Reg Edwards wrote:
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?

It may just be semantics. I call the first parallel self-
resonant point the *1/4WL* self-resonant point. At its self-
resonant frequency, a mobile loading coil is 1/4WL long. I
am using that same convention for the coax choke. For instance,
my 75m bugcatcher mobile loading coil is self-resonant at
~6.6 MHz. That's the frequency at which it is 1/4 wavelength
long and presents a high impedance. At some higher frequency,
it is 1/2WL long and presents a low impedance.
--
73, Cecil http://www.qsl.net/w5dxp

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

Will wrote:
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.

My rule of thumb is for RG-213 wrapped around a 4" dia.
2L pop bottle and thus applies mostly to HF. Using a 2"
dia. coil form (pill bottle) with RG8X, my EXCEL
program says 2-3 turns is about right for 2m.

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

That works. Just make sure the ferrite is functional at
146 MHz, maybe #67 material.
--
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

"Will" wrote in message
oups.com...
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?


Hi Will

I have been using some affordable ferrites for "keeping the RF from the
outside of the coax" at about 2 meters. All my crude measurements indicate
that the Very affordable ferrite tubes from All Electronics
http://www.allelectronics.com/ work OK. I figured the tubes were intended
to keep RFI from conducting down the cables inside the *All ferrite tubes*,
so I tried them, and they work for me. They have some snap-on models that
might have an ID big enough to fit over your RG8X.

Jerry

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.

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

It's at http://www.qsl.net/w5dxp/Corum1.xls

Can you run EXCEL, Reg, or do you need it in a different form?
--
73, Cecil, http://www.qsl.net/w5dxp

Cec, I can't run Excel.

I need something which will run just by clicking on it.
----
Reg.
==============================


"Cecil Moore" wrote in message
...
Reg Edwards wrote:
Let us know when your rule-of-thumb is available from your
website.
I'm looking forward to seeing the details.

It's at http://www.qsl.net/w5dxp/Corum1.xls

Can you run EXCEL, Reg, or do you need it in a different form?
--
73, Cecil, http://www.qsl.net/w5dxp

Will wrote:
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?

Digikey carries a pretty broad line of Steward long form ferrite bead
cores. You can check the specifications at http://www.steward.com/

For instance, core HFB143064-300 has an impedance of about 180 ohms at
146 MHz. Type HF is their highest frequency material.

Core 28B0562-200 (same size in the lower frequency, 28 material)
produces an impedance of about 360 ohms per core.

I think both of these would fit over RG8X and are about an inch and an
eighth long.