End-feeding dipoles
 

On Tue, 19 Jun 2007 16:45:15 -0400, Chuck
wrote:

Hello Richard,

The complaints made here are far from sparse.

I've seen no complaints here at all
about matching difficulties. Even
posters who had used the antenna omitted
matching difficulties from their
reported experiences.

As I said, those that don't notice, don't complain. This is the human
condition. If they didn't notice, it must have matched (or they
didn't measure it, which is the same thing as not noticing).

We've seen plenty of complaints. Certainly they didn't lead with
their chin, the symptoms bore out the problem and they were
complaining of something they thought was remote from tuning; but not
far remote - meaning they "thought" it was tuned, but their rig was
going whacko.

On the other hand,
those who don't notice, don't complain. I will bet you have one
outlet in your home with inverted neutral/hot and a floating ground.
Does it bother how your lamp works?

I understand. But I really hoped to talk
about matching difficulties and find
myself awash in discussions of potential
common mode currents, about which I have
no truck. ;-)

You simply have to recognized the symptoms. If there are no symptoms,
there are no complaints. However, that doesn't mean their systems are
free of common modalities. It simply means the currents/voltages are
below the threshhold of notice.

Common Mode currents/voltages exist in EVERY system. It is merely the
degree and tolerance that become the issue.

Some folks have common mode complaints, others don't.

Sure. And I'd extend that to real,
center-fed dipoles with less than
perfect transmission line/antenna
symmetry. All a matter of degree?

Yup, as I anticipated in my earlier comment.

In that
vein, you stand to come out ahead if you seriously examine your
shack's quality of ground for all applications.

I'd do it immediately if it would help
explain the alleged matching
difficulties. ;-)

One solution for common mode problems is a ground tuner. This is also
called a virtual ground if the wire terminates in an open instead of
going to ground. What this does is references your rig/bench/room to
RF neutral. In that condition you don't notice that slight tingle
from the chassis as you brush the back of your fingers over it; or the
sizzle from the mike when your lips touch it. If your shack is
relatively close to the service ground, and the wire from your
rig/bench/ground runs only several feet; then everything should be
hunky dory. That is: up to a point where that length becomes a
significant fraction of the wavelength with a sizeable energy content.

At that point, you want to reduce the reactance of that wire by making
it one big Honker! Or, for a bench, you use a conductive sheet and
tack your equipment to the sheet. Usually a star (branching) system
of grounds is the best, but our equipment rarely exists in isolation
and there are cross connects. This can lead to ground loops (common
mode really rears its ugly head in this circumstance). So in that
instance, you cross connect like mad (and hence build your own mesh of
that sheet you should have laid down in the first place).

Most folks we hear from who don't complain about tuning (it loaded
fine, works fine, and lasts a long time) wail a tale of grief about RF
getting into their speaker, bathroom fault isolation, hall dimmer, VCR
- you name it, but it isn't called a tuning problem fer sure.

We ask them to jumper in an extra few feet of transmission line and
check their SWR. They usually are astonished that their antenna needs
tuning again. This is a slam dunk indication of common mode problems.

Like I said, these complaints are not uncommon. On the flip side,
some folks think more tingle on the lips is simply their excitment of
working DX (whose going to complain about that?).

73's
Richard Clark, KB7QHC

End-feeding dipoles
 

Thanks for the elaboration, Richard.

73,

Chuck

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End-feeding dipoles
 

Richard Clark writes:

On Tue, 19 Jun 2007 16:45:15 -0400, Chuck

I'm not trying to argue against your main points, but I have a problem
with this:

We ask them to jumper in an extra few feet of transmission line and
check their SWR. They usually are astonished that their antenna needs
tuning again. This is a slam dunk indication of common mode problems.

Unless the SWR on the line is 1:1, changing the length of transmission
line *will* change the impedance seen. Whether there is common mode
current or not.

73
LA4RT Jon

End-feeding dipoles
 

"Jon Kåre Hellan" wrote in message
...
Richard Clark writes:

On Tue, 19 Jun 2007 16:45:15 -0400, Chuck

I'm not trying to argue against your main points, but I have a problem
with this:

We ask them to jumper in an extra few feet of transmission line and
check their SWR. They usually are astonished that their antenna needs
tuning again. This is a slam dunk indication of common mode problems.

Unless the SWR on the line is 1:1, changing the length of transmission
line *will* change the impedance seen. Whether there is common mode
current or not.

73
LA4RT Jon

True it will change the impedance seen but not the SWR so unless you are
checking your line with an impedance bridge you should see no change except
that due to cable loss and normally a few feet of feedline does not add any
significant loss. If you do see a change it will be due to currents on thec
coax.

Jimmie

End-feeding dipoles
 

"Jimmie D" writes:

"Jon KÃ¥re Hellan" wrote in message
...
Richard Clark writes:

On Tue, 19 Jun 2007 16:45:15 -0400, Chuck

I'm not trying to argue against your main points, but I have a problem
with this:

We ask them to jumper in an extra few feet of transmission line and
check their SWR. They usually are astonished that their antenna needs
tuning again. This is a slam dunk indication of common mode problems.

Unless the SWR on the line is 1:1, changing the length of transmission
line *will* change the impedance seen. Whether there is common mode
current or not.

