[R-390] Loop antenna details

[Tim Shoppa]

Several have already written asking for details of my loop antenna.

Very simple:

Get one ten-foot length of 1/2" copper pipe.
Also 2 copper 1/2" elbows, one copper 1/2" Tee, and one PLASTIC 1/2"
elbow. (Not sure exactly about the sizing of the plastic elbow - it's
the one that 1/2" copper pipe fits into snugly.)

In principle I suppose you could use any sort of small diameter pipe
or conduit.

I use twinax that I bought for a couple of bucks on E-bay for
the lead-in. Twinax sometimes shows up in large quantities if you
watch mainframe computer deinstallations.

Cut the ten-foot length of pipe into 4 equal pieces.

Lay out the 4 pieces on the floor like a diamond. At the top
put the plastic elbow. At the sides put the copper elbows. At
the bottom put the Tee. DO NOT SOLDER or securely fasten
any of the corners yet, if you do so it'll be very hard to run
the wire inside the copper pipe!

The copper pipe is there to provide electrostatic shielding (it'll
be hooked to ground) and the plastic elbow at the top breaks the
circuit (so it is not a magnetic loop).

Run some insulated wire around the loop one or two or three times
and have the two ends come out of the Tee on the bottom. 

Run twinax from the 390A's balanced antenna jack to the antenna.
Center two conductors go to the loop of wire. The shield goes to
the copper.

It has some directionality and works from BCB up through at least
20MHz pretty well. It is NOT nearly as sensitive as a random longwire
strung out for even a short distance. But it does an excellent
job of suppressing household RFI (mostly computer monitors and
AC blower motors) that plagues my location.

The directionality isn't awfully strong in the "forward" direction
but there is a sharp null to the sides. My antenna just sits in
the attic and I try pointing it in different directions every once
in a while, but I suppose you could add some supporting structure
(wood? plastic?) and put it on a rotator.

I experimented with different numbers of turns and also with adding
a resonating tuning capacitor and while there was some difference
it was not profound. There are several sites on the net that
talk about similar construction loops and they found for transmitting
at least that the resonating capacitor was essential.

I suspect that those who are not as plagued by computer and blower
RF hash as I am will not get much advantage from the loop over a
random longwire. But for me it's the difference between a million
screaming banshees and clarity!

Tim.