Hi Roelof,

Some interesting comments there.

On the topic of feeder pick up Alan G3NYK and myself have had many
conversations over the years about ways to deal with the problem and a
method we've never got around to trying but I now have some components
to try (when I get a "roundtooit") is to convert (E/O) electrical to
optical at the feedpoint and then conduct by fibre through the noise
field converting back to demodulated baseband in the shack. The problem
of powering the feedpoint equipment may be overcome by using batteries
(possibly solar-charged) thus avoiding further noise pick up and
re-radiation from power source cabling.

73




On Fri, 2011-01-14 at 13:53 +0100, Roelof Bakker wrote:
> 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
> 
-- 
73 es gd dx de pat g4gvw
 qth nr felixstowe uk
(east coast, county of suffolk)