Owen Duffy wrote:
Perhaps if you want to receive normal signals, you just need an antenna
that will deliver 6dB more noise (ie from ambient noise) to the receiver
than its equivalent internal noise. In that case, the received S/N will
not be more than 1dB worse than with a lossless antenna system.

Owen

Andy writes
A good point..... The rule of thumb I have always used is that if
you turn up the receiver to hear the noise, and THEN connect the
antenna,..... if the noise level goes up a lot, then everything is
working just fine...... At HF, it isn't unusual to have 30uv/m of
"noise", from cosmic or galactic or whatever... A 20 db noise
figure is just fine for a receiver in those ranges...... The biggest
challenge is man-made interference ......

I have seen several articles using ferrite loops at HF, mostly
for building direction finders.... If the ARRL handbook has a
heading under "direction finders", or Hints and Kinks, or
the ARRL Antenna Handbook, they can give the OP some ideas....

Andy W4OAH in Eureka, Texas