"Cecil Moore" wrote

Wouldn't the resonant circulating currents be at their
highest magnitudes at the parallel resonant point and
therefore the I^2*R losses in the coil be greater at
that frequency than on either side of self-resonance?
(I explained that avoiding self resonance is only a
personal preference.)
======================================
Hi Cec,

The exceedingly small power loss in the choke depends on the impedance
of the power source which is driving it. It is a question of
non-conjugate mis-matching. But at self-resonance, and indeed
anywhere else, you can forget all about power loss. Especially as no
ferrite material is involved.

If you have bothered to check your "rule-of-thumb" and
"turns-per-metre-of-wavelength" against program SELFRES3, then,
provided results are in the same ball-park, all will be more than
satisfactory. SELFRES3 is itself only approximate. The choke's
self-resonant frequency is somewhat indeterminate as soon as anything
is connected to it.

A hank of a bunched number of turns is quite good enough as a coaxial
choke. It is only necessary to know the number of turns and the
diameter. Construction neatness helps with estimating the number of
turns but has hardly any effect on performance.

Coaxial chokes are a relatively unimportant circuit component.
Exactly what they are supposed to do is aways doubtful unless they are
used as baluns. I wonder why I am spending so much time discussing
the subject. Praps it's because I have nothing better to do.
smiley
----
Reg.

Jerry Martes wrote:
Doesnt the increase of impedance of the parallel resonant "balun"
discourage current from flowing into it?

My bad for confusing 1/4WL self-resonance with 1/2WL
self-resonance. A choke is essentially non-functional
at the 1/2WL self-resonant point. That's the configuration
to be avoided.
--
73, Cecil, http://www.qsl.net/w5dxp

Reg Edwards wrote:
A hank of a bunched number of turns is quite good enough as a coaxial
choke.

I got the 1/4WL and 1/2WL self-resonant points confused
in my mind. A "bunched number of turns" causing 1/2WL
self-resonance would function very poorly as a choke.
--
73, Cecil, http://www.qsl.net/w5dxp

Cecil,

A choke consists of an inductance in parallel with its stray
capacitance. It has only two terminals.

Its loss can be represented, either by a small resistance in series
with the inductance, or a very high resistance in shunt with the
inductance and its shunt stray capacitance.

What you mean by 1/4-wave and 1/2-wave resonance I have no idea and,
in any case, has nothing to do with how the extremely simple circuit
behaves.

The best thing to do is to forget the whole thing and find something
else of greater consequence to argue about. smiley
----
Reg.

While you guys were chewing on this I decided to repair a broken coax
on the 80 meter array for beaming europe... Temp 96 degrees... Wind
blowing like snot with a storm front coming in and cumulus building to
the West and getting ready for a climax... I climb slowly to 100 feet
(puffing and sweating.. It can't be cause I'm out of shape! 'Must be
the heat', I mutter)...
After a bit of maneuvering, and using one foot to snag a drooping wire
I untangle the 70 feet of coax that had flopped back against the tower
and managed to wrap itself on everything in sight... I capture the
center insulator of the driven element that is waving widly in the
wind... After some ten minutes of stripping and clipping and clamping
I have the coax reattached... I drag out the propane soldering
pencil... I can't keep it lit in the wind... I even resort to lighting
it inside my tool pouch.. All I succeed in doing is burning a hole in
the pouch... Jeez Louise... I give up and climb down... I left the
connected parts taped to the tower leg... I'll have to solder on a
calmer day... The saving grace is that I don't have a coaxial choke to
deal with...

denny

Reg Edwards wrote:
What you mean by 1/4-wave and 1/2-wave resonance I have no idea and,
in any case, has nothing to do with how the extremely simple circuit
behaves.

The circuit is extremely simple only in the human mind, Reg.
It's not so simple in reality. Given a real world choke
wound out of coax, as one increases the frequency of the
signal generator, one will encounter resonances. Some will
look like parallel resonances and some will look like serial
resonances. The first resonance is the 1/4WL self-resonance.
--
73, Cecil, http://www.qsl.net/w5dxp

Cec,

Can't you see that the first resonance corresponds to the parallel
resonance of the inductance with its stray capacitance, and is what
you call its 1/2-wave resonance?

At your 1/2-wave resonance the input impedance is very high regardless
of what the load impedance is on the other side. It does not behave
anything like a transmission line.

Higher frequency resonances, IF they exist, will be above and outside
the amateur bands and of no interest to anybody.

Interest lies only in either side of the first resonance. That is in
the wide band of frequencies at which the impedance is greater than
1000 ohms or so.

See program SELFRES3.
----
Reg.

Cecil Moore wrote:
Based on some calculations I did today, I am offering
an original rule of thumb (as far as I know). Regarding
a coax choke, when deciding how many turns of coax to put
on a 2 liter pop bottle at two turns per inch, one needs
to avoid the self-resonant frequency. So don't put more
turns on the choke than the number of meters in a wavelength,
e.g. no more than 20 turns on 20m, no more than 6 turns on
6m. Backup calculations will be published on my web page.
--
73, Cecil http://www.qsl.net/w5dxp

20 Turns is way too much for 20 through to 10.

http://www.k1ttt.net/technote/airbalun.html

Some actual measurements with a Network Analyzer. It shows 6 turns is
enough for 20 through to 10.

I have just improved and added to the notes to program SELFRES3.

The issue date on the first page of operating notes has been changed
to 4th August 2006.

I suggest you download SELFRES3 again and use it overwrite your old
copies.
-----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

That is neet. Now I just got to figure a choke of RG8X at 146mhz to
keep the RF from comming back down the outside of the coax cable from
the J pole.
The VSWR is realy good at 1:1.1, and it talks realy well.

What about ferrite beads/chokes over the coax instead of the coil of
coax cable?