I plan to feed the loop at the side opposite to the side containing the
tuning capacitor. Note that the capacitor is inserted in series with the
loop and is used to tune the loop to resonance at the operating frequency. A
gamma match is used to connect the coax to the loop, and the loop is NOT cut
on the feed side.

"Frank's" wrote in message
news:r5LLf.1227$dg.71@clgrps13...
This winter I have been working on the design and construction of a
compact loop for 80 meters. The loop will be constructed from 72 ft of
3/4 in copper pipe and will be in the shape of an octagon. The plane of
the loop will be vertical, and the loop will be remotely tuned by a
vacuum variable capacitor coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the
system. Constructive comments will be appreciated.

John, N9JG

John, how high above ground is the antenna?

Frank, VE6CB

John, just for interest I placed the top of the antenna at 30 ft; which
puts the base at about 8' 3". Using NEC's Sommerfeld/Norton ground with:
sigma = 5 mS/m, and Er = 13. The input impedance computes to 6.57 +
j855.6 ohms. Since this is very close to the edge of the Smith Chart, your
1 ohm figure is probably within a reasonable margin of error. However I
cannot see how you can match such an impedance with a single element.

The structure appears to be a very efficient, physically small, antenna.
NEC computes 0.84% copper loss.

Frank