Hello Petr,
Thank you for your kind words.
When I became serious about listening at LF, I messed around a long time
with loop antennas.
Unfortunately these pick up too much noise for weak signal work at my QTH.
As a last resort I tried my HF doublet with the open feeders strapped
together and though it spans the block of houses I live in, performance
was much better. That lead to trying an active whip and these worked even
better than the 11 meter long toploaded T-antenna.
The whip length was one meter and tests were performed to establish what
should be the minimum length without impairing peformance.
I used a Wandel & Goltermann SLM SPM-3 with a tuneable pre-amplifier to
measure the band noise and signal strength.
It appeared that the whip length could be reduced to 30 cm without
decreasing the signal to noise ratio.
The tests were made at 137 kHz, 300 kHz and 400 khz.
It also became clear that at these frequencies noise from within the house
in the E-field was screened by the walls by some 30 dB.
The only local noise perceived traveled along the shield of the coax feed
line to the outside world and was received by the antenna. Grounding the
shield at the bottom of the mast and near the house solved that problem.
At the time I was reading that at LF a whip antenna acts as a capacitor
and the idea struck home that the whip element could be replaced by a
capacitor, e.g. a piece of copper clad PCB. I started with a 30 x 140 mm
long piece of PCB and it became immediately apparent that rather than lack
of signal strength, the problem was way to high signal levels. After each
measurement I cut of a piece of PCB until the electronics and my receiver
could cope with it.
I have written it all up in an article in Dutch for "Electron" a Dutch ham
radio magazine.
You can find it here:
http://www.ndb.demon.nl/DXA/
There is a photo of the prototype clamped in a vice.
You asked for improvements. Many people have taken great effort to improve
the buffer amplifier in regard to strong signal handling. However, the
current design is a good balance between strong signal handling capability
and avoiding receiver overload.
This week I measured my noise floor on a quiet band (347.2 kHz, -125 dBm
in 10 Hz) and found that it is 18 dB above the noise floor of my PERSEUS
SDR.
73,
Roelof, pa0rdt
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