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Roy Lewallen · October 2nd 06, 12:25 PM posted to rec.radio.amateur.antenna

wrote:
. . .there is foil on the back of my attic roofing. . .

I want to operate most VHF but I like 20 CW and 10 FM and 6 meters -
440 mhz both SSB and FM. So I've been trying to come up with an antenna
to cover that. I have the antennas pretty much decided but Bingo -- the
roofing material problem pops up and I don't have any idea how much
signal I will lose through that stuff. If it is a few DB and messes the
pattern up a little I don't care, but if it is 20 db and won't tune I
do care. . .

Does anyone have any experience with this kind of attic. I have heard
everything from it wont work to people that said it worked better than
in other standard plywood attics.

The whole range of reports could be completely true. There are many
variables involved, including but not limited to the width of the batts,
whether and how much they overlap or how far they're spaced, how far and
how they're oriented relative to the antenna, the type of antenna, the
frequency, possibly the thickness of the aluminum, and the directions
and elevation angles of desired communication.

So any binary answer of "works" or "doesn't work" has a 50% likelihood
of being right. I generally use a coin to make this sort of decision.

I thought a shield had to be at "ground" potential to effectively block
a signal (eg Faraday shield).

Nope, not at all. To be effective, an RF shield has to be continuous,
that is completely enclose what it is you're trying to shield. Shielding
DC electrostatic fields is another matter and not applicable here. Your
foil insulation makes a lousy shield at HF. But that's not to say it
won't have some profound effects, as I elaborate below.

For instance shielded audio cable remains
unshielded unless grounded at one end. Does the same hold true for this
foil barrier heat shield?

Nope. A shielded audio cable works somewhat differently than a coax
cable carrying RF or an RF shield.

An HF antenna will be close to the foil in terms of wavelength. It will
induce a current on the surface of the antenna side of the foil. This
will flow around the edges of the foil to the outside, where it'll
become the real antenna and radiate. The field from this conductor will
(vectorially) add to whatever field escapes from the intended antenna
itself, so the net field will be a combination of the two. The only
practical way to find out what the result will be is to try it. The
proximity of the foil will also modify the impedance of the antenna,
very possibly in a major way. So classical formulas for antennas in free
space might be way off, and if so you'll probably need a tuner to match it.

What I don't know is whether the radiant barrier, which apparently is
often much thinner than a conventional foil insulation moisture barrier
and perforated to allow moisture through, might be thin and irregular
enough to be lossy. But I kind of doubt it.

Good luck!

Roy Lewallen, W7EL