[Lowfer] WA test coil.... also loop size radiation resistance

[Lyle Koehler]

That formula quoted for loop radiation resistance has been around for many
years, and as far as I know it correctly predicts the Rr for any shape of
loop. Well, any reasonable shape; it won't work for figure-8 loops, for
example! Since the ac resistance of a loop (neglecting ground proximity) is
proportional to the square root of the area (in other words, directly
proportional to the length of wire), I would expect the radiation from a
10-meter loop to be about -9 dB with respect to a 20-meter loop of the same
wire size, rather than -12 dB.

If you include an N(squared) term in the numerator, the formula works for
mulit-turn loops as well. That has prompted many people to believe that they
can quadruple the radiation, rather than doubling it, by doubling the number
of turns, but it doesn't work that way. Except for the fact that the
impedance may be more convenient to work with, the efficiency of a two-turn
loop is the same as that of two single turns of the same wire size in
parallel, assuming that the turns are spaced enough to get rid of proximity
effects.

Antenna modeling software using the method of moments can be used for
predicting the radiation resistance of both loops and verticals. Procedures
for using the NF.ZIP freeware (which I think is still available on the ARRL
ftp site), or other NEC or MININEC-based modeling software, are described on
my web site at www.computerpro.com/~lyle/models/lftxant.htm  Once the
radiation resistance is known, you can plug in measured current values to
obtain a comparison of your own antenna against a "Brand X" loop or
vertical.

Lyle, K0LR