On Apr 2, 7:04*pm, Richard Clark wrote:
On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne
wrote:

Richard
I didn't ignore choking the feed line but I will readily confess that
I did not choke it twice. *Starting immediately below the bottom of
the matching stub I followed the recommendation of the various authors
and wound a multi-turn coax balun with a six inch diameter coils of
coax. *They call for ten turns if I recall correctly. *

Hi Tom,

This sound like very common advice - so common that it begs
investigation because it is common advice for HF Choking, not 2M, and
certainly not 70cm. *However, this common advice acknowledges the need
for choking.

You would be better served using ferrites (W2DU style BalUn/Choke),
or, if you really wish to stick with wound coax, then use a Grid Dip
Meter to test its resonance (which should reveal you can't serve both
bands). *Using an antenna analyzer to do this will give you measurable
Z, and that may give you the data to see how well you are doing. *You
may wind a lot of chokes to discover that the diameter is
extraordinarily huge (or so the same for turn count - one or the other
or both).

I was not aware
of the need for a second choke at one quarter wavelength away. *Do you
have the energy to explain why that is necessary? *

[Do I have the energy.... *With all the junk cluttering this space
outside of this thread, I have plenty of energy.]

The antenna fields will try to excite the transmission line's common
mode (the shield of the coax that it sees). *This will induce currents
that will become radiative (just like stacked elements in a vertical
multi-element radiator - sound familiar?). *The use of chokes at
quarterwave intervals snubs these currents.

Sometimes more chokes are needed. *I use a 20' long line with a bead
every four inches or so. *The intent is to create a very long resistor
(very wide bandwidth) with very short leads (very high frequency)
between its distributed resistance.

How critical is the
length between the two chokes. *

Not particularly, you would be well served to attempt to make it at
the interval of quarterwave at the highest frequency used (70cm).
This, then, would snub lower frequency (2M) common mode currents.

Do I use the middle of the two chokes
as my measuring points? *

This sounds deceptively exact (and probably a consideration for those
enormous chokes of common advice vintage). *No, such exactitude is
going to be lost at the 70cm scale anyway which will be perturbed by
other factors (have I talked about environment?).

Could I substitute a one to one current balun
built of ferrite beads? *That would have a less intrusive appearance
and accumulate less ice in the winter.
If you check the link that I gave for Ed Fong's dual band j-pole;
available here
http://f1.grp.yahoofs.com/v1/
kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKk c
\FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just
look it up in the files section of the Yahoo reflector; *

I would prefer not to add yet one more account registration to simply
view this, sorry. *However, the narrative here should tell you if we
agree.

you will see
that he did test it in the field with fairly sophisticated
instrumentation. *He uses a trapped radiator to obtain similar gain on
seventy centimeters as a simple J-pole without the trap gets on two
meters. *

My background was working in the standards laboratory system of NBS
(my speciality was RF measurement to the highest accuracies). *I know
that sophisticated instrumentation and quality results don't
necessarily track each other. *My point was about environment, not
instrumentation anyway. *You don't need $1000 meters, but you might
need $1,000,000 environment.

The description of Apple's echoless environmental chamber used to test
their iPod is a marvel of engineering, and cost a stack of dollars.

His work was published in QST and I didn't find any
authoritative repudiation, or even strong criticism, of his design.

The same could be said of publishing it in Playboy. *QST needs content
to offer subscribers. *Vanity articles satisfy that need. *Think of
these articles as the introduction to a topic, not the final word.

Why would the presence of the trap in the lower half wave of the two
meter collinear half wave J-pole wreck the tuning on two meters.

The trap's tuning is heavily influenced by the geometry of the
elements because they are also part of the tuning. *The trap disturbs
that symmetry. *You have also introduced new L and C components that
further upsets the total circuit. *These things are not super-critical
when you consider that moving a resonance (say) 145KHz out of 145MHz
is only a shift of 0.1%. *That isn't super-critical either.

