[600MRG] Hunting the noise source - loop mods.

[Kenneth G. Gordon]

Today, since I am still waiting for parts for the NM-20B, I went back to that 
36" Stoddart loop which I mentioned here a while back.

It had been mounted on a 12" diameter heavy steel plate, and the loop itself 
had been modified. The bottom of the steel plate had been covered with a 
thick piece of felt.

The loop had had a BNC connector added opposite to the original twin-ax 
connector.

I had previously determined that the loop was resonant at about 500 KHz.

There had been added to the loop two switch boxes, one of which contained 
a three stage FET preamp of some sort, and both of which contained the 
necessary switchable capacitor bank to resonate the loop to the various 
Navy VLF transmitting frequencies.

I removed those two switch boxes today, and then carefully opened up the 
loop, removing the compass (on top) and the bottom cover, to discover how 
it had been modified.

The tech had added the BNC connector, two resistors, and a 0.01 mfd 
metalized film capacitor.

The 0.01 mfd capacitor was soldered directly to one lead of the loop 
connection at one side of the twin-ax, and then a series resistor of 470 ohms 
led from the other end of that capacitor to the center-conductor of the BNC 
connector.

>From the OTHER loop/twin-ax connection, a 10 ohm resistor was soldered to 
a ground lug which was bolted to "ground".

I then connected the BNC connector through a short length of 50 ohm coax, 
as a test, to the FT-890's antenna input, tuned the FT-890 to 500 Khz, and 
found a HUGE noise source.

Even in my basement shack, by rotating the loop, I found a very, very deep 
null in the noise in an ESE-WNW direction. Rotating the loop 90 degrees 
resulted in a very strong peak in the noise.

Although I am not one bit sure what the two resistors and the capacitor are 
really doing to the loop pattern, if anything, I am going to go triangulating 
anyway as soon as I reassemble the loop.

I'll build an opamp loop interface asap. But in the meantime, I suspect I will 
find what I am looking for.

Keeping my fingers crossed.

I am thinking that I should "un-modify" the loop connections, remove all the 
bits that connect to the BNC connector, revert to the twin-ax, and build that 
OP-27 interface box that was suggested here.

Can anyone here foresee any bad effects by KEEPING that kludgey BNC 
connection?

Ken W7EKB