"Tom Bruhns" wrote in message
m...
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


Thanks for that info, Tom.

If resonance is not necessary, then I need only one turn of .75 diameter to
make sure resonance is higher than my operating frequency. That would make
the coil with parasitic capacitance look inductive.

73,
John

"John Smith" wrote in message ...
...
Thanks for that info, Tom.

If resonance is not necessary, then I need only one turn of .75 diameter to
make sure resonance is higher than my operating frequency. That would make
the coil with parasitic capacitance look inductive.

??? A single turn 3/4" diameter won't give you much reactance at 440.
Looks to me like if you used about three turns with that ID with a
total length around 1.5 inches, you'd be a lot better off. That'll be
about 300 ohms inductive reactance, shunted by enough parasitic
capacitance to come close to resonance (thus a higher net reactance).

Cheers,
Tom

"Tom Bruhns" wrote in message
m...
"John Smith" wrote in message
...
...
Thanks for that info, Tom.

If resonance is not necessary, then I need only one turn of .75 diameter
to
make sure resonance is higher than my operating frequency. That would
make
the coil with parasitic capacitance look inductive.

??? A single turn 3/4" diameter won't give you much reactance at 440.
Looks to me like if you used about three turns with that ID with a
total length around 1.5 inches, you'd be a lot better off. That'll be
about 300 ohms inductive reactance, shunted by enough parasitic
capacitance to come close to resonance (thus a higher net reactance).

Cheers,
Tom

Actually, Tom, my GDO measures about 440 with two tight turns. Add a couple
of inches for leads and it goes lower. So, I figured I needed something
slightly higher in resonance to be sure it is on the inductive side of 440.

No?

John

The objective is to get a sufficiently high value of impedance. It
doesn't matter whether the impedance is inductive, capacitive, or
resistive. You'll get the highest impedance at resonance, so that's the
best choice for a single band and there's no point in modifying the
design to "be sure it is on the inductive side". Typically, the
impedance is higher even considerably above resonance than it would be
with an inductive choke of fewer turns. With HF air-core coax chokes,
the impedance is very high at resonance and on the bands immediately
above and below the resonant band -- and adequately high for some or
most applications at frequencies above and below that, since the
resonance is quite broad.

Roy Lewallen, W7EL

John Smith wrote:

Actually, Tom, my GDO measures about 440 with two tight turns. Add a couple
of inches for leads and it goes lower. So, I figured I needed something
slightly higher in resonance to be sure it is on the inductive side of 440.

No?

John