Or maybe it is.

I'm only trying to learn here.

Let me know if this helps.

73's
Richard Clark, KB7QHC

Richard
Ed Fong's article can be found on the ARRL website at
http://www.arrl.org/files/file/Technology/tis/info/pdf/
0302038.pdf.
I was able to access the above link without any password or other
challenge. I just reread it and found that he does address the
changes in the two meter element brought on by the inclusion of the
seventy centimeter trap. For one thing it shortens the two meter
element somewhat. So let me propose a modification process for my
collinear dual half wave two meter j-pole and you can tell me if you
can think of ways to rearrange or modify the steps so as to move
towards by hoped for results. What I hope to achieve is to maintain
the gain of the stacked, out of phase, dual half wave antenna on two
meters AND trap the lower have wave to a single half wave on seventy
centimeters.

Here are the steps I was thinking of:
Fabricate a trap from 1/2" copper pipe coming out of a tee that has
been cut in half through a plane perpendicular to the foot of the
tee.
Remove the top half wave and the phasing loop from the existing
collinear dual half wave antenna
Attach the stub to the lower half wave so that the open bottom end of
the stub is one half wave on UHF from the top of the matching stub.
Measure the SWR on 446 MHz. of the newly trapped two meter radiator.
Adjust stub location for minimum SWR at 446 MHz.
Measure SWR at 146 MHz.
Shorten that radiator to lowest SWR on two meters or until the two SWR
readings for VHF & UHF are equal.
Reattach the upper two meter half wave and phasing stub.
Measure antenna for SWR on both VHF & UHF again
Adjust length of of upper two meter half wave if necessary.

The antenna analyzer that I will use will be an AEA Technology SWR
Meter: 140-525. It happens to be what I have available to me.

Does my process seem to be along the right line or would you suggest a
different order of operations?

--
Tom Horne, W3TDH

On Sun, 03 Apr 2011 10:19:34 -0400, John Ferrell
wrote:

What I need to know is
1- When to use them. RF in the Shack is pretty obvious

Hi John,

Sometimes it is not always that obvious, but it can be made obvious. I
say this because the symptoms do not always draw our attention to the
problem arising from Common Mode (CM) current.

One possible obvious symptom is that you can feel the current on the
shielded components of your system - like a microphone shell when your
lip brushes along it (which can sometimes give a nasty shock); or when
you pull a connector and get that tingle of feeling (which might kill
you if conditions were worse). "Obvious" can vary from subtle to
deadly.

Another possible obvious symptom is you are moving a transmission line
while keyed down, and you notice your SWR shifts at the same time. Is
it because of a faulty connection? Sometimes yes, but we are talking
about CM; and when it is CM, then that shift in SWR is because a tuned
circuit (the line is now part of a dynamic system) has been changed.

* * * * * * * * *

Let's set aside in-the-shack symptoms and look at the antenna end of
what might be obvious.

For beam antennas, through careful testing with known remote stations
(beacons, for instance) you find that you have high sidelobes, almost
no back rejection, and a broad response off the front. The
transmission line coming down is acting like a vertical antenna, and
is feeding its drive into the antenna feed point (two antennas in
parallel). That extra antenna is picking up stations that would
normally be suppressed by beam design.

Some folks never notice this - and it shows how benign CM can be (it
is there full blown, but it is NOT obvious at all).

For wire antennas, you cut and cut and trim and trim your antenna to
frequency, but even though it is book perfect (using the usual
formulas for length), it is wildly out of whack and a bitch to tune
with your tuner. Your dipole without choking is actually a tripole:
two horizontal elements with a third vertical element (which is
variable in length and relationship to ground/environment as it swings
in the wind). You raise it higher to see if that does anything, and
the tune shifts to the moon because the third element has gotten
longer.

Again, some people never notice this, we have seen them; others do
notice it and fulminate against the dipole as being a useless
invention where their endfire (which suffers the same issues, but is
magical by comparison) antenna is the cat's meow.

