of the antenna high enough to let the cows graze underneath without getting zapped. Somewhere I've got charts showing attenuation version frequency verses wire size for steel and iron wire in open twin lead form. For #10 AWG, that's about 8.0 inches center to center. For 100 KW, that's 7.75 KV at 12.9 Amps. Mind the zaps please.

No, I don't think so. It was located between the highway and the coast, but the antennas were quite different. I recall two enormous VHF broadside arrays - I dunno, maybe 60 elements wide, and ten tall pointing in the direction of Hawaii - up fairly close to the road, and maybe four long wires or rhombic pointing in the same direction. They were in the aeronautical band (118 - 136 MHz), as well as a half dozen HF freqs from

3.47 to 17.91 MHz I've looked through my old aeronautical stuff, but they don't give me coordinates. I'm pretty sure it was close to San Gregorio rather than up towards Half Moon. I've got some ancient maps, and it doesn't look as if there is room for a rhombic in to many places.

That wouldn't surprise me. I've got friends who were working at Ames who were commuting from Piller Point, while other are commuting from Los Banos.

East of the Bayshore, between San Antonio and the Embarcadero - know it well.

I didn't have any high power dummy loads, but I always had attenuators which worked just as well.

Old guy

Reply to
Moe Trin
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Web Results 1 - 10 of about 163 for 'Microwave Theory and Applications' Stephen+F.+Adam. (0.49 seconds)

Product search results for 'Microwave Theory and Applications' Stephen+F.+Adam [shopbag.gif] Microwave Theory and Applications - $24.95 - biblio.com Microwave Theory and Applications (hardcover) - $42.69 - BestPrices.com Microwave Theory and Applications - $45.95 - The BookCellar.com

Geez that thing has gotten expensive. That's a text book that been out for quite a number of years. My copy looks to be from 1976, bought for US$ 16 for a class at HP taught by the author.

Hope you're viewing this with a mono-spaced font. Let's define a two-port device as having four points - two incident, and two reflected:

--> a1 ----- S21 -->-- b2 --->

| ^ | | S11 S22 | | V | SWR and you would not get a 99% reflection See that diagram above? You know we can use other values of S parameters to describe other things. For example, a 'short' or 'open' would have S21 = S12 = 0, S11 = 1.0, and S22 not relevant. Now, if you measured the power going in verses the power coming out, they would be the same, is that correct? It's all reflected. Do the math, and that's '[ 1 + 1 ] / [ 1 - 1]' for a SWR of infinity.

Now let's cascade the 20 dB attenuator with this short/open, and look what we see from the left side of the attenuator. You have

Attenuator Short/Open In 0.1 0 0 0 1.0 who_cares Out 0.1 0

We can ignore the zeros in this exercise, so we see 0.1 * 1 * 0.1 BECAUSE we go through the attenuator two times - once enroute to the total reflection, and once on the way back... at least we do with every regular attenuator I've ever worked with. (Yes, I've also worked with ferrite isolators, but they're really three port devices, not two.)

Not to sure where you found that decibel value, but you are forgetting that the indicent power takes a loss going in, and the reflected power takes a loss coming back through the attenuator. The reflection would not be 1.0, but would be the product of the losses through the attenuator times the reflection of the short/open - or 0.1 * 1.0 * 0.1 - or if you want it in decibels, (-20) + (0) + (-20) [negative because they are a loss] or -40 dB. The SWR would be '[ 1 + 0.01 ] / [ 1 - 0.01 ]' or 1.02:1

The above explanation is a simplification, for conceptional use. In the real world, things get horribly complicated by the fact that S11 and S22 have values other than zero, and phase angles get involved, and there are more than two devices involved. The Adams book noted above is a good college text book that goes into a LOT more detail.

Old guy

Reply to
Moe Trin

I can just see the guys up at PARC cringing. And yes, I did work with the original 3Base5 that really did use RG-8/U, and none of that nambie-pambie fire marshal approved crap ;-)

Old guy

Reply to
Moe Trin

$3 to $16 used on Amazon. |

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My copy walked away at a consulting job about 10 years ago. I miss it. I think I paid about $50 in about 1980.

Simple to anyone with a few years of network analysis.

Worse. The leaky coax is a distributed attenuator with a combination of leakage and dielectric attenuation. The leakage (radiation) is fairly uniform along the length but tends to leak more at the source end due to dielectric attenuation effects. Modeling doesn't look too horrible.

PREDICTION OF INDOOR WIRELESS COVERAGE BY LEAKY COAXIAL CABLE USING RAY TRACING

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A Leaky Feeder Model for Indoors Impulse Response Modelling
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Reply to
Jeff Liebermann

There've been at least five printings of it - probably a lot more. My copy is a 4th printing, and has a LoCUS catalog number of 76-82902 and what I take to be an ISBN number of 13-581488-X, though it's copyright date is 1969.

As you know, it's very hard to provide a decent background/explanation on Usenet. That post was what, a hundred lines? The class that Adam taught was two semesters two nights a week. I got conned into it by the company requirements of "continuing education", and the class (from Foothill-DeAnza) was held at 1500 Page Mill Road, in his lab area. And of course, he's got all kinds of nifty toys laying about - some of which probably retailed for something like my annual salary at the time. But he's a good teacher and knew what he was talking about, and had the gear to be able to demonstrate an answer to a question. Quite formidable. I don't know about the rest of the students, but _I_ certainly got a lot out of the class.

Fun case one time - I'm visiting a site that is unstable, and no one knows why. It's a C-Band rig with multiple antennas, ferrite switches, and automated RF monitoring up the whazoo. Finally I notice that if I open the cabinet door while monitoring the reported VSWR, I see that the angle the door is open has a huge effect. WTF??? Finally spot the fact that one of the waveguides way down in the back of the cabinet is wrapped in masking tape over what appears to be aluminum foil... Some unknown _factory_ tech had damaged the flexible (corrugated) guide - and "repaired" it by tightly wrapping the tear with foil, and holding that in place with masking tape.

Ghod, I'd need a drink after that. ;-)

Old guy

Reply to
Moe Trin

Thanks for that.I shall dust off my calculator and do some reading.

Bob

Reply to
Bob II

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