Arie:

The following formula is taken from the ARRL Amateur Radio Handbook and is 
for the inductance of a single-layer air-core:

    L = d(squared) n(squared) / 18d + 40 l

        L = inductance in microhenrys
        d = coil diameter in inches
        l = coil length in inches
        n = number of turns

Notation in connection with the use of this formula says that this formula is 
a close approximation for coils having a length equal to or greater than 0.4d.

There are formulas for winding loop antennas that take into account the wire 
diameter.

Hope this information is helpful.

Bob Laney
Herndon, VA
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