More could be said....

2- When not to use them. There must be some negatives.

That is a natural question, but I cannot think of any down-side at
all.

3- "Compare and Contrast" the Inductor-capacitor choke with the
Ferrite Choke(W2DU). Are they electrically equivalent?

They are NOT electrically equivalent. The ARE functionally equivalent
- that is, they both suit the same purpose of introducing a very hi Z
into the current path of CM.

The coil of transmission line is, on first pass approximation, an
inductor whose Z climbs with applied frequency. The presumption is
that it can only get better for the higher bands. This works to a
point, but the same coil of TL also has loop-to-loop capacitance, and
end-to-end capacitance. These two capacitances, at some frequency,
become resonant and offer you a tank circuit. This works even better
to isolate the current at the drive point from the rest of the line.
However, a tank is not a wide band device by definition. It can
exhibit an extremely hi Z at one frequency, and often high enough Z in
neighboring bands (hence it is useful if you understand this). As you
go higher in frequency, and for the very large coils suggested
earlier, then the capacitances begin to dominate and actually kill any
sense of choking.

The ferrite choke is a resistor, plain and simple. This means it is
wide-banded - although its frequency characteristic is not universally
applicable (this widebandedness usefully covers two, maybe three
octaves). The formulation of the ferrite determines the coverage, and
so you can either throw more beads into the mix, or use beads of
different formulations to create a more flattened frequency response
(over decades of frequency change).

4- Are either of them a multi band solution?

Yes, as described above.

5- Are there Upper and lower frequency limitations for using them.

That too is described above.

As usual, a Google search provides an overwhelming set of responses
that most likely will cover the subject in greater detail than I can
digest.

Early in the list provided by the search I discovered
http://www.audiosystemsgroup.com/RFI-Ham.pdf

That may have all the answers but I have only glanced at the beginning
of the 66 page document.

It is a very good and broad discussion. The downside is its enormity.
However, this is not a simple topic and understanding comes slow.

The reason for that is that Common Mode analysis as distinct from
Differential Mode analysis immediately brings up the specter of WTF?
There are TWO currents flowing at the same time in the same circuit?

This perspective is usually very foreign and takes deep meditation to
come to grips with. It is difficult for the majority of the Pros too.

I am going to prompt readers for the next question:
"What is common mode current?"
If any one cares to indulge me by asking, we can go further, but the
material in this posting is enough to digest in one sitting.

73's
Richard Clark, KB7QHC

On Sun, 3 Apr 2011 09:43:27 -0700 (PDT), Tom Horne
wrote:

What I hope to achieve is to maintain
the gain of the stacked, out of phase, dual half wave antenna on two
meters AND trap the lower have wave to a single half wave on seventy
centimeters.

Hi Tom,

Right off the bat (yes I can see the article now) you offer a
conflict. Out of phase blurs gain (lowers it). Your statement
suggests you are losing from the start.

However, it could simply be wording. Let's make this simple:
1. State the gain at the frequencies of interest before the change;
2. State the gain at those frequencies you expect after the change.

Here are the steps I was thinking of:
Fabricate a trap from 1/2" copper pipe coming out of a tee that has
been cut in half through a plane perpendicular to the foot of the
tee.

Lost already - sorry.

Remove the top half wave and the phasing loop from the existing
collinear dual half wave antenna
Attach the stub to the lower half wave so that the open bottom end of
the stub is one half wave on UHF from the top of the matching stub.
1Measure the SWR on 446 MHz. of the newly trapped two meter radiator.
2Adjust stub location for minimum SWR at 446 MHz.
3Measure SWR at 146 MHz.
4Shorten that radiator to lowest SWR on two meters or until the two
SWR
readings for VHF & UHF are equal.

Probably not. You are going to be repeating steps 1-4 to even
approach this desired end condition.

