I was trying out a simple construction idea for a shielded loop antenna
for SW and found that it worked well but it worked even better for BCB.
I have not tried it out on long wave but it might work well there also.

I made two antennas one 25 foot and the other 40 foot in diameter. Both
are vertical and are made of 50 ohm coax. The basic construction is to
use part of the cable as lead in to the radio and the rest is in a loop
shape of a square. It could be other shapes like circle or triangle. In
order to complete the loop the center conductor on the end of the coax
cable is connected to the shield of the coax about 10 foot from the end
connected to the radio. The shield on the far end of the coax from the
radio is not connected to anything. The part of the coax that becomes
the antenna is the part that is the cable end to the place that the
center conductor is connected to the shield.

I found I could make the response a little broader and impedance flatter
by connecting the shield on the cable end with 50 ohm resistor to the
same shield point as the center conductor was grounded to on the shield
with some loss of signal.

The loop has a real nice null. I found that I could null BCB stations
and even WWV at 5MHz with the 25 foot loop. The null at 10MHz was not
very good and the null was non existent at 15MHz. The nulling was tried
out on daytime signals.

These small loops picked up BCB stations very well and yet local noise
that I have around here aplenty is practically absent. The AM broadcast
band has always been noisy at my location and this shielded loop works
wonders on my table top radios. These loops need no amplification, no
tuning and are very easy to make.

--
Telamon
Ventura, California