coiled and folded antennas

It's more simple than that: the coiled antenna is an inductor that's coupled to free space; the important parameters are it's inductance, radiation resistance, and it's actual resistance. The radiation resistance is a measure of how well it's coupled to free space, and it isn't very high -- in fact, it'll be much less than the actual resistance of the wire, so losses will be high.

Winding the coil with silver wire would help, or using a high-temperature superconductor (and a non conducting vessel for the LN2). Best would be a bigger antenna...

Antennas that are "small" compared to the wavelength are pretty much dominated by their effective area.

But there is the old AM resonance ploy.

Resonate a fairly large loop antenna nearby, and the AM reception sensitivity will sharply increase.

Sort of a Q multiplier effect.

No. The area surrounded by the coil is a lot more important.

No. The coiled length is usually a lot less than the long wire, uncoiled length.

An antenna is a transducer than connects to the energy passing through some volume of space. The length of the conductor is not so important as how the energy is coupled to it.

More like inductively coupling a larger antenna to the loop in the radio.

At 300kHz my understanding is that the best antenna for a receiver is a one meter probe feeding the gate of a FET. It'll certainly pick up enough atmospheric noise to overwhelm the thermal noise in a modest quality receiver.

Nope. The critical thing is resonant frequency. In other words, there are parasitic capacitors between the close-spaced wires. EM energy can jump across.

In older AM radios, the ferrite coil antenna is tuned by one section of the main tuning capacitor. When in resonance with the desired AM station, the antenna receives lots more RF power than with a simple untuned coil of the same size.

No, but it's not that far different. It depends on the spacing of the turns, and the inductance of the loops formed.

Maybe. Or perhaps it falls as the number of turns squared. You reduce the gain by making the antenna physically smaller, but you increase the gain because the Q is higher.

The gain rises. If multiple waves must pass, that means the peak at resonance is sharper. Higher Q gives better coupling and larger received power. But a resonant antenna must be tuned to the desired frequency, not like an untuned quarter-wave antenna.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty chem.washington.edu UW Chem Dept, Bagley Hall RM74 billb eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066