M2's Dutch Windmill
This is a photo of my infamous 12-inch miniature square loop antenna. The total boom length is 12 inches (translates into a side dimension of 8.5 inches !). There are 24 windings of 26 gauge enamel wire (occupying 1.5 inches depth) for the primary, and 2 windings for the secondary (placed 0.25 inches from the primary). The primary is connected to a 365 pF variable capacitor. I used stained and polyurethaned pine for this project (but used walnut for my larger versions). The number and spacing of the windings was based "loosely" on the formulas found in J. Carr's book on loop antennas, but I had to initially experiment with a "cardboard" version of this antenna to get the dimensions correct...
The antenna works pretty well but has very sharp tuning characteristics (almost too sharp - hard to fine tune without a vernier), and covers a frequency range of approximately 450 to 1500 kHz (basically the AM broadcast band). It exhibits a little hand capacitance problem when you bring your hand close to the tuning knob, but it is minor and only noticeable at higher frequencies (above 1300 kHz). Directionality is decent, and sensitivity is comparable to a Marconi antenna of short length (40 feet).
I have this antenna connected to a American Bosch Magneto 48 TRF radio (vintage 1930). If I turn the volume (really the RF sensitivity control, in those days) up too high on this radio with the 12-inch loop connected, I get a slight squeal (probably from regen osc). This squeal is not present with a long wire antenna.
I have found no literature on these small (less than 24 inch) type of loop antennas, but this one seems to work pretty well. Its design came out of an aesthetic need to be placed in a living room (and satisfy my wife). My daughter calls it the "Dutch Windmill"