5Reattach the upper two meter half wave and phasing stub.
6Measure antenna for SWR on both VHF & UHF again
7Adjust length of of upper two meter half wave if necessary.

This will disturb the work done in steps 1-4. You cannot rely on that
configuration being isolated from the work in steps 5-7 which will
also demand repeated attention. However, proceed with 5-7 repeats
until you obtain the desired end condition.

NOW, revisit steps 1-4 with everything attached. I may be wrong, but
I suspect you will need to trim things here again. And guess what?
This means you need to visit and repeat steps 5-7 again.

And guess what? You will be visiting steps 1-4 AND 5-7 again, and
again.

Take heart, these revisits "should" result in ever smaller
adjustments. That means you will approach your desired end condition
asymptotically.

The antenna analyzer that I will use will be an AEA Technology SWR
Meter: 140-525. It happens to be what I have available to me.

Does my process seem to be along the right line or would you suggest a
different order of operations?

The order is quite rational and fully expected. You are entirely on
the right track. Your experience will reveal how much they are
interactive and how sensitive all this is. If you anticipate the
amount of repeated operations and plan accordingly, it will go far
more quickly.

Epilogue

I would note that your writer does not offer any testing of sufficient
caliber to support the claims of the EZNEC model. He merely provides
signal reports using a sophisticated instrument (a spectrum analyzer
that is not being used for spectrum analysis). Those reports also
tell me that there is at best 1.2dB gain over a 1/4 wave design - very
underwhelming. The only SWR data is reported as "The SWR was low."

73's
Richard Clark, KB7QHC

John Ferrell wrote:
On Sat, 02 Apr 2011 16:04:27 -0700, Richard Clark
wrote:

On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne
wrote:

Richard
I didn't ignore choking the feed line but I will readily confess that
I did not choke it twice. Starting immediately below the bottom of
the matching stub I followed the recommendation of the various authors
and wound a multi-turn coax balun with a six inch diameter coils of
coax. They call for ten turns if I recall correctly.
Hi Tom,

This sound like very common advice - so common that it begs
investigation because it is common advice for HF Choking, not 2M, and
certainly not 70cm. However, this common advice acknowledges the need
for choking.

You would be better served using ferrites (W2DU style BalUn/Choke),
or, if you really wish to stick with wound coax, then use a Grid Dip
Meter to test its resonance (which should reveal you can't serve both
bands). Using an antenna analyzer to do this will give you measurable
Z, and that may give you the data to see how well you are doing. You
may wind a lot of chokes to discover that the diameter is
extraordinarily huge (or so the same for turn count - one or the other
or both).

This thread has shown me that I don't know enough about Common Mode
Chokes. I think they might also be referred to as transmission line
chokes.

What I need to know is
1- When to use them. RF in the Shack is pretty obvious
2- When not to use them. There must be some negatives.
3- "Compare and Contrast" the Inductor-capacitor choke with the
Ferrite Choke(W2DU). Are they electrically equivalent?
4- Are either of them a multi band solution?
5- Are there Upper and lower frequency limitations for using them.


As usual, a Google search provides an overwhelming set of responses
that most likely will cover the subject in greater detail than I can
digest.

Early in the list provided by the search I discovered
http://www.audiosystemsgroup.com/RFI-Ham.pdf

That may have all the answers but I have only glanced at the beginning
of the 66 page document.


That document certainly has a LOT of the answers... and good practical
data to use, as well.

On Sun, 03 Apr 2011 10:40:45 -0700, Richard Clark
wrote:

On Sun, 03 Apr 2011 10:19:34 -0400, John Ferrell
wrote:

What I need to know is
1- When to use them. RF in the Shack is pretty obvious

Hi John,

Sometimes it is not always that obvious, but it can be made obvious. I
say this because the symptoms do not always draw our attention to the
problem arising from Common Mode (CM) current.

