On Mon, 27 Sep 2004 11:11:16 -0500, "John Smith"
wrote:

Thanks, Richard. I'll give this a shot. Might not get to it until the
weekend, though.

Hi John,

Good. Let us know of your results whenever you finish. This feed
back encourages others who might not be participating in this thread,
but who are interested nonetheless.

73's
Richard Clark, KB7QHC

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)

"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

Roy, W7EL wrote:
"The objective is to get a sufficiently high value of impedance. It
doesn`t matter whether the impedance is inductive, capacitive, or
resistive."

True, at a particular frequency. But, inductive impedance increases with
frequency. Capacitive impedance decreases with frequency.

The parallel resonant circuit may have extreme impedance, but at higher
frequencies it may have a very low impedance, especially if the resonant
circuit is a high-C circuit.

Best regards, Richard Harrison, KB5WZI