RadioBanter - View Single Post - End-feeding dipoles

Richard Clark · June 21st 07, 05:27 PM posted to rec.radio.amateur.antenna

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