One possible obvious symptom is that you can feel the current on the
shielded components of your system - like a microphone shell when your
lip brushes along it (which can sometimes give a nasty shock); or when
you pull a connector and get that tingle of feeling (which might kill
you if conditions were worse). "Obvious" can vary from subtle to
deadly.


Been there, done that...
although my particular case was trying to cathode modulate a home brew
final about 50 years ago...

Another possible obvious symptom is you are moving a transmission line
while keyed down, and you notice your SWR shifts at the same time. Is
it because of a faulty connection? Sometimes yes, but we are talking
about CM; and when it is CM, then that shift in SWR is because a tuned
circuit (the line is now part of a dynamic system) has been changed.


I had not thought of that. I will have to check that out.

* * * * * * * * *

Let's set aside in-the-shack symptoms and look at the antenna end of
what might be obvious.

For beam antennas, through careful testing with known remote stations
(beacons, for instance) you find that you have high sidelobes, almost
no back rejection, and a broad response off the front. The
transmission line coming down is acting like a vertical antenna, and
is feeding its drive into the antenna feed point (two antennas in
parallel). That extra antenna is picking up stations that would
normally be suppressed by beam design.

Some folks never notice this - and it shows how benign CM can be (it
is there full blown, but it is NOT obvious at all).

For wire antennas, you cut and cut and trim and trim your antenna to
frequency, but even though it is book perfect (using the usual
formulas for length), it is wildly out of whack and a bitch to tune
with your tuner. Your dipole without choking is actually a tripole:
two horizontal elements with a third vertical element (which is
variable in length and relationship to ground/environment as it swings
in the wind). You raise it higher to see if that does anything, and
the tune shifts to the moon because the third element has gotten
longer.


You have just described my recent attempt on 160 Meters.
I will put it on my "TO DO" list and try again.

Again, some people never notice this, we have seen them; others do
notice it and fulminate against the dipole as being a useless
invention where their endfire (which suffers the same issues, but is
magical by comparison) antenna is the cat's meow.

More could be said....

2- When not to use them. There must be some negatives.

That is a natural question, but I cannot think of any down-side at
all.

3- "Compare and Contrast" the Inductor-capacitor choke with the
Ferrite Choke(W2DU). Are they electrically equivalent?

They are NOT electrically equivalent. The ARE functionally equivalent
- that is, they both suit the same purpose of introducing a very hi Z
into the current path of CM.

The coil of transmission line is, on first pass approximation, an
inductor whose Z climbs with applied frequency. The presumption is
that it can only get better for the higher bands. This works to a
point, but the same coil of TL also has loop-to-loop capacitance, and
end-to-end capacitance. These two capacitances, at some frequency,
become resonant and offer you a tank circuit. This works even better
to isolate the current at the drive point from the rest of the line.
However, a tank is not a wide band device by definition. It can
exhibit an extremely hi Z at one frequency, and often high enough Z in
neighboring bands (hence it is useful if you understand this). As you
go higher in frequency, and for the very large coils suggested
earlier, then the capacitances begin to dominate and actually kill any
sense of choking.

I thought this was a likely scenario but you are the first I have
encountered that passed that along. Somewhere I read about a fellow
who was using spool of coax in the shack for a choke on 160M...
Well intentioned, but clearly not really a solution!


The ferrite choke is a resistor, plain and simple. This means it is
wide-banded - although its frequency characteristic is not universally
applicable (this widebandedness usefully covers two, maybe three
octaves). The formulation of the ferrite determines the coverage, and
so you can either throw more beads into the mix, or use beads of
different formulations to create a more flattened frequency response
(over decades of frequency change).

4- Are either of them a multi band solution?

Yes, as described above.

5- Are there Upper and lower frequency limitations for using them.

That too is described above.

As usual, a Google search provides an overwhelming set of responses
that most likely will cover the subject in greater detail than I can
digest.

Early in the list provided by the search I discovered
http://www.audiosystemsgroup.com/RFI-Ham.pdf

That may have all the answers but I have only glanced at the beginning
of the 66 page document.

