Richard Clark wrote:


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.


Thank you for that
clarification/correction, Richard. What
I should have said was that with CM
effects, the SWR meter will not show the
actual SWR "in" the coax, which could be
wildly different from the 1:1 measured
for the entire antenna/feedline system.

73,

Chuck

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On Wed, 20 Jun 2007 09:20:15 -0400, Chuck
wrote:

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.

Hi Chuck,

I can report a bright side. During one field day, years ago, one
fellow brought in a baloon and hoisted enough wire into the sky to
work it as a halfwave 160M vertical. He used an optical rangefinder
to measure the height. He had this Army surplus tuner that tuned it
up to his rig. The 160M contacts he made told him he was the
strongest signal on the band (working 100W). Perhaps it helped that
we were in a school ball field on the top of a hill. To further add
to the mix, I suggested he tie the tuner to the fence line and
backstop.

73's
Richard Clark, KB7QHC

On Thu, 21 Jun 2007 03:30:53 GMT,
wrote:

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

Hello Richard,

Richard Clark wrote:
It is merely an off-center dipole that hasn't come out of the closet.
The wiring in your shack supplies that other half, and supports the
common mode current/voltage.


Interesting concept. Not technically a
dipole, though. But similarly true of
less-than-perfect, center-fed dipoles, no?

Seperate generic dipoles from resonant dipole halfwave antennas
or other resonat half wave antennas with unusual feeds.
It's possible to have a nonresonant dipole that is an effective
antenna but that a whole different animal.

This doesn't make much sense. A dipole is a generic term for two
poles. There are very few monopoles in nature in spite of the term
being used generally for a vertical. Even then, there is the plane of
ground (or a ground plane when elevated) that serves as the other half
fulfilling the sense of "dipole" (two poles, i.e. points that are
separated by 180 degrees of voltage sense).

Irrespective of being a true or other dipole, resonance comes with the
nature of the system, which may include the feedline, and any wiring
that is substantially (in terms of wavelength) remote from ground. It
takes extreme effort to remove these effects of elevated wiring. These
same wires radiated, even if ineffectively (or even effectively) or,
rather, unmanageably (the operator never intended, or considered them,
part of the radiator).

Be careful as the average OCF is an antenna that longer than a
halfwave. Though for instructional value there is one aspect there
in that at the center the feed point in free space is 72ohms (or there
abouts). As you move the feed point to the ends the feed resistance
goes up untill you reach the and where it's the highest at that point.

That, in fact, is not true. You may find certain "sweet" or "sour"
spots along the length. Some can effectively render a 50 Ohm match
(if you take great care to both choke the feed and isolate it from the
imbalanced fields). You can also elevate the Z too. Transformation
along the length of a wire renders many Rs and Zs. The OCF will
generally resonate at the same frequencies regardless of feed point.
So a half wave of wire can be induced to match different Zs and Rs by
feed placement (although maintaining isolation, as mentioned, can be
exceedingly difficult).

Shack wiring is not RF.

This is the desire of the operator, but rarely given enough
consideration to be generally true. The further your operating
position is from ground, in terms of wavelength, the higher potentials
your position supports in the presence of Common Mode excitation.
Second story shacks are notorious for these problems.

The difference if the home power was DC
it will still require a return path and there is no way to virtualize
it. At RF ground is/can be a virtual thing.

You can force the solution through tuning the ground wire, or
providing for a virtual ground, certainly. This, however, is rarely
practiced. Faith does not replace ground.

Having built end fed half waves (and full waves) for 40-6m
I can say I've had RF problems. There is a reason for that.
If the radiator is not a resonant half wave there will be problems
and that length is affected by position and environment
just like any dipole.

Common mode cares not a whit about resonance. It is a condition
driven by imbalance. That imbalance arrives either electrically or
physically (both of which are so intimately intertwined as to be
inseparable in the common antenna installation).

That is the first step, if the radiator is
not resonant it will induce reflections in the system just like a half
wave dipole.

All antennas that are short are by definition (the halfwave and even
full waves discussed here) are Standing Wave antennas. They ALL
reflect regardless of resonance. You only encounter non-reflective
antennas by one of two means:
1. Large dimensions (Rhombics are classic) that are terminated;
2. Short antennas provided with buffering resistors.

The second reason I've had problems is
going from 50 ohms to around 3500 to 5500ohms (assuming a
resonant radiator) and the network for that if wrong will induce
all manner of problems.

Of course. A Hi-Z feed point is nearly impossible to choke. That
feedpoint looking back at the entire system will endeavor to feed
everything, including the transmission line and thus induce Common
Mode currents. These currents are a random and opportunistic fact of
life, and across the variety of situations will either present
problems or appear to be benign.

When I have it right there is no common
mode RF on the outside of the coax (other than induced by the near
field of the antenna) and this is similar to what would be expected
from a properly fed half wave dipole.

