Rule of Thumb for coax chokes
 

On Sat, 12 Aug 2006 18:53:06 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
But Cecil, I thought this thread was about chokes to prevent common-mode
currents from flowing on the feed line.

Now yer talking about loading coils for mobile whip antennas.

Yes, carrying the subject over from an earlier thread on loading
coils. There is a master's thesis paper authored by the Corum
brothers, K1AON and KB1EUD, and sponsored by the IEEE at:

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

which deals with RF coils. Although aimed at Tesla coils, it
contains lots of useful information for hams. In particular,
it predicts the VF for large real-world coils which is very
useful for me. It essentially shoots down the argument that
the current through a real-world loading coil is the same
at both ends of the coil, i.e. the delay through the coil
approaches zero as presented by the lumped circuit model.

As I understand
the issue, one is 180° from the other. For the choke you want a high resistance,
which is what you get at the self-resonant frequency. But for the loading coil
you want the lowest series resistance possible, which you don't get when
anywhere near the self-resonant frequency.

My point is that the same laws of physics apply to loading coils
and coaxial coil chokes even if the applications are different.
And we do, quite often, operate our 75m loading coils fairly
near their self-resonant frequencies - like your Hustler example.

Like I said above, the Hustler 80m loading coil achieved 'low swr' by making the
coil self resonant slightly above 4 MHz, with a series resistance of 31 ohms.
Now you are suggesting a bugcatcher coil self-resonant at 6.6 MHz, which means
yer coil is going to give you a nice low swr, but yer losing half of yer power
in the coil because of the high series resistance you can't avoid. Yer also
losing yer mind.

Well, that is the measured self-resonant frequency of my often
glorified 75m Texas Bugcatcher coil supposed to be one of the
highest-Q coils available.

Yeah, but Cecil, have you ever actually MEASURED the series resistance? The
slope of the resonance curve that peaks at 6.6MHz ain't gonna be low enough at
4.0. MHz to make an efficient mobile antenna. The Hustler coils, with 31 ohms
series resistance was a hoax on the average ham who didn't know the real reason
the Hustler gave them a low swr, which is what they mistakenly thought was
paradise. When it comes to efficiency in an antenna with a loading coil, the
best efficiency comes with the highest swr in absence of any attempt to match
the terminal impedance to 50 ohms.

IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz
was a high-Q coil at 4 MHz.

Walt

Rule of Thumb for coax chokes
 

On Sat, 12 Aug 2006 15:14:10 -0400, Walter Maxwell wrote:

On Sat, 12 Aug 2006 18:53:06 GMT, Cecil Moore wrote:

Walter Maxwell wrote:

IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz
was a high-Q coil at 4 MHz.

Walt

What I mean't to say, Cecil, is that you've been hoaxed if you thought a coil
self-resonant at 6.6 MHz intended for use at 4.0 MHz was a high-Q coil.

Walt

Rule of Thumb for coax chokes
 

Walter Maxwell wrote:
IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz
was a high-Q coil at 4 MHz.

All I know is that Texas Bugcatcher coils tend to be
near the top of the the 75m mobile shootout results.
Using base loading on a GMC pickup, it is resonant on
3.8 MHz with a six foot stinger. Remove the stinger and
it is self-resonant at 6.6 MHz.

My 75m Texas Bugcatcher coil was a gift from K7JEB.
It is 26.5 turns at 4 tpi on a 6" air core form.
Calculates out to be about 70 uH. I don't know how
to make it higher Q.
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

On Sat, 12 Aug 2006 23:32:09 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
IMO, Cecil, you've been hoaxed if you thought a coil self-resonant at 6.6 MHz
was a high-Q coil at 4 MHz.

All I know is that Texas Bugcatcher coils tend to be
near the top of the the 75m mobile shootout results.
Using base loading on a GMC pickup, it is resonant on
3.8 MHz with a six foot stinger. Remove the stinger and
it is self-resonant at 6.6 MHz.

My 75m Texas Bugcatcher coil was a gift from K7JEB.
It is 26.5 turns at 4 tpi on a 6" air core form.
Calculates out to be about 70 uH. I don't know how
to make it higher Q.

I'm guessing #12 wire in the coil, which leaves lots of spacing between turns ,
which should make the distributed capacitance very small. I haven't made any
calculations, but a seat of the pants estimate would say the self-resonant
frequency would be much, much higher than 6.6 MHz. (As I remember, the Hustler
used #14 close spaced on about 2" diameter to obtain a self-resonant frequency
at 6MHz.)

