"Cecil Moore" wrote in message
. ..
Reg Edwards wrote:
Why avoid the self-resonant frequency?
That's the frequency at which the choking impedance is a maximum.

Personal preference - I would rather the choking impedance
be at maximum reactance rather than at maximum resistance.
I think that saves a little bit of power.
--
73, Cecil http://www.qsl.net/w5dxp
=====================================

But Cecil, maximum resistance, maximum reactance and maximum impedance
all occur at the same resonant frequency.

And at the resonant frequency minimum loss occurs in the choke anyway.
There's negligible loss of power in the choke left to save.
----
Reg.

Reg Edwards wrote:
But Cecil, maximum resistance, maximum reactance and maximum impedance
all occur at the same resonant frequency.

Reg, if you would learn the Smith Chart, you would know
that is not true. :-) At self-resonance, the inductive
reactance cancels the capacitive reactance leaving a
pure resistance usually of a few thousand ohms. But I
agree with you that a purely resistive self-resonant
choke is not a bad thing.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
...
Reg Edwards wrote:
But Cecil, maximum resistance, maximum reactance and maximum
impedance
all occur at the same resonant frequency.

Reg, if you would learn the Smith Chart, you would know
that is not true. :-) At self-resonance, the inductive
reactance cancels the capacitive reactance leaving a
pure resistance usually of a few thousand ohms. But I
agree with you that a purely resistive self-resonant
choke is not a bad thing.
--
73, Cecil http://www.qsl.net/w5dxp
======================================

Cec, I don't need a Smith Chart, and neither does anybody else, to
work out the resistance at resonance.

I don't think you quite know what you are doing with elementary
circuit analysis. The Smith Chart is giving you the wrong answer. The
resistance of a choke at HF at self resonance is in the order of a
megohm or more, not just a few thousand ohms.

By the way, what is the diameter and winding length of a USA pop
bottle.
----
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.

Bindy wrote:
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.

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

I didn't say anything about "20 through to 10". Here's what
my rule of thumb says:

No more than 20 turns on 20m, no more than 10 turns on 10m,
and no more than 6 turns on 6m.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
. ..
Bindy wrote:
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.

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

I didn't say anything about "20 through to 10". Here's what
my rule of thumb says:

No more than 20 turns on 20m, no more than 10 turns on 10m,
and no more than 6 turns on 6m.
======================================
Cec,

Does all the above apply to the diameter of a pop bottle?

And what is the diameter over the plastic jacket of the coax?

The length of the coil will be N times the jacket diameter, assuming
the N turns are close-spaced.

Don't forget, mean coil diameter will be pop bottle diameter plus coax
jacket diameter. It does make a difference.
-----
Reg.

Reg Edwards wrote:
Does all the above apply to the diameter of a pop bottle?

Yes 4" dia. and 2 turns per inch using RG-213.

And what is the diameter over the plastic jacket of the coax?

0.405" for RG-213, approximately 1/2".

The length of the coil will be N times the jacket diameter, assuming
the N turns are close-spaced.

Yes, that's why there are 2 turns per inch for RG-213.

Don't forget, mean coil diameter will be pop bottle diameter plus coax
jacket diameter. It does make a difference.

Maybe a 10-20% difference. Doesn't make much difference to a
rule of thumb. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Cec,

Let us know when your rule-of-thumb is available from your website.
I'm looking forward to seeing the details.

I guess there will be the usual collection of over-meticulous
nit-pickers.
----
Reg.