[600MRG] Noise - an interesting find.

[Kenneth G. Gordon]

On 11 Mar 2014 at 18:58, Brian Pease wrote:

> Try grounding one of the 2 pins to the shell and using that as the 
> shield of your coax, with the other pin to the center.  Don't expect 
> great sensitivity.  If you get nice nulls off each side on a ground wave 
> station then you are good to go.

Thanks, Brian.

Well, it is interesting what one finds in one's "junque box" when one 
investigates.

I find one of my excellent Stoddart stainless steel 3 foot diameter loops 
has been, obviously, modified.

It is mounted to a steel plate, about 12" in diameter, the bottom of 
which is covered with a layer of felt, and there are two mini-boxes 
bolted to the top of the plate.

One minibox has a twin-ax, made of two short pieces of RG-58, leading 
into it. There is a multiposition switch with a big knob mounted on top. 
Inside, the switch, obviously, connects various values of fixed 
capacitors in parallel with the loop's inductance. Some of the values are 
quite large. One of the pieces of RG-58 has come unsoldered from 
whereever it originally was connected. That connection is probably to the 
switch common.

I suspect that this was done in an attempt to tune the loop to the  
frequencies of the various VLF stations which were on the air at the time 
(1970). The reason I suspect this is that this loop was part of an 
experiment in VLF propagation done then. The experimenter was using the 
various Navy VLF stations to provide a known signal and location for his 
research.

The second mini-box has two of the multiposition switches mounted in it, 
and two, obviously, slug tuned inductors.

Inside the box, there is a three stage FET amplifier (using ancient 
GEFET2s), each stage being separated from the next by a copper-plated PC 
board divider.

Input and output are both BNCs, and there were lengths of RG-58 connected 
to both connectors.

Like the first box, the switches connect various capacitors in parallel 
with the large slug-tuned inductors which are in the first two sections, 
but there are no tuned circuit components in the last section. Its BNC is 
labeled "OUT"

Lastly, there has been a BNC connector added to the OTHER side of the 
loop's base, opposite to the twin-ax connector. In addition, the "top" of 
the loop, where the opened section of the shield normally is, has 
obviously been opened up at some time in its past. It was then wrapped 
with "Coax-Seal" sealant when whatever operation that was performed was 
completed.

Since this is a shielded loop, use of a GDO to figure out the resonance 
points is futile. Perhaps a one or two turn link connected to first the 
BNC then to the twin-ax connector will allow use of the GDO to 
find....something.

I can check continuity and inductance through the two connectors.

I can check the resonance points of the two tuned stages in the amplifier 
box, but will have to re-open the loop to see if, perhaps, another loop-
winding, connected to the BNC, was added in addition to the original one 
which is connected to the twin-ax.

This is going to take some research.

Gee...has anyone here heard of such a device?

What I suspect this is, is an inductively coupled shielded dual-loop, one 
inductance leading to the twin-ax, and the other one to the BNC. I 
suspect the purpose of this setup was to DF the VLF stations, or possibly 
noise on those frequencies.

Does anyone here know if an inductively coupled dual-loop was ever used, 
and what might have been its purpose?

Ken W7EKB