From: David Barts (davidb@ce.washington.edu)
Date: March 16, 1995
Original source: Usenet's rec.radio.shortwave

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.