An ordinary biquad antenna (two squares in the same plane) has a feed
impedance of about 50 Ohm when placed about lambda/8 in front of a
reflector.

But what is the corresponding feed impedance of a double biquad ( four
squares = "double double quad") like each stacked element in this
one?:

http://www.vhfman.freeuk.com/radio/23cmantennas.html


Intuitively I would guess it is 100 Ohm. But I may certainly be wrong,
how the individual squares interact is over my head. Anybody got a
more authoritative guess?

Jeppe