TheRadioBoard

RF resistance of a smooth tubular conductor (as in a magnetic loop/small transmitting loop antenna) is comparatively low. I'm wondering about the extreme opposite: a highly irregular, rough, and crumpled surface.

In particular this individual has used, for a small transmitting loop antenna, a single long sheet of aluminum foil crumpled rather arbitrarily into a loop:


http://mashoo.org.uk/radio/antennas/mag ... ntenna.php

Does anyone have any notion of how much RF resistance is introduced by the crumpling? Is it in the "too low to worry about" range (few milliohms), or in the "significantly detrimental" range (hundreds of milliohms)? I don't have a sense of intuition about how surface area roughness affects RF resistance - can anyone offer any opinions?

"Depends." There are going to be frequency-dependent current irregularities along the way, which could cause irregularities in the field and phase cancellations (and enhancements!). At HF, the effects are probably slight, as the surface variations are small relative to wavelengths.

73,

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Now that is a very interesting argument and quite encouraging if true. Tenuously (very tenuously) extending this logic would imply that the loop itself in the picture might achieve RF conductance on the same order as an aluminum tube of equivalent diameter. This would be great because such a loop is ridiculously simple and cheap to fabricate: just wrap a continuous length of aluminum foil in a tube-like shape around a non-lossy circular central support. Easier and cheaper than aluminum tubing, and the foil can also serve as the capacitor!

I skimmed a fairly thorough US Navy report on multi-turn wire loop antennas (I can provide the reference if anyone's interested) where significant attention was given to the proximity effect of current in adjacent conductors introducing loss, and an optimum of conductor diameter and spacing was discussed. In the jumbled mishmash of a crumpled conductor, I wonder if such proximity effects will also be present, or if the whole mass tends to act (on the whole) like one single conductor, again using the argument that the surface variations are small compared to a wavelength.

Practically speaking, though, if crumpled foil transmitting loops worked well, I would seriously expect to have seen more reports about them, as hams are an inventive bunch. That I've only seen the above referenced web page may be a warning that unknown other losses limit the efficiency of such a loop.

Only one way to find out .

I'd be concerned about the resistance of the connections to the foil. Most of what I've read wants very good electrical conections with a small transmitting loop.
Mike AD1OS

I agree. The loop design presented above is sub-optimal in that regard, using mechanical pressure connections between the loop's foil conductor and the variable capacitor.

The thing though that I find infinitely cool about magnetic loops is that if you use the loop conductor as the capacitor, then you don't need any electrical connections at all to the loop conductor! (The coupling loop also needs no electrical connection.)

To use the loop conductor itself as a capacitor you just keep the ends of the loop conductor in proximity to each other forming a sort of large gimmick capacitor. I've tried this and it works with 50mm-wide copper tape. Others have inserted copper tubing into itself to form a trombone capacitor - again, with no solder or mechanical connections.

However, once we start to use the crumpled foil not only as the loop inductor but also as the loop capacitor, then we need to worry about the RF losses introduced by the irregular (and large) capacitor surface area - there was talk in another thread about "environmental" losses if capacitor plates are too large, and larger plates also means more stray inductance in what should be a capacitor. Then there's loop balance to worry about as well... still, all in all, if a single piece of foil can make a reasonably efficient small transmitting loop, that would be really, really neat in my book. Consider the usual alternative: silver-soldered copper pipe and a vacuum variable capacitor - orders of magnitude more expensive and troublesome.

Now that you mention it, maybe leaving the ends "uncrumpled" and dressed inside and outside of a suitable insulative tube would do as the cap, with a chance of actually calculating it's value. The tube maybe easier to handle and could act as part of the support. Never did it, just guessing.
Mike AD1OS

Yeah, I'm starting to think the trombone capacitor idea is the most mechanically stable solution for a crumpled-foil-wrapped-around-hollow-tube magnetic loop. I had been trying to avoid a trombone capacitor, as I understand they can be quite lossy compared to a butterfly cap or vacuum variable cap, but mechanically it seems like the easiest solution. The other option I was considering was leaving the ends of the foil loop flat so they could act as flat capacitor plates, but this requires some supporting and hinged structure for the foil plates, which negates the simplicity of the foil loop design.

EDIT: specifics about lossiness in trombone capacitors: http://www.eham.net/ehamforum/smf/index ... #msg246697 .
Interesting, no? A capacitor with a wiper contact may be just as good as, or better than, the oft-proposed trombone capacitor!