It is a very good and broad discussion. The downside is its enormity.
However, this is not a simple topic and understanding comes slow.

The reason for that is that Common Mode analysis as distinct from
Differential Mode analysis immediately brings up the specter of WTF?
There are TWO currents flowing at the same time in the same circuit?

This perspective is usually very foreign and takes deep meditation to
come to grips with. It is difficult for the majority of the Pros too.

I am going to prompt readers for the next question:
"What is common mode current?"
If any one cares to indulge me by asking, we can go further, but the
material in this posting is enough to digest in one sitting.


It is you who are indulging me!

Google and other Internet resources are very helpful but they are a
poor substitute for direct answers to direct questions.

As the sailor told the Captain, "I won't abandon the ship until the
keel comes through the foredeck!"

It seems to me that I might get an indication of current on the
outside using a Grid dip meter as a detector at various points on the
transmission coax.

73's
Richard Clark, KB7QHC

On Sun, 3 Apr 2011 09:43:27 -0700 (PDT), Tom Horne
wrote:

On Apr 2, 7:04*pm, Richard Clark wrote:
On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne
wrote:

Richard
I didn't ignore choking the feed line but I will readily confess that
I did not choke it twice. *Starting immediately below the bottom of
the matching stub I followed the recommendation of the various authors
and wound a multi-turn coax balun with a six inch diameter coils of
coax. *They call for ten turns if I recall correctly. *

Hi Tom,

This sound like very common advice - so common that it begs
investigation because it is common advice for HF Choking, not 2M, and
certainly not 70cm. *However, this common advice acknowledges the need
for choking.

You would be better served using ferrites (W2DU style BalUn/Choke),
or, if you really wish to stick with wound coax, then use a Grid Dip
Meter to test its resonance (which should reveal you can't serve both
bands). *Using an antenna analyzer to do this will give you measurable
Z, and that may give you the data to see how well you are doing. *You
may wind a lot of chokes to discover that the diameter is
extraordinarily huge (or so the same for turn count - one or the other
or both).

I was not aware
of the need for a second choke at one quarter wavelength away. *Do you
have the energy to explain why that is necessary? *

[Do I have the energy.... *With all the junk cluttering this space
outside of this thread, I have plenty of energy.]

The antenna fields will try to excite the transmission line's common
mode (the shield of the coax that it sees). *This will induce currents
that will become radiative (just like stacked elements in a vertical
multi-element radiator - sound familiar?). *The use of chokes at
quarterwave intervals snubs these currents.

Sometimes more chokes are needed. *I use a 20' long line with a bead
every four inches or so. *The intent is to create a very long resistor
(very wide bandwidth) with very short leads (very high frequency)
between its distributed resistance.

How critical is the
length between the two chokes. *

Not particularly, you would be well served to attempt to make it at
the interval of quarterwave at the highest frequency used (70cm).
This, then, would snub lower frequency (2M) common mode currents.

Do I use the middle of the two chokes
as my measuring points? *

This sounds deceptively exact (and probably a consideration for those
enormous chokes of common advice vintage). *No, such exactitude is
going to be lost at the 70cm scale anyway which will be perturbed by
other factors (have I talked about environment?).

Could I substitute a one to one current balun
built of ferrite beads? *That would have a less intrusive appearance
and accumulate less ice in the winter.
If you check the link that I gave for Ed Fong's dual band j-pole;
available here
http://f1.grp.yahoofs.com/v1/
kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKk c
\FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just
look it up in the files section of the Yahoo reflector; *

I would prefer not to add yet one more account registration to simply
view this, sorry. *However, the narrative here should tell you if we
agree.

you will see
that he did test it in the field with fairly sophisticated
instrumentation. *He uses a trapped radiator to obtain similar gain on
seventy centimeters as a simple J-pole without the trap gets on two
meters. *

My background was working in the standards laboratory system of NBS
(my speciality was RF measurement to the highest accuracies). *I know
that sophisticated instrumentation and quality results don't
necessarily track each other. *My point was about environment, not
instrumentation anyway. *You don't need $1000 meters, but you might
need $1,000,000 environment.

