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