"John N9JG" wrote
You seem to be exceedingly knowledgeable about loop antennas.
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It's just the way I write it.
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I have limited
space available for a 160 meters antenna. What advice do you have
about a
loop antenna for 160 meters constructed out of 3/4" copper pipe with
an
octagon shape, and a perimeter of 100 feet. The plane of the loop
would be
vertical with the bottom of the loop about one foot high. The loop
would be
fed at the bottom, and the remotely tuned series capacitor would be
centered
in the top side of the loop. Does this project seem doable and does
it make
sense?
John, N9JG
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It makes good sense. Depending on your resources there will be
mechanical problems to solve. The weight of the large value,
motor-driven tuning capacitor, plus that of copper pipe will need
supporting. Plastic guy ropes may be needed. And ideally the whole
thing should be manually rotateable through 90 degrees.

Electrical comment : Do NOT cut the copper pipe to feed it at the
bottom of the loop. The balanced feedpoint input impedance will be
extremely small and impossible to match efficiently.

The best method of feeding is via a small loop of wire inside the main
loop in the same plane. The small loop is approx 1/5th of the diameter
of the main loop and is insulated from the main loop. The small
coupling loop is just a self-supporting wire between the inner and
outer conductors of a 50-ohm coaxial feedline and can be located at
the bottom of the main loop.

Compared with main loop diameter, the 1-foot height of loop above
ground is very low. Loss in the ground will be rather high. Try to
obtain a height of 6 feet for a not very great improvement.

With a perimeter of 100 feet, on 160 meters performance will be about
1 S-unit worse than a 1/2-wave dipole. On 80 meters performance will
be about the same as a full-size 1/2-wave dipole at the same height.
The perimeter is too long to work on 40m.

Further performance details can be obtained from program MAGLOOP4 from
website below.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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