David Harmon wrote:
When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)


So, these guys are telling you that the antenna you describe is a
dipole? Funny, you call it a ground plane antenna (gpa) and I can draw
a mental picture of exactly what you are attempting to build. If you
had called it a dipole, I'd have expected the common half wave wire fed
in the middle.

You get my drift, nice to have two different names for the two vastly
mechanically-differing constructions of metal and insulators.

By the way, have you ever heard a yagi called a "Multiple Dipole Antenna
Working in Tandem?" GRIN!

As to your question, cut the radials ~5% longer than the vertical
element, droop them at a angle which give you a good match to your 50
ohm coax (or ~45 degrees.)

Better yet, consider a half wave end fed antenna which requires no
counterpoise (radials, check out the radiation pattern of the halfwave
endfed):
http://www.70mhz.org/halfwav.htm
Simply change dimensions to fit your freq(s), much easier to hang a
single vertical wire in the air than one vertical and four
horizontal--no matter what the freq.

Best regards,
JS