On the face of it, an OCF (or end fed) antenna is distinctly different
from a "properly fed half wave dipole" by simple, geometrical
analysis. The Common Modality follows this with strong correlation. A
standard dipole, "properly fed" assumes a strong symmetry which
precludes Common Mode induction when also properly choked. An OCF
does not exhibit symmetry, and typically suffers considerable Common
Mode induction which often precludes attempts to suppress conduction
through choking.

Voltage fed antennas
(halfwave) have too low a current at the feed point to have common
mode problems

This is simply wrong. There is nothing inherent about Common Mode
that is extinguished by any distinction of being "voltage fed." You
may experience a condition that supports this statement as an example,
but it does not create a rule. Luck does not define the condition.

Allison, you mix so many examples of problematic episodes and
conflicting advice as to negate most of what you offer. Common Mode
is dominated by balance or imbalance. There are certainly
expectations of resonance that follow from need, but they have
absolutely nothing to do with the CM problems that are encountered,
nor dodging the CM bullet through happenstance.

Common Mode issues are induced, controlled, or cured through very
simple considerations and they never appeal to resonance. The problem
for the typical Ham is these simple considerations are not simply
solved. Balance is not achieved by the wave of a hand. You cannot
force CM away through careful tuning. If you don't notice CM, all
fine and well, but that does nothing to solve an imbalance either -
you simply don't care about it is all.

73's
Richard Clark, KB7QHC

To further add
to the mix, I suggested he tie the tuner to the fence line and
backstop.

73's
Richard Clark, KB7QHC

That was canny advice, Richard... The top rail of the fence and
backstop 'could' have been an NVIS antenna - which will get you
smokin' reports out to a few hundred miles...
I do this routinely for Field Day with a horizontal loop for 80 meters
being strung about 20 feet high over a low, wet, fertilized field next
to a river... The guys run QRP off a battery and are amazed how they
can break pile ups... I have never bothered to point out to them that
the vast majority of their contacts are within a 500 mile circle...
They are happy and I believe in ignorance being bliss...


denny / k8do

On Thu, 21 Jun 2007 11:49:39 GMT,
wrote:

Half wave antenna can be a monopole however the reverse is
can only be true if its resonanance is at 1/2 wave.

Hi Allison,

This still lacks common sense. What is the reverse? What are you
comparing? Apples to Apples and saying one is not an Apple?

Irrespective of being a true or other dipole, resonance comes with the
nature of the system, which may include the feedline, and any wiring
that is substantially (in terms of wavelength) remote from ground. It
takes extreme effort to remove these effects of elevated wiring. These
same wires radiated, even if ineffectively (or even effectively) or,
rather, unmanageably (the operator never intended, or considered them,
part of the radiator).

Explain the EDZ which is a doublet and the 5/8s wave both of wich do
not resonate at the operating frequency.

What is there to explain? Resonance is remarkable only to the
measurement of Z. None of this has anything to do with either
encountering Common Mode nor imbalance, nor feedline problems.

Your questions would be more meaningful if they contained some
context. I don't see that in relation to my comments.

Physical imbalance is not an issue. Electrical imbalance is.

How you can separate the two in a structure of several tens of meters
is a painful conceit. Physical balance with even the best of dipole
implementations is a rarity. Lacking that balance, electrical
imbalance follows to the same degree. This is simple physics.

What is required for balance is less understood.

I understand it quite well, and I have designed instrumentation that
requires it without fail. To extend this knowledge to common Ham
situations is not a leap of faith, merely the practice of common
sense.

Ground however is not required.

The dismissal of ground is specious. No one here has any power to
achieve that or have options in that regard.

No it's easy to deal with. Chokes however to be effective at hi-z
must also exhibit hi-z.

You have some really easy answers to difficult situations; and yet you
describe situations fraught with challenge and unknowns. For instance
this effective choke for feedpoint Zs in the 5000+ Ohm region: you
don't describe it.

I dare say your best approximation would be a line that is halfwave
long itself. In that case, the choking action would, at best, equal
the feedpoint Z. Not much of a choke there; you simply end up with a
new, lazy dipole.

OK then, the matching tank? You still need to choke the line going to
the tank and the loaded Q of the tank does not present choking to the
extent of the unloaded Q.

In fact, the only saving grace is that your matching tank is located
at the base of the antenna (not uncommon), and the path back to the
transmitter is immaterial because of the proximity of lossy earth
snubbing the Common Mode. This Hail Mary solution inverts your dictum
above: an effective choke for Hi-Z should be low and lossy.

Unfortunately this fails for end-fed horizontals on high, and
achieving the Hi-Z choking is not guaranteed of being achieved nor a
guarantor of being effective.

By and large, success in these scenarios is often a greater part of
luck, or the skill of practicing luck.

On the face of it, an OCF (or end fed) antenna is distinctly different
from a "properly fed half wave dipole" by simple, geometrical
analysis. The Common Modality follows this with strong correlation. A
standard dipole, "properly fed" assumes a strong symmetry which
precludes Common Mode induction when also properly choked. An OCF
does not exhibit symmetry, and typically suffers considerable Common
Mode induction which often precludes attempts to suppress conduction
through choking.

