[Lowfer] VLF

Wed Dec 25 10:43:15 EST 2013

Andy

After many years chasing VLF signals I've come to the conclusion that the best approach is to use 
the smallest antenna possible ... the main requirement being that the antenna should be able to 
produce sufficient 'band noise' to overcome the receiver noise. A small antenna minimizes the amount 
of  noise pickup thereby easing the noise mitigation that must be carried out. Small 'amplified' e 
probe or loop antennas are ideal for this application.

For optimum reception, one should be prepared to try both types of antennas at different locations 
within the boundaries of receiving site. Picking one type and mounting it in a convenient spot is 
unlikely to produce optimum results. At this location loop antennas have, in general, proven noisier 
than e probe antennas for VLF reception. Sighting of the antenna (e probe in my case) was initially 
carried out with battery operated receiving equipment while roaming the property. Several favorable 
areas were identified for further investigation and ground/mast support rods were driven into the 
ground to facilitate testing at those locations. Further testing with the 'full blown' setup using 
SpectrumLab was carried out to identify the best location. There can be 'sweet spots' where noise 
pickup is at a minimum.

In addition to selection and sighting of the antenna, noise mitigation techniques must be carried 
out. This includes isolation transformers and/or common mode chokes and even battery/solar power for 
the antenna vs. power over the coax. At my location conventional light dimmers and CFLs were removed 
years ago. Several brands of LED lightning are currently used and are rf quiet.  New arrival 
electronic 'gadgets' (specifically their power supplies) are subject to testing and replacement with 
low-noise equivalents. Transmit antennas, not necessarily resonant at the frequencies of interest, 
may have to be disconnected (left floating) to completely eliminate re-radiation.

This all may sound a bit extreme but it is what has made the difference at this location. YMMV as 
all locations have their own individual characteristics. As one moves up in frequency through LF, MF 
and into the HF spectrum the noise problems typically become less making an effective receiving 
setup less critical.

Jay

----- Original Message ----- 
From: "Andy - KU4XR" <ku4xr at yahoo.com>
To: "1750 Meter List" <lowfer at mailman.qth.net>
Sent: Tuesday, December 24, 2013 1:04 PM
Subject: [Lowfer] No beeps and no SAQ here in Friendsville

Admittedly excited, then utter disappointment..At Jays recommendation, I listened for the
Alpha beeps for over an hour.. I tried every trick on hand, but not a single beep heard....
As I expected; the static level did rise quite noticably as the night went on, but with the
right amount of clipping, and noise blanking in Spec Labs filtering; I could almost eliminate
the static bursts, except for the strong ones..  I started listening on 17.2 KHz with a
100 Hz BW, and filtering engaged, at 2:25 am EST, just after 2:30 am, I " thought " I heard
a carrier come on.. From past comments, I listened to see if it drifted around as SAQ
reportedly does during tune up...  No Drift.. ( disappointment starts ) , with no drifting of
the signal, probably not SAQ... I did not log the time, but " close to " 2:35 to 2:40-ish ,
I again " thought " I heard a faint CW signal, and heard what I thought was " 73 " ...
John Andrews - W1TAG mentioned to me in a post quite some time back, that the human
brain is quite good at trying to make order out of chaos.. Mine was in sleep deprivation overdrive..
By 2:45 am; nothing but static.. I anxiously waited for 3 am, had Audacity timer recording,
just in case I fell asleep at the mouse... 3 am came... nothing !! around 3:06 am, I thought I
heard a very faint CW note, but not above the noise enough to make out anything.. I started
quickly changing settings, turning off the clipper, then the noise blanker.. That only clobbered
me with complete static bursts.. I turned off one bandpass filter, still no signal, then the last
bandpass filter.. I was wide open with noise.. but no signal from SAQ.. I was using my own
custom user file for Spec Lab that I have spent a couple of weeks - peaking, and tweaking..
As a last gasp attempt; I opened Spec Labs " SAQrcvr "  file, and listened for about 5 more
minutes.. But by 3:15 am ; I knew it wasn't happening here this year either.. I turned
Audacity off, and shut down, and went to bed... I have not listened to Jays audio file, so
This report is what I actually " didn't hear " without trying to confirm something by someone
else's reception.. Now for the Techie notes, and just for the sake of posting them..
I used my 160 meter dipole, in a " T " configuration.. ran it thru a 33 mH choke.. I had an
air variable with a total of .0012 µF.. Peaked amount for 17.2 KHz was .001 µF +/- a few pico's..
I ran this thru a 2 section Hi-Pass Filter to lower the AC buzz.. I am not " scientific " , I just
lashed up something, and then changed values until I got the best results from my viewpoint..
I ran the antenna thru a .047 µF disc cap, with a 4 mH adjustable Inductor to ground, this
then went thru a .01 µF cap, with a 4 mH Inductor to ground.. The variable inductors were
adjusted for peak at 17.2 KHz.. L-1 had an in circuit value of 3.17 mH for 17.2 KHz...
L-2 had an in circuit value of 3.26 mH... The air variable from the antenna to ground would
move the resonant point back and forth, and adjusting the Inductors in the Hi-Pass filter
would peak the " noise level " for the frequency I was listening on.. The Hi-Pass filter gave
me a 20 dB reduction in AC Buzz,  At 6 KHz and down, it was almost gone.. the noise
level on 17.2 KHz peaked on Spec Labs graph with the settings I had, at -45 dB , and rolled
off fairly well to about -60 dB at 10 KHz , and -70 dB at 6 KHz .. Naturally the roll off was not
that sharp above 17.2 KHz, but was quite noticable on the spectrum graph.. This arrangement
then went into the sound card mic input ...  I had mentioned in an earlier post that I have the
best VLF reception that I have had to date, and I believe that to be correct, even though I did
not hear SAQ last night.. I know the military stations are " mega-powered " , but I have seen,
And Heard ! audibly, stations that I have not in the past... Remembering a recent comment
from Jay - W1VD; " almost any antenna will capture enough signal, and noise ; the issue
is the noise " ... To be serious with this ; that must be my focus.. an antenna that gathers
much less noise than my 160 meter noise catcher... I need to seriously consider getting
out the wellbroop loop I have stored in the garage, and get it set up, and use it...

Thanks for reading... And I wish all of you, a very Merry Christmas, and the best year you
have ever had in 2014..

73 Friends :

Andy - KU4XR
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