[Milsurplus] Simple, Effective Loop Antenna for MF-LF

[David Stinson]

A Simple, Effective Loop Antenna for Your LF Receiver.

On LF-MF, a well-designed ferrite loop receiving antenna will run rings 
around random wires and "E-Field Probes," because they can be easily 
rotated and tilted to "null-out" noise sources and interfering signals.  
There are a few commercial receiving loop antennas available, but they 
are laughably expensive and no better than the simple design presented 
here.

The Loop Amplifier:
I use the Palomar LA-1 Loop amplfier with all the plug-in coils. This is 
no longer being manufactured.  It was insane expensive when it was and 
still is on the pre-owned market, going for $150 with only one coil.    
Inside is a tuning capacitor, a board about two inches square,  a 
"stereo" quarter-inch jack for connecting the coil, switches and  a 
nine-volt battery.  Here is the circuit diagram:

https://photos.app.goo.gl/emcNuKcRe4XFZ5yu9

The Coil:
The original LA-1 coils used a 5/8 diameter, 7-inch ferrite cores with 
u=125.  The coil is bifilar wound.  There is an excellent, short and 
readable paper on this type of loop and specifically this type coil 
published by NASA at:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800006804.pdf

The discussion of loop design begins on page 2.   They used a 5/8th 
diameter coil and that #22 speaker wire we all got from Radio Shack to 
wind the coil, looping-back the end of wire "A" to the beginning of wire 
"B" and grounding this point, makeina a dual winding with center tap 
grounded.

I did a little fiddling on this today.  Didn't have the big ferrite rod, 
but I do have a bunch of those 5/16ths diameter, 5 inch long rods used 
in early transistor radios.  I bundled three of them with tap and wound 
14 spaced turns evenly spaced, securing them with tape:
https://photos.app.goo.gl/VFEBYyiw62JUxqN29
The start of wire "A" is one side of the loop coil (left).  The end of 
wire "A" is brought back and connected to the start of wire "B."  This 
becomes the grounded center tap of the loop.  The end of wire "B" 
becomes the other side of the coil (right).
https://photos.app.goo.gl/2aYiRse3Jg252KX18

The loop center tap goes to the grounded "sleeve" of the stereo plug.  
One side of the coil winding goes to the connector tip and one to the 
connector ring.  With 14 evenly spaced turns, the loop tuned sensitive 
and sharp from about 700 KC up to the 160 Meter Band.  Compressing the 
turns together moved it down to cover about 500 KC to about 1600 KC.   
Turning the loop displayed excellent nulls and peaks.  Once you find the 
"azmuth" null, lift one end to deepen it with an "elevation angle."  You 
can lean it against a block of wood with an angle cut or just prop it on 
something.



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