TheRadioBoard

If online skin depth calculators can be believed for a flat, wide conductor like aluminum foil, then the skin depth at 7 MHz should be 31 microns. Most foil sold on shelves here seems to be 10-20 microns, but I found thicker aluminum foil at 60 microns available online (probably also locally if I looked hard enough). So a thick foil should, in terms of skin depth, be usable at 7 MHz and above. I still don't know how bad the effects of wrinkles and sharp edges will be - but heck, aluminum foil is cheap, so it's worth a shot.

Could you fold the edges over? If using thicker foil, a light fold (non-compressed) of (say) 5mm would leave you with a rounded edge exposed to the "aether".

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I'm not so clear on the whole issue. My tentative understanding is that sharp edges can cause coronal discharge and impede RF current flow because electrons (and there are huge quantities of them sloshing around in a magnetic loop antenna) tend to pile up at sharp edges.

First, where are the sharp edges of concern? Compared to a smooth tubular conductor, a rectangular strip conductor has intrinsically four sharp edges, two along the strip length and two along the strip width. Additionally the surface will have numerous wrinkles and sharp peaks (consider the extreme case of a crumpled piece of foil).

Ignoring for the moment the surface irregularities and focusing on the rectangular edges, if we fold the edges over as you suggest, would this prevent the "piling up" of electrons that impedes flow and causes coronal discharge?

Practically I think edge folding along the length would be impossible (as the conductor couldn't be formed into a loop then). Edge folding along the width (at the loop ends) seems possible, but it's not clear to me how this would help and if the benefit would be significant compared to the surface irregularities which will still remain.

It could if the edge is folded right over? which was what I envisaged.
You are not dealing with deadly accuracy if wrapped around some cartons?

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OK, let's assume it is possible to fold all along the length of the strip (one fold along the long top edge and one along the long bottom edge) and to form this into a loop without introducing more sharp points.

Then, wouldn't the sharp edges still exist, with only their position being changed? Or are you saying that by folding in this manner current will not flow along the sharp edge anymore, effectively stopping at the (soft) fold?

Capacitor plates on high quality caps used to be "tumbled" to remove all sharp corners and edges. I guess if a corona was a problem then you would have no option but to try it.

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When referring to skin depth on a flat conductor, they are referring to how far in from the edge you will have current flow. The flat face of the foil contributes virtually nothing, which is why flat conductors are almost never used for RF. So, in your case, if skin depth is 31 microns, and your foil is 20 microns thick then you will have current flow at each edge over a cross section of 20x31 microns for a total of 2x20x31 (since it has two edges) or 1240 square microns. This means that using foil any wider than 62 microns is a waste of copper. Thin wall copper tubing is far more efficient.

Some cause for hope is given by this paper:
http://fermi.la.asu.edu/w9cf/articles/equiv.pdf

According to the formula in that paper, the equivalent conductor diameter of a flat strip conductor of 300mm width and 0.060mm thickness is 56mm. That should be usable for a magnetic loop antenna.

One of the assumptions of the calculation is that "the strip is much thicker than a skin depth". If the skin depth is 31 microns and the thickness is 60 microns, I wonder if this qualifies.

Don't know if this helps: "Current distribution in HF conductors"

http://www.eetimes.com/design/power-man ... conductors

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Most "flat conductor" coils are wound on edge like this:

http://www.eham.net/data/classifieds/images/324035.jpg

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Why are flat conductor coils wound that way?

I recall having read that for a magnetic loop antenna made of strip conductor the coil should not be wound on edge, but can't recall the reason. I think it was something about the non-uniform effects it would have on the current distribution in the strip (the outer edge represents a greater path length than the inner edge).

The speed of light being constant and the velocity of propagation in the copper probably also being constant, there would be a slight variation in phase across the strap width. This might be a useful quality for an inductor, but not for a radiator. In the case of the rotary inductor, the thin edge is easier to contact with a pulley-type contact.

73,

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Yet another niggling little detail to consider is that there is apparently such a thing as a "too wide" strap conductor, at which point the material starts to act like a capacitor instead of an inductor:
http://www.eham.net/ehamforum/smf/index ... #msg395335
(see Reply #11)

Anyone have any more details about this phenomenon?

I'm slowly getting to the point however that I think I may be entering "good enough" territory. If the equivalent conductor diameter is somewhere around 56mm, and the capacitor losses aren't severe, my feeling is I will be able to have some QSOs on this antenna (particularly on the higher bands) even with my tiny 5W. I hope to report back with on-air results soon. Gotta build that noise bridge first though.

Hmm, maybe. But he makes some assumptions that I'm not totally comfortable with. Rather than comparing to an "equivalent round conductor" it would be far more useful to use one of the existing well recognized formulas to calculate the actual RF resistance so that you know what numbers you're really dealing with.

Also, you say that your strip is 300mm wide. That's approximately 12 inches, which is a lot wider than the strip that's shown in your photos. It looks closer to 30mm.

The photo uses 50mm-wide copper tape. The next planned version will use 300mm-wide aluminum foil. There is apparently some danger in using an overly-wide strip (stray capacitance, if I understand correctly), but I'm planning to proceed anyway.