Cecil, I suggest you re-measure the self-resonant frequency of the coil by
itself, and if you have the means to do it, also measure the series resistance
at both the self-resonant frequency and at 4.0 MHz. I'm betting there will be a
large difference in the resistances, and that the self-resonant frequency will
be much greater than 6.6 MHz.

Walt

Rule of Thumb for coax chokes
 

The self-capacitance of a multi-turn solenoid has little to do with
spacing between turns. Self-c depends on coil length and diameter.

The self-capacitances between adjacent turns are all in series with
each other. Resulting capacitance across coil is negligible.

----------------------------------------------------------------------

A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500 at
3.8 MHz, and self-resonant frequency = 9 MHz.
----
Reg.

Rule of Thumb for coax chokes
 

Walter Maxwell wrote:
Cecil, I suggest you re-measure the self-resonant frequency of the coil by
itself, and if you have the means to do it, also measure the series resistance
at both the self-resonant frequency and at 4.0 MHz. I'm betting there will be a
large difference in the resistances, and that the self-resonant frequency will
be much greater than 6.6 MHz.

I'm not sure how to measure the 1/4WL self-resonant frequency
with an MFJ-259B without a ground plane. I suppose it could be
done using a 1/4WL counterpoise in free space. Let me just
report what the MFJ-259B readings are with the isolated 75m
Texas Bugcatcher coil across the MFJ-259B terminals.

The first dip in impedance is at 14.7 MHz where the MFJ reads
620+j0 ohms. The second dip in impedance is at 24.4 MHz where
the MFJ reads 88+j0 ohms. Is the first dip the 1/4WL self-
resonant point and the second dip the 1/2WL self-resonant point?

I want to make it clear that the previously reported 6.6 MHz
self-resonant measurement was made with the base-loaded coil
mounted a few inches away from my GMC pickup ground plane. The
ground plane was no doubt in the field of the coil at the bottom
end so the coil was certainly not isolated as it is in the above
reported measurements.
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

Reg Edwards wrote:
A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500 at
3.8 MHz, and self-resonant frequency = 9 MHz.

Wouldn't mounting the coil four inches above a GMC pickup
ground plane reduce the Q and the self-resonant frequency?
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

"Cecil Moore" wrote
Reg Edwards wrote:
A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500
at
3.8 MHz, and self-resonant frequency = 9 MHz.

Wouldn't mounting the coil four inches above a GMC pickup
ground plane reduce the Q and the self-resonant frequency?
--
======================================
Cec,

Very likely. But not very much.

It would not be the self-resonant frequency any more. The srf never
changes. And neither does the intrinsic coil Q.

We must be careful with our descriptions. Slackness leads to
misunderstandings, arguments and fights.
----
Reg

Rule of Thumb for coax chokes
 

Reg Edwards wrote:
It would not be the self-resonant frequency any more. The srf never
changes. And neither does the intrinsic coil Q.

So what would you call the frequency at which a coil alone
is resonant when mounted as a base-loading coil over a
ground plane?
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

On Sun, 13 Aug 2006 15:02:03 +0100, "Reg Edwards"
wrote:


"Cecil Moore" wrote
Reg Edwards wrote:
A coil 6.6" long, 6" diameter, 26.5 turns, has L = 68 uH, Q = 500
at
3.8 MHz, and self-resonant frequency = 9 MHz.

Wouldn't mounting the coil four inches above a GMC pickup
ground plane reduce the Q and the self-resonant frequency?
--
======================================
Cec,

Very likely. But not very much.

It would not be the self-resonant frequency any more. The srf never
changes. And neither does the intrinsic coil Q.

We must be careful with our descriptions. Slackness leads to
misunderstandings, arguments and fights.
----
Reg

Reg, I had never given much thought to the series relationship of the
capacitance between turns. I had always considered them as being in parallel,
thus the honeycomb, or the basket-weave configurations to minimize the interturn
capacitance. Have I misconstrued the purpose of those configurations?

Do I also understand you correctly that with a specified length of the solenoid,
and a given diameter, the total interturn capacitance is independent of the
number of turns, because the capacitance between turns adds in series to the
same value regardless of the number of turns?

Please educate me.

Walt