Hello Dennis,
Though vertical antennas (and active whips) have a bad name in regard to
local noise, this is not necessarily true for reception at LF. Local noise
in the electric field is attenuated by the walls of the building (except
for a wooden one), whilst the magnetic field of a local noise source is
not attenuated at all. I have measured 30 - 40 dB of electric field
attenuation at 400 kHz.
The bad name of verticals in regard to local noise is not in the antenna
itself, but due to bad engineering practices in the feedline system.
So called common mode noise travels along on the shield of the feedline
from inside the house towards the outside world and is received on the
antenna.
A few simple precautions can solve this problem: use a rf-isolating
transformer between the feedline and the receiver and/or connect the
shield of the feedline at the bottom of the mast to a groundstake.
I have also used a couple of twisted pairs of a CAT5 network cable to feed
DC and RF separately to an active whip with excellent results. The
balanced twisted pairs offer some 40 dB attenuation against noise ingress.
Loops are excellent reception antennas. However in my location they are
quite useless, whilst I have stellar low noise reception using an active
whip. (I have logged a NAVTEX message from Taiwan on 518 kHz).
Best regards,
Roelof Bakker, pa0rdt
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