[Lowfer] Morning lowfers

[Garry]

Hi Mark,

One thing I found surprising about LF was that you didn't need acres of 
land to put up an antenna that could hear things.  That's not to say 
that 2000' beverages and K9AY directive loops don't do better, they do, 
much better.  But the point is even with antenna probes a few feet in 
length and elevated only 10' above ground LF listeners can log hundreds 
of aircraft beacons, DGPS transmitters, Navtex RTTY stations, military 
MFSK stations, time signals, experimental stations run mainly by hams, 
etc. below 540 kHz.

Jay, W1VD at http://www.w1vd.com/modifiedeprobe.pdf has a nice 
description of an e-probe, an antenna/amplifier that allows one to copy 
signals clear down to VLF.  Jack of Clifton Labs offered similar gear 
commercially but unfortunately has health problems. Nonetheless, his 
Z1501D manual at 
http://www.cliftonlaboratories.com/Documents/Z1501D%20Instruction%20Manual-01.pdf 
has a wealth of information about e-probes and how to install them 
(especially see page 31).

E-probes are omnidirectional and that sometimes is a plus, sometimes a 
minus. When strong co-channel signals are present, a loop antenna might 
allow nulling unwanted signals so a wanted signal can be copied. VE7SL 
at http://members.shaw.ca/ve7sl/loop.html describes how to homebrew one 
type of loop antenna, including a preamp for it.

Initially I used an Icom IC-706MkIIG radio as the receiver. It is quite 
deaf below 1 MHz but that can be overcome with sufficient preamplifier 
gain or one can mix the LF signals up to an HF IF frequency where the 
radio is decently sensitive. For some years now though I've used an 
SDR-IQ for my receiver. That is now old technology and much better 
software defined radios exist. But I've been happy with the 
performance/price of the SDR-IQ and am glad to have two.

Whether you use an analog radio or an SDR, it is a big plus to interface 
it with a sound card and PC. It's amazing when digital processing can 
pull out of the noise. Must have software in my opinion includes ARGO 
(http://www.weaksignals.com/) and Spectrum Laboratory 
(http://www.qsl.net/dl4yhf/). The latter program has a steep learning 
curve but it can do practically anything so it's well worth spending the 
time. In fact, it can even tune the SDR-IQ and other software defined 
radios, as can a number of other programs, although I continue to use 
the original SpectraVue software for that task.

My original setup involved placing an eprobe as high as possible up on a 
roof tower. I was skeptical that performance could be as good at just 
10' off the ground. I also used the full length of my 10' telescoping 
probe. Both actions are not necessary and perhaps outright wrong. The 
key to good reception is to try many locations to discover what works 
best for you. I found that a spot in the back yard where an eprobe up 
only 10' often gave 3 dB better SNR than the rooftop location. I use 4' 
probe lengths now and probably wouldn't suffer detection degrade even 
with shorter lengths. As the length is shortened the signal coupled to 
the receiver goes down, but so does the noise. SNR is really what's 
important so that likely won't hurt and it will definitely help reduce 
the second and third intermodulation problems that eprobes produce. My 
particular QTH is not far from high power AM broadcast stations WGN on 
720 kHz and WBBM on 780 kHz. They mix to produce a signal at 60 kHz that 
is so strong the only way I can copy the WWVB  time standard is to use 
the loop antenna aimed to null the AM sources.

I'm fortunate to have neighbors that don't have noisy TVs, LED bulbs, 
etc. I do have a substantial picket fence of 60 Hz harmonics at VLF. 
They can often be tamed by including chokes on the coaxial feedline 
between eprobe and DC inserter. I've done so but they provide no real 
benefit for me, presumably because the harmonics aren't getting to the 
receiver via the feedline shield.

That's enough of a dump from me. Perhaps others will chime in with their 
favorite DOs and Don'ts.

73, Garry, K3SIW, EN52ta, Elgin, IL