In article P2G1e.110904$Ze3.11791@attbi_s51,
wrote:

By all means put me on your kill file. I asked a simple question and you
want to reply to a different question of your liking and place your
question
as one preferable to mine.

Art, in cases like this, you keep asking "simple questions" which
imply, by their very wording, a whole bunch of assumptions about how
things work which just ain't so.

The fact that you keep getting answer after answer, from a lot of
knowledgeable people, which you either don't understand or "blow past"
or that you feel evade the point of your question, ought to be saying
something to you: that there's something wrong with the questions you
ask.

No one has been able to supply the answer to my question,
Using your words ,what steers the pattern away from a circular
form from a natural circular form.

That depends on what you mean by "a natural circular form."

If you're referring to the fact that the main lobe of a dipole tends
to look circular on many of the commonly-used plots, then the pattern
isn't "circular" in any cosmic sense of the word. It's just as
correct to say that it's elliptical, or bumpy, or squashed, because
that's exactly how it will look on plots which use different circular
axes (linear, logarithmic, etc).

To try it again, though: you're asking why the pattern appears to be
compressed, as the gain increases. Fundamentally, it's due to the
fact that the antenna is sending more power out in the desired
directions (more gain), at the expense of sending less in other
directions. This is done by creating multiple radiators, which are
offset in power and location and phase so that their individual
radiation wavefronts reinforce in the desired directions, and cancel
in the undesired directions.

When we plot the resulting RF strengths, the RF in the desired
direction is stronger (we got the gain that we want). Let's assume
that (as is common practice) we continue to plot the signal in the
strongest direction on the outer circle of the graph.

Now, one of two things will have to be true:

[1] Every other direction in the main lobe had its power "scaled up"
by the same amount... the increase in gain worked the same for all
directions within the main lobe. In this case, the shape of the
main lobe will not change at all.

In this case, the additional power required to achieve the
increase in gain in the main lobe will have had to come for
somewhere. Since it didn't come from the main lobe, it will have
had to come either from the sidelobes, or from the rear half of
the antenna's pattern.

There's a limit to how far you can take approach [1]. It stops
working when your sidelobes and rear half of the pattern drop to
zero... and it becomes rather ineffective some time before that,
when the largest of the side/rear lobes is maybe 10-15 dB down.
Beyond that point, there just isn't enough power left in those
backlobes to be useful.

[2] The other possibility is that you didn't manage to boost the gain,
uniformly, in the entire main lobe.

In this case, if you're still plotting the strongest signal on the
outermost circle of the graph, you'll notice that the shape of the
main lobe has changed. Any direction in which the gain increase
was less than the maximum you achieved, will be closer to the
center of the circle than before. [Another way of looking at this
is that by increasing your maximum directional gain, you've
"enlarged the circle" on which you're plotting it, but that some
points didn't move outwards by the same ratio.]

In the common case of a Yagi, when you boost the gain (say, from 10 dB
to 15 dB) there just isn't enough power available in the side and rear
lobes to make up this gain... you can't 'rob' enough directivity from
the sidelobes and rear lobe. Instead, you 'rob' the power from the
outer edges of the main lobe, and shift it in towards the center.

You do this, most commonly, by adding additional parasitic elements,
whose location and phasing are such that their radiation reinforces
that moving in the "forward" direction, and interacts destructively
with (cancels) radiation moving outwards at an angle.

When you plot the resulting pattern, and scale it so that the
strongest signal is on the outer circle of the plot, you find that the
main lobe looks narrower. Part of this is due to the actual
redirection of power, and part of it is due to the fact that you've
re-scaled the graph.a

.. I am not asking for the extraneious
information
that all feel they are compelled to supply to make their posting look
informative.

[invokes several wrathful deities...]

Art, if you continue to ask "simple questions", and you continue to
get back complicated and detailed answers, it really ought to convey
to you the possibility that your "simple" question is oversimplified.
Or, perhaps, that you've been given the actual (simple) answer three
or four or five times already, have rejected it, and people are trying
to explain to you why it's actually correct.

Goodbye, Art. This is/was my last attempt, I think. I doubt I'll try
again.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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