Rule of Thumb for coax chokes
 

Reg Edwards wrote:
Cec, I can't run Excel.
I need something which will run just by clicking on it.

Reg, did you get the BASIC program I sent to you?
If not, please send me an email so I can reply.
--
73, Cecil, W5DXP

Rule of Thumb for coax chokes
 

John Popelish wrote:

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.

By the way, if you want the convenience of the snap around cores in a
plastic retainer, part # 28A0593-0A2 provides 450 ohms per core at 146
MHz.. They cost about $2 plus shipping. They are about 2 and 1/4
inches long.

Rule of Thumb for coax chokes
 

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

The rule of thumb is pretty simple and is for 2L pop bottles.
Wrapping at 2 turns per inch (RG-213) around a 4 inch diameter
pop bottle, the optimum number of turns for a particular band
is equal to the numbers of meters in a wavelength. In other
words, use 40 turns on 40m, 20 turns on 20m, and 10 turns on
10m. It's not actually linear - just a rule of thumb. It is
most accurate around 20m-17m.

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

Well, here's some data points that will probably surprise you.
I don't have a pop bottle so I am using a Quaker's Oats box
for a coil form. It is 5 inches in diameter which makes the
diameter of the coil using RG-400 about 5.4 inches. I wrapped
8 turns at ~4 turns per inch. My EXCEL spread sheet indicates
that coil is 1/4WL self-resonant at about 22.5 MHz making it
a good choke on 15m. Doubling that 1/4WL self-resonant frequency
to estimate the 1/2WL self-resonant frequency gives 45 MHz.
Now brace yourself.

The series impedance of that choke falls below 650 ohms at
about 27.2 MHz. 650 ohms is the maximum impedance that my
MFJ-259B will display. The choking impedance is never again
higher than 650 ohms as frequency is increased. It goes to
a minimum of 49 ohms at the 1/2WL self-resonant frequency of
45.6 MHz. Note that is reasonably close to double the 1/4WL
self-resonant frequency of 22.5 MHz *calculated* by my EXCEL
spreadsheet and by the BASIC program that I sent to you.

There is a one-wavelength self-resonant point at ~88 MHz and
a 1.5WL self-resonant point at ~122 MHz.

These measurements seem to prove that the coiled coax choke
acts more like a transmission line than like a lumped
inductance. Conclusion: A coiled coax choke designed for 20m
doesn't function very well on 10m or at any higher frequency.
(A coiled coax choke designed for 10m also doesn't function
optimally on 80m.)
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

Cecil,

What makes you think your EXCEL spread sheet (whatever that is) gives
the right answers?

Have you ever made any measurements of the harmonic resonant
frequencies?

If so, how did you do it?
----
Reg.

Rule of Thumb for coax chokes
 

Reg Edwards wrote:
What makes you think your EXCEL spread sheet (whatever that is) gives
the right answers?

Because it agrees within 3% of my actual measurements.
It's the same formula covered by the BASIC program
that I emailed to you.

Have you ever made any measurements of the harmonic resonant
frequencies?

The posting to which you are responding has three of
those measured harmonic resonant frequencies.

If so, how did you do it?

I put the coaxial choke across my MFJ-259B terminals and
looked for low impedances. They occurred at 45.6 MHz, 88 MHz,
and 122 MHz corresponding to 1/2WL, 1WL, and 1.5WL. That
proves that the VF given by Corum's equation is correct
because it predicted the 1/4WL point at 22.5 MHz, within
3% of the measured results.

And if the Corum equation is valid for coaxial chokes, it
is probably also valid for mobile antenna loading coils.
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

Cecil,

You have convinced me Corum's formula is in the right ball park. I
have not found time to study how it has been derived. It doesn't
appear to be particularly useful.

I will now tell you how to obtain ALL resonant frequencies, both
1/4-wave and 1/2-wave as you call them.

Place a single turn link winding around the CENTRE of the coil under
test. Between the link winding and the MFJ-259B connect a loosely
twisted pair (or a short length of speaker cable). The whole caboodle
can be made from a single length of thin, insulated, stranded wire.

I've a feeling that the length of the connection should not be too
long. But the 259-B should not be very near to the coil to keep the
meter outside the field of the coil. 6" or 10" seems about right
depending on the size of the coil. Ideally, length should be much
less than 1/4 wavelength at the test frequency.

Begin at a low frequency and search for the first high impedance on
the moving coil meter on the 259B. The first high impedance resonance
corresponds to the self-resonant frequency of the coil.