You've convinced yourself. And an end fed is not an OCF.

I fully know what I am convinced of. Your statement is merely a
mis-interpretation without explanation.

Luck is random. The condition achived is not and is predictable as
well as reproduceable.

I roll the dice, 7 comes up, I win from that being the highest
probability and that is an achieved condition. It is predicable too.
Reproducing it conforms to common odds. Winning does not explain HOW
to win.

No you wish that. However there is no confilct in what I gave. If
you do it worng it doen't work right. IF you do it right the most if
not all of the issues will not arise.

This is an homily, not an explanation.

They were examples of things done wrong and in most cases seriously
wrong. I may point out the people that had problems I'd helped could
not get a basic monoband dipole to work because the idea of keeping
the antenna away from the trunk of a tree or measuring accurately was
not taken seriously. I offered those to point out that many who claim
"It doesnt work" did enough wrong that the expectation of proper
operation was unreasonable.

Certainly problems are invested with this common condition.

Generally I've had more problems with
RF getting into places I didn't want it due to radiation (desired)
than common mode currents in a working system of any sort.

This is exactly the situation of Common Modality. It arrives by
induction or conduction. Over the air, or over the wire. Both over
the air AND over the wire. The path and the imbalance define the Z
and the linkage excites it to exhibit voltages and currents consistent
with that impedance.

Conceptually it is very simple to understand CM: imbalance. One can
start with a "perfect" dipole in free space. At a great distance, and
at a point along the same axis you can not perceive any signal from
it. Move off bore-sight and the signal becomes perceptible to the
degree of the imbalanced condition you have now entered into.

Place the same dipole over a "perfect" ground, and you will never find
a "perfect" null because the "perfect" (or even imperfect) ground
disturbs the balance. Common Mode, by definition, arrives over the
air. In a real world, it is impossible to remove. You can reduce the
effects to below the threshold of irritation, certainly.

Stand below the "perfect" dipole on a "perfect" ground, and feed it
with a balanced line and you will obtain a Common Mode free condition.
Draw the line to one side by a degree, or droop an element of the (now
non) "perfect" dipole one degree, and that condition will be lost - to
that "degree." Physical symmetry informs the degree of balance.
Electrical balance slavishly follows physical balance, there are no
solutions that can repair this without answering the physical state.

Now, excite an end-fed anything (it is immaterial how long the wire
is) that travels at some height over a real ground (it is also
immaterial that it be ground, it can be in relation to an airplane, or
a Zeppelin, or lunar lander). You must have a dipole somewhere. Most
ignore the obvious: the bulk of the transmitter is enough, even it if
it is battery operated and self contained and remote from any operator
or shack. A current will not flow without a potential difference, and
that difference requires some anchor - however small. Common sense
informs us that there is not such thing as a monopole generator.

The average Ham transmitting into an end-fed antenna does not do it in
such isolation. The bulk of the transmitter is not without wiring
that can be substantially elevated from the ground below the antenna.
That complete circuit is not balanced. The lack of balance
necessarily injects Common Mode into the picture. The degree by which
it interferes (in all senses of the word) is found in the combination
of balance and the Z presented by this random path. If the operator
can tolerate it, it isn't a problem. Not being a problem does not
wish away CM.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

Half wave antenna can be a monopole however the reverse is
can only be true if its resonanance is at 1/2 wave.

...
Richard Clark, KB7QHC

Richard:

I think he means a half-wave monopole is quite possible (I use 'em all
the time!), and a half-wave dipole can exist with each leg being 1/4
wave ... both (having in common) being a half-wave antenna.

Or, at least, that is the way I read it ...

Regards,
JS

John Smith I wrote:
I think he means a half-wave monopole is quite possible (I use 'em all
the time!), and a half-wave dipole can exist with each leg being 1/4
wave ... both (having in common) being a half-wave antenna.

"Pole" can have two different meanings in this context.
A physical fishin' "pole" is different from an electrical
or magnetic "pole". Seems the word "pole" requires an
adjective to define the context for antennas.
--
73, Cecil, w5dxp.com

Cecil Moore wrote:

...
"Pole" can have two different meanings in this context.
...
73, Cecil, w5dxp.com

Cecil:

You'd love to meet my mom; she is still alive, yanno?

Anyway, her good advice was to always keep my "pole" in my pants, said
something about children not needing to be raising children ...

Anyway, the word "pole" has always been confusing to me ... grin

Regards,
JS

Cecil, W5DXP wrote:
"Seems the word "pole" requires an adjective to define the context for
antennas."

It should not hurt. My dictionary says a dipole antenna is a straight
conductor usually fed in the center.

However, dipole characteristics are determined by current distribution,
not the feeding if the reeder doesn`t radiate.

Best regards, Richard Harrison, KB5WZI

John Smith I wrote:

[stuff]

Anyway, the context used here relates to north pole vs. south pole, up
vs. down, right vs. left ...

Regards,
JS