[TowerTalk] 2000' Yagi redux

[L. B. Cebik]

Several folks--more than several--have replied to my little tongue in
cheek exercise in using scrap from those fallen 2000' towers, correcting
the modeling done in a 2 minute period to lighten the discussion of crash
radii.

About half picked up on the fact that something was wrong.  NEC was
misleading me by virtue of using too small a resolution for patterns.  Had
I used a resolution of 0.1 degrees, I would have seen that there are lobes
every so often at points between whole numbers, and nulls that might come
closer to the whole numbers used for 1 degree resolution.  After all, NEC
is an excersise, when generating plots, of connect the dots, using only
the dots it has to use at the points we tell it to plot.

So for those truly planning that 2000' tower, here is a better story,
using 0.1 degree resolution.  There are lobes at about 0.5 degree
intervals at low angles, including 2.2, 2.7, and 3.2 degrees.  Beamwidth
of the lobes is about 0.2 degrees.  From the 8.11 dBi free space test
model, you can get 13.95 dBi at 2000', but if you add a 10' mast, you can
get 14.00 dBi.

At certain heights, the problem would have shown itself in an erratic
pattern of lobes when using 1 degree resolution, as some lobes came near
the 1 degree mark and others did not.  However, the recurrent pattern of
lobes peaking at nearly 0.5 degrees produced a smooth set of lobe curves
at 1 degree resoution.  The coincidence with the resolution came at 2000'
at the 23 degree mark with adjacent lobe sequences falling sufficiently
off the 1 degree mark in regular increments to give a "convincing"
pattern, as the dots got connected.

Lesson #1:  if you model, then just to check what you think you have,
check it at a higher resolution just to be certain.

A few asked if the ground would still have an effect.  The answer is yes,
but not the one of the exercise originally.  The ground still reflects.
And lots of power gets wasted with a standard single Yagi by going off at
very high angles.  At 2000' feet it goes off as a large series of spiky
lobes rather than as a couple of wide blobs, as it does at low altitude.

In fact, if we ignore all the other advantages and disadvantages of
placing the antenna at 2000' (for example, those related to physical
survival, cable length, etc.), one could raise the question of whether the
lower lobes with their narrow beam width would add up to a significant
advantage over a single beamwidth of several degrees width when centered
in the middle of the range of angles taken by incoming skip signals.  I am
not certain whether incoming signal angle resolutions are refined or
refinable enough to answer that question, but it would be interesting from
a curiosity perspective.  In other words, is there a practical limit to
the height at which vertical beamwidths of elevation become too narrow to
offer an advantage in reception, despite the slowly increasing gain of
each low angle lobe.

Oh, yes, there is another lesson to all this:  if you are going to say
something ignorant, a. try to be sure to say it around April 1; b. say it
during an idle practice exercise; and c. say it among friends who will do
some thought redirection for you.

Kudos to those who caught the nature of the error of the previous
exercise; and congratulations to those who had doