Active magnetic loop, how big should it be?
The loop must have the next characteristics:
Frequency of coverage: 100KHz-30MHz
Design:
A preamplifier with about 20db of gain will be used to make it an active loop http://www.g8jnj.net/activeantennas.htm
To be "magnetic" at HF and below, the loop should not be big. But how small can I make the loop, so as not to lose the directivity (nulling) feature at these frequencies and at the same time provide reasonable reception sensitivity (based on my 20db preamplifier)?
Magnetic-loop antennas can have the circumference one lambda or a fraction. Best used at resonance which limits noise bandwidth and increases the low radiation efficiency.
Making the loop one or up to four turns allows to diminish the size. Directivity is affected by loop position above ground.
At short waves using a low-noise preampliier makes no sense due to atmospheric noise. Receiver sensitivity is rather affected by noise bandwidth, therefore use the resonant loop.
Details can be found in "Burger, Dvorsky, Magnetic Loop Antenna" book, available at www.pollabs.com
To make it a magnetic loop, the circumference should be small compared to the wavelength. I would start with lambda/10 or so at the highest frequency, and see if capacitance values are practical.