Quad Loop
 

I have been designing a 10MHz quad loop. It is roughly rectangular and
the bottom wire is about 3m above the ground. EZNEC shows the feed
impedance to be about 130 Ohms (verical side). Now going to reality -
I have put it up and, at the end of a quarterwave transformer (75 ohm)
I measure 93 Ohms at resonance. Back calculating gives me a feed
impedance of 60 Ohms - half what I was expecting. Any ideas?

Richard,
The shape of the loop, and the height above ground has
quite a lot to do with the input impedance of the antenna.
Your loop is wider than it is tall, right? If you made the
loop twice as tall as it is wide, and mounted it the width
of the loop above ground, the input impedance would be very
close to 50 ohms, feeding it at the bottom center. The
shape...
'Doc

Richard, G3CWI wrote:
"---a feed impedance of 60 Ohms - half what I was expecting. Any ideas?"

The ancestor of the quad loop is the folded dipole. 72 ohms is a
free-space impedance and the impedance of any antenna may be affected by
proximity to nearby objects.

3 meters elevation above the earth is only 0.1 wavelength at 10 MHz, so
poor earth absorbs much energy from a low element which is nearly
parallel with the earth. This is the likely cause of low drivepoinnt
impedance and of low performance too.

ON4UN in "Low-Band DXing" has a chapter (10) on large loop antennas,
which will probably answer your questions on quads. ARRL is the
publisher of ON4UN`s book. Another good reference is William Orr,
W6SAI`s "All About Cubical Quad Antennas".

Best regards, Richard Harrison, KB5WZI

I have now solved the problem. The line that was described as 75 Ohms
measures 120 Ohms. That threw the matching out!