[BC] am receiver for monitoring at a 20 KW site...

[Glen Kippel]

On Wed, Jun 6, 2012 at 10:45 AM, Burt I. Weiner <biwa at att.net> wrote:
 
<snip>
 
>I built a two turn Faraday shielded loop about 1 foot or so in diameter made out of 8751 balanced audio wire.  It's important to make the loop as physically flat (symmetrical) as possible in order to get the best null.  I connected the two 8751 conductors in series and split the shield at the center of the loop, and tied the bottom ends of the loop's shield to the coax shield.  Rather than going for maximum signal of the desired stations, I went for minimum pickup of the 1300 kHz signal.  The loop was installed in the same room as the transmitter and even though it was not at maximum for the desired signals, the results were very good and I had no problem with reliable reception and no cross-talk from the local transmitter.
 
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For the past 50 or so years I have used "floppy loops" made of whatever coax I could get my hands on -- mic cable, RG-58, etc.  I even made the first "hula loop" by putting the coax inside a 10-foot length of flexible irrigation tubing bent into a circle, with a tee for the cable to come out of.  Just expose the center conductor, cut a slice out of the outer jacket and insert the end of the center conductor between the outer jacket and the shield braid.  Tack-solder it or just wrap it up with electrical tape to hold it together.  I have used these, supported by wire brads on a wall, to make a loop of approximately 8 ft. by 10 feet.  A loop of RG-58 comes out, as expected, at 50 ohms, as measured on a GenRad impedance bridge.  Even though the loop is unbalanced, the Faraday shield works well enough to cut through most electrical noise.  I have also used balanced, unshielded loops for DXing.  You just need to wind a toroid transformer to make the loop balanced.  One advantage of these is that they are untuned, not to mention simple to make.