From HB9ASB, JN36pt

Hello Peter and the Group
I'm really amazed how good the helical works on 73kHz. I've tried also
two other antennas while Peter, G3LDO, called me. The first is a tuned
loop nailed on the wall of my shack, 16 turns with about 2m diameter and
oriented north-south.
The second is an untuned active Loop, away from the house in a garden
shelter. It has about the same diameter and orientation.
The loop in the shack was useless. It picks up too many RF from my
computer, but I could see Peter's signal on the loop in the garden. Not
as good as on the helical, but with the advantage that I could receive
on 73 during my transmission on 136khz without any disturbances - full
duplex!
I also have to say, that the 73khz Band is very quiet here. There is no
Luxembourg - Effect as on 136.
The receiver was the Harris RF590. The AF goes trough the Timewave
DSP599 where I use it's noise reduction algorithm before feeding the
sound card of the computer. This gives a considerable additional noise
reduction!
So far I've tried different receivers for the LF work, besides some
brands of commercial receivers also amateur equipment.
The problem with many amateur transceivers is, that they have a built in
attenuation pad for the LF/MF range, so they are quite deaf down there.
If you're not living in proximity of a MF/LF broadcast station you don't
need this attenuator. It's easy to locate on the schematic diagram. It's
often right in front of the LF/MF low pass filter and I've eliminated it

73 de Toni