If you are searching for nulls, I find a "shielded" 0.5-meter loop made of 0.5-inch copper tubing hard to beat.? Five internal turns, the center of which is cut and brought out to a home brew and characterized (using an HP VNA) broadband transformer.??
We have a "blowtorch" AM transmitter in Boulder, Colorado on 760 kHz which measures -24 dBm on any of my outdoor dipoles (not cut to frequency - they're all quite short for the frequency).? I'm only some 25 or so miles from the transmitter on the Front Range of the Rockies.? The null with that small loop is so sharp and deep that I'm able to null out that "blowtorch" during day or night (no reduction in power after sunset) to receive other distant stations.? I haven't measured the null depth, but it must be in excess of 40 to 50 dB.