73
LA4RT Jon

True it will change the impedance seen but not the SWR so unless you are
checking your line with an impedance bridge you should see no change except
that due to cable loss and normally a few feet of feedline does not add any
significant loss. If you do see a change it will be due to currents on thec
coax.

Jimmie

If you only measure SWR, you will see no change. But the text I
commented said "their antenna needs tuning again". You *will* have to
adjust your antenna tuner. Antenna tuner settings don't only care
about the magnitude of SWR, but also about the specific resistive and
reactive impedances.

73
LA4RT Jon

End-feeding dipoles
 

Chuck,
I just recently finished a round of antenna tuner thrashing that
included some vertical, half wave wire, bottom fed antennas... This
was through a tuner(s) of my own design and construction including
hand built variable caps, with the feed points being head high and
the 1/2 wave antenna worked against a half wave elevated counterpoise,
with the coax dropping straight to the ground and running on the
ground hundreds of feet to the shack...... The ground was wet with
half melted snow and rain during most of the test... I had to stand
in a flowing stream to make tuner adjustments - snow melt water will
get your attention when it runs over the top of your boots!

While I got the tuner design to work - which was the whole reason for
the exercise as opposed to being primarily an antenna test - I was not
impressed with the half wave, end fed, vertical antenna overall - 80,
40, and 20 meter antennas were tested...
They were distinctly more noisy than ground mounted quarter wave
antennas for the same bands... Often, deafeningly more noisy...
The recovered signal strengths we
1. often less than for the quarter waves -
2. sometimes comparable -
3. the strong signal exceptions being the times that the very low
arrival angles were exactly what the half wave vertical wanted to
see...
(you can never have too many antennas)

On 20 meters the separation between the two antennas was 500 feet,
and expanding to some 900 feet for 80 meters test antenna being the
half wave end fed, and the reference antenna being 1/4 wave ground
mounted... I feel that the distances were sufficient that mutual
coupling was minimized enough as to not skew the results - it
certainly was not eliminated, however...

The circulating tank current on a tuner used transform 50 ohms to an
end fed half wave is impressive - often melting the dielectrics used
for the variable caps...

denny

End-feeding dipoles
 

Jon Kåre Hellan wrote:

If you only measure SWR, you will see no change. But the text I
commented said "their antenna needs tuning again". You *will* have to
adjust your antenna tuner. Antenna tuner settings don't only care
about the magnitude of SWR, but also about the specific resistive and
reactive impedances.

There are some, like Cebik,

http://www.cebik.com/trans/cmp.html
A Common-Mode Current Picture Show

who believe common mode RF getting into
some electronic metering circuits
produces erroneous SWR readings. Could
that be the reason small changes in
transmission line length sometimes
result in apparent changes in SWR?

Of course, if there are common mode
currents on a coax transmission line,
the impedance of the line seen by the
SWR meter (50 ohms in parallel with the
impedance between the shield's outer
surface and ground) is no longer 50 ohms
and the meter calibration is no longer
correct regardless of whether RF is
getting into the electronics of the
meter. Changing the length of the coax
gives you a new, out-of-calibration
measurement. ;-)

73,

Chuck

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End-feeding dipoles
 

Denny wrote:


The circulating tank current on a tuner used transform 50 ohms to an
end fed half wave is impressive - often melting the dielectrics used
for the variable caps...


Thanks for the report, Denny. Some of
the T- and L- network tuners also pass
some hefty currents at high power.

It is difficult to find much enthusiasm
for the performance of half-wave
verticals from folks who have actually
tried them. Your experience sure
supports that.

73,

Chuck

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End-feeding dipoles
 

On Wed, 20 Jun 2007 06:49:46 -0400, "Jimmie D"
wrote:

Unless the SWR on the line is 1:1, changing the length of transmission
line *will* change the impedance seen. Whether there is common mode
current or not.

True it will change the impedance seen but not the SWR

You are both wrong.

73's
Richard Clark, KB7QHC

End-feeding dipoles
 

On Wed, 20 Jun 2007 09:14:04 -0400, Chuck
wrote:

who believe common mode RF getting into
some electronic metering circuits
produces erroneous SWR readings.

This would be an exceedingly strange meter. Of course, there are any
number of ways to do something wrong. If this were the case (the
"infected" meter) then the antenna's being tuned is also suspicious.

Could
that be the reason small changes in
transmission line length sometimes
result in apparent changes in SWR?

No.

Of course, if there are common mode
currents on a coax transmission line,
the impedance of the line seen by the
SWR meter (50 ohms in parallel with the
impedance between the shield's outer
surface and ground) is no longer 50 ohms
and the meter calibration is no longer
correct regardless of whether RF is
getting into the electronics of the
meter.

What you have done is tuned the entire antenna/feedline system to 50
Ohms (this includes the common mode effects). Changing the length of
the line (which should not change the SWR in a CM free system) also
changes the reactance of the this length that was formerly tuned out.

Changing the length of the coax
gives you a new, out-of-calibration
measurement. ;-)

By giving you an out-of-50-Ohm load (antenna plus unchoked line).

Fellows, this is all classic stuff and has been fodder for discussion
for years. The solutions have met the test of time.

73's
Richard Clark, KB7QHC