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

Why avoid the self-resonant frequency?

That's the frequency at which the choking impedance is a maximum.
----
Reg

Cec,

Suppose you havn't got a pop bottle but only a length of plastic drain
pipe of different diameter.

You will need a different rule of thumb for every contingency.
----
Reg.

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

Reg Edwards wrote:
Suppose you havn't got a pop bottle but only a length of plastic drain
pipe of different diameter.

Then I use my EXCEL spreadsheet which allows variation
in the number of turns per inch and diameter of the coil.
If you would like a copy, it can be downloaded from

http://www.qsl.net/w5dxp/Corum1.xls

You will need a different rule of thumb for every contingency.

2 liter pop bottles are in abundance in the US. If a coil
form is used for a coax choke, that's the most likely.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Reg Edwards wrote:

Suppose you havn't got a pop bottle but only a length of plastic drain
pipe of different diameter.


Then I use my EXCEL spreadsheet which allows variation
in the number of turns per inch and diameter of the coil.
If you would like a copy, it can be downloaded from

http://www.qsl.net/w5dxp/Corum1.xls

You will need a different rule of thumb for every contingency.


2 liter pop bottles are in abundance in the US. If a coil
form is used for a coax choke, that's the most likely.

But then you have to drink 2 liters of wet, sticky, gooey, sickeningly
sweet, soda pop.
73,
Tom Donaly, KA6RUH

"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.

I like to drink soda water from 2l bottles with a splash of lemon
juice; sounding pretty good right now given that it's 97F in Maryland
and the weather this week is, as per the NWS, "Excessive Heat"

Dan


But then you have to drink 2 liters of wet, sticky, gooey, sickeningly
sweet, soda pop.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
2 liter pop bottles are in abundance in the US. If a coil
form is used for a coax choke, that's the most likely.

But then you have to drink 2 liters of wet, sticky, gooey, sickeningly
sweet, soda pop.

Lipton and Nestea tea also come in those bottles as does
lots of sugar free diet stuff.
--
73, Cecil http://www.qsl.net/w5dxp

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