Increase frequency to find a low impedance resonance. Continue to find
the next high impedance resonance, etc.

The resonant frequencies may not be closely harmonically related.

You may not find very close agreement with the results obtained by
connecting the 259B directly across the coil. But both sets of
results are equally valid (or invalid).

I leave it to you to draw conclusions from the sequence of high-Z and
low-Z resonant frequencies.

Greatest accuracy is obtained by using the link coupling at the high
impedance resonances because the coil is then more isolated from its
environment. Its environment includes the input impedance of the
259-B itself.

The lower the self-resonant frequency, the greater the accuracy.

I found a coil in the junk box, 2.7" diameter, 4.0" long, 44 turns,
which has a self-resonant frequency of 13.6 MHz. I would have liked
it to be as low as 2 MHz.

I agree, a coil at sufficiently high frequencies begins to behave
something like a transmission line with a very low velocity factor.

To investigate what is really happening requires an instrument capable
of measuring impedance versus frequency from HF to VHF. It probably
doesn't exist.

When a coil is used to load a short HF vertical, it operates at a
frequency much lower than its self-resonant frequency and transmission
line effects don't matter two hoots.
-----
Reg, G4FGQ.

Rule of Thumb for coax chokes
 

On Sat, 12 Aug 2006 14:30:34 GMT, Cecil Moore wrote:

Reg Edwards wrote:
I agree, a coil at sufficiently high frequencies begins to behave
something like a transmission line with a very low velocity factor.

Just below its self-resonant frequency, it behaves somewhat
like a transmission line of less than 90 degrees.
snip
But maybe one hoot. :-) My 75m bugcatcher coil is operated
relatively close to its measured self-resonant frequency of
6.6 MHz. If I wound a bugcatcher coil to be self-resonant
on 4 MHz and then used 2/3 of that coil for a loading coil
on 4 MHz, its VF would not change and its electrical length
would be 60 degrees accompanied by the appropriate 60 degree
delay through the coil.

Hi Cecil & Reg

Sometime during the '70s I measured the self-resonant frequency of the 80m
Hustler loading coil, 6MHz. The series resistance of that coil was 31 ohms at 4
MHz. That is why they claimed 'lower swr than with othe brands'. What a fraud.
On the other hand, I also measured the Webster KW-80, self-resonant at 14.0 MHz,
with a series resistance of 8 ohms at 4 MHz. I reported this on Page 6-12 in
Reflections.

So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for
operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking
just to heat the coil instead of radiating the energy into space?

Walt, W2DU

Rule of Thumb for coax chokes
 

Walter Maxwell wrote:
So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for
operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking
just to heat the coil instead of radiating the energy into space?

Sorry I wasn't explicit, Walt. I use only 2/3 of the coil and
chop the other 1/3 off and discard it. That ensures that the
VF of the coil of 2/3 length is the same as the VF of the
whole coil at the frequency of operation.

The alternate approach would be to extend the windings on a
75m bugcatcher coil until self-resonance was reached at 4 MHz.
The VF could then be calculated and the extra windings
removed.

The purpose of the two above exercises is to determine the
VF of the coil *at the frequency of operation*. The VF of
large real-world loading coils changes with frequency. Knowing
the self-resonant frequency of a 75m bugcatcher coil is
6.6 MHz doesn't (necessarily) yield the correct VF at 4 MHz.
--
73, Cecil http://www.qsl.net/w5dxp

Rule of Thumb for coax chokes
 

On Sat, 12 Aug 2006 15:54:24 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for
operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking
just to heat the coil instead of radiating the energy into space?

Sorry I wasn't explicit, Walt. I use only 2/3 of the coil and
chop the other 1/3 off and discard it. That ensures that the
VF of the coil of 2/3 length is the same as the VF of the
whole coil at the frequency of operation.

The alternate approach would be to extend the windings on a
75m bugcatcher coil until self-resonance was reached at 4 MHz.
The VF could then be calculated and the extra windings
removed.

The purpose of the two above exercises is to determine the
VF of the coil *at the frequency of operation*. The VF of
large real-world loading coils changes with frequency. Knowing
the self-resonant frequency of a 75m bugcatcher coil is
6.6 MHz doesn't (necessarily) yield the correct VF at 4 MHz.

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. 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.

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.

Walt

Rule of Thumb for coax chokes
 

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.
--
73, Cecil http://www.qsl.net/w5dxp