The description of Apple's echoless environmental chamber used to test
their iPod is a marvel of engineering, and cost a stack of dollars.

His work was published in QST and I didn't find any
authoritative repudiation, or even strong criticism, of his design.

The same could be said of publishing it in Playboy. *QST needs content
to offer subscribers. *Vanity articles satisfy that need. *Think of
these articles as the introduction to a topic, not the final word.

Why would the presence of the trap in the lower half wave of the two
meter collinear half wave J-pole wreck the tuning on two meters.

The trap's tuning is heavily influenced by the geometry of the
elements because they are also part of the tuning. *The trap disturbs
that symmetry. *You have also introduced new L and C components that
further upsets the total circuit. *These things are not super-critical
when you consider that moving a resonance (say) 145KHz out of 145MHz
is only a shift of 0.1%. *That isn't super-critical either.

Or maybe it is.

I'm only trying to learn here.

Let me know if this helps.

73's
Richard Clark, KB7QHC

Richard
Ed Fong's article can be found on the ARRL website at
http://www.arrl.org/files/file/Technology/tis/info/pdf/
0302038.pdf.
I was able to access the above link without any password or other
challenge. I just reread it and found that he does address the
changes in the two meter element brought on by the inclusion of the
seventy centimeter trap. For one thing it shortens the two meter
element somewhat. So let me propose a modification process for my
collinear dual half wave two meter j-pole and you can tell me if you
can think of ways to rearrange or modify the steps so as to move
towards by hoped for results. What I hope to achieve is to maintain
the gain of the stacked, out of phase, dual half wave antenna on two
meters AND trap the lower have wave to a single half wave on seventy
centimeters.

Here are the steps I was thinking of:
Fabricate a trap from 1/2" copper pipe coming out of a tee that has
been cut in half through a plane perpendicular to the foot of the
tee.
Remove the top half wave and the phasing loop from the existing
collinear dual half wave antenna
Attach the stub to the lower half wave so that the open bottom end of
the stub is one half wave on UHF from the top of the matching stub.
Measure the SWR on 446 MHz. of the newly trapped two meter radiator.
Adjust stub location for minimum SWR at 446 MHz.
Measure SWR at 146 MHz.
Shorten that radiator to lowest SWR on two meters or until the two SWR
readings for VHF & UHF are equal.
Reattach the upper two meter half wave and phasing stub.
Measure antenna for SWR on both VHF & UHF again
Adjust length of of upper two meter half wave if necessary.

The antenna analyzer that I will use will be an AEA Technology SWR
Meter: 140-525. It happens to be what I have available to me.

Does my process seem to be along the right line or would you suggest a
different order of operations?

I used to use a 48 element Collinear antenna on 439.25 MHz for Amateur
TV in Ohio. It was designed by WA8RMC and constructed by WB8LGA.

The procedure to verify that each element was contributing something
to the array was to power it with a small signal then watch a field
strength meter as far away as you could see it and touch each element
to see if it would produce a wiggle on the meter.

No wiggle means that particular element was not doing anything. I
think you could do something similar to verify things are coupling.

That particular antenna was a collection of 24 dipole/reflectors in
harness. I have been thinking about building another. It was only 4
feet by 6 feet but a whole lot of wind resistance.

I intentionally did not clip this post to keep things in context...

John Ferrell W8CCW

On Tue, 05 Apr 2011 10:58:25 -0400, John Ferrell
wrote:

Another possible obvious symptom is you are moving a transmission line
while keyed down, and you notice your SWR shifts at the same time. Is
it because of a faulty connection? Sometimes yes, but we are talking
about CM; and when it is CM, then that shift in SWR is because a tuned
circuit (the line is now part of a dynamic system) has been changed.


