Radiation pattern of the reference antenna EFHW varies with frequency.
Loops however are very predictable and the pattern is well understood and
does not change with frequency until it no longer a small loop, its why they
A multi turn loop has a lower frequency to size (total lenght) where the pure loop
behavior departs from the predicted. Its easy to see that as the deep nulls are no
longer along the though axis. By then a smaller loop is likely desirable.
The whole 2M thing was a frequency where testing can be done without great
time and expense. One does not have to be interested in the higher frequency
to get useful and accurate data. It is a common practice in the industry to scale
to a frequency where its easy to do the needed testing in smaller spaces or even
a metallic or wood work surface.
I do have a 144mhz (4 inch diameter) tuned loop for noise DF (RX only or under 1W)
and the nulls allow me to determine where on a pole a noise source is once I know
which pole. Very sharp nulls.
In general two costly items are the cap and if padding caps are used suitable high
voltage and high current fixed value caps of decent Q. The element is copper
tube and not a high price item considering how much is required. Copper is
preferred over aluminum as aluminum has a higher resistance and if oxidized
(its normal state) the oxide surface s a near insulator. Also copper can be hard
soldered (silver alloy) using MAPP gas torch.