Mark Kinsler (kinsler@bobcat.ent.ohiou.edu) wrote:
In accordance with instructions imperfectly recalled from an ancient electronics magazine I once wound a sort of "super loop" antenna around the periphery of a closet door. I don't know how many turns of what kind of wire were supposed to be used, but I did think it was cool that you could swing the door so as to orient the loop properly.
This, plus a variable capacitor, should make a pretty good broadcast-band antenna. Does anyone know the proper specifications, assuming a non-ferrite closet door?
I worked through the math last July, and it turns out that if you use
a square form and a standard 0-365 pF tuning capacitor, using about 80
feet (25 meters) of wire will do the trick (irrespective of the size
of the square you choose).
A closet door is of course not square, so the wire length will be
different (probably longer, but it's been a while since I worked out
the math and I can't exactly remember). You could always drill holes
in the door in a square pattern, insert wooden pegs, and use those to
wind the loop.
Take care to separate the turns of wire (1 cm or more between adjacent
turns is best) to minimize self-capacitance inside the loop. If you
don't, the extra unwanted capacitance will prevent the antenna from
resonating at the upper ends of its intended range.
There are also companies that sell pre-built loop antennas. However,
this is one area where you can save a LOT of money by building your
own. I've seen tuned loops that sell for $100 or more, and I spent
around $5 on mine.
BTW, the closet door idea is very clever! I may re-build my loop in
that way; currently the most awkward thing about it is rotating and
orienting it properly without it falling over.