I had not thought of that. I will have to check that out.

Hi John,

Another method (that I have not tried, and may be apocryphal) is to
take large piece of tinfoil in your hand (the tin foil is gripped
around the line) and move it along the line while watching the SWR
meter.

The tin foil is used as a means to increase YOUR surface area to the
line because you are the major conductor that is moving. You have
diverted the Common Mode path away from where it was going, by moving
into your hand, through your body, to your feet, and ground.

One very sure method is to insert an additional length of transmission
line in series (will require a barrel connector). Some lengths are
more dramatic in the shift of SWR than others (again, this is about
wavelength).

This method has been reported to us here more than several times -
unfortunately the reporters hadn't connected the dots to recognize
this was a confirmation of CM.

It seems to me that I might get an indication of current on the
outside using a Grid dip meter as a detector at various points on the
transmission coax.

Get a split core ferrite. Snap it around the coax. Add a short length
of wire through the core. Connect the ends of the wire to an LED (now
a complete circuit). Move the core/LED along the line during
key-down. Did the LED glow?

Make sure to test the LED first to see if it glows (I can't imagine
why it would not). Test the LED after this survey to see if it glows
(I can imagine that CM current could have fried it). Play with adding
more turns of the wire through the split core.

You have a simple portable RF transformer/indicator.

73's
Richard Clark, KB7QHC

On Tue, 05 Apr 2011 11:00:18 -0700, Richard Clark
wrote:

On Tue, 05 Apr 2011 10:58:25 -0400, John Ferrell
wrote:

Another possible obvious symptom is you are moving a transmission line
while keyed down, and you notice your SWR shifts at the same time. Is
it because of a faulty connection? Sometimes yes, but we are talking
about CM; and when it is CM, then that shift in SWR is because a tuned
circuit (the line is now part of a dynamic system) has been changed.


I had not thought of that. I will have to check that out.

Hi John,

Another method (that I have not tried, and may be apocryphal) is to
take large piece of tinfoil in your hand (the tin foil is gripped
around the line) and move it along the line while watching the SWR
meter.

The tin foil is used as a means to increase YOUR surface area to the
line because you are the major conductor that is moving. You have
diverted the Common Mode path away from where it was going, by moving
into your hand, through your body, to your feet, and ground.

One very sure method is to insert an additional length of transmission
line in series (will require a barrel connector). Some lengths are
more dramatic in the shift of SWR than others (again, this is about
wavelength).

This method has been reported to us here more than several times -
unfortunately the reporters hadn't connected the dots to recognize
this was a confirmation of CM.

It seems to me that I might get an indication of current on the
outside using a Grid dip meter as a detector at various points on the
transmission coax.

Get a split core ferrite. Snap it around the coax. Add a short length
of wire through the core. Connect the ends of the wire to an LED (now
a complete circuit). Move the core/LED along the line during
key-down. Did the LED glow?

Make sure to test the LED first to see if it glows (I can't imagine
why it would not). Test the LED after this survey to see if it glows
(I can imagine that CM current could have fried it). Play with adding
more turns of the wire through the split core.

You have a simple portable RF transformer/indicator.

73's
Richard Clark, KB7QHC
When TV was in in its infancy and in Ham TV we frequently used 300 ohm
twin lead transmission lines. The effect was dramatic when you wrapped
foil around the twin lead and moved it up an down the feedline. If a
half wave of line was accessible, You had the full range of tuning.

I also have a meter stick with a pair of wires on which I can slide a
shorting bar. The usual indicator is a 50 micro amp meter with a 1n34
germanium diode and either a dipole or sense loop attached. Nearly all
of that work was done at 440 mhz.

I will think about that LED indicator. Usually I keep the power low
when doing this sort of thing. The threshold required to trigger the
led. Also, once it triggers the abrupt transistion would probably
affect the operating conditions. Perhaps it is time to increase the
power a bit for such experiments...

John Ferrell W8CCW