I've made coiled-coax choke baluns at 440MHz. Seems to work fine.
Generally expect to use pretty small line, to be able to make a small
enough coil. The self-resonance is nice but not critical. You're
just looking for a moderately high impedance. I have a program that
does a good job of estimating the self-resonant frequencies of coils,
but it's not on this machine so I can't check for you...I think one of
Reg Edward's programs also estimates self resonance for coils. If you
can get an impedance that's a few times the load impedance, you should
be OK. In feeding an antenna, it may be as important to use a couple
such chokes, spaced about a quarter wave apart, to keep the antenna
from coupling strongly to a possibly resonant section of feedline.

Cheers,
Tom

"John Smith" wrote in message ...
Hi again!

Has anyone tried making the coiled coax balun work at UHF? Specifically, on
the 70 cm band.

In "Antennas for All Applications", they say the coil should be resonant at
the frequency of operation. The ARRL book(s) do not cover this band. Using
RG58 and a GDO, I've learned that one cannot leave long leads on one's coax
coil as it will lower the resonant frequency drastically. Apparently, the
long leads in conjunction with the coil forms an antenna/inductor/capacitor
combination.

It appears to be difficult to obtain the required resonance repeatably. Or,
am I missing something (as usual)?

The diameter seems to be somewhat critical. Does the diameter need to be
much, much smaller than a quarter wave? What diamter would be a maximum?

Thanks for any help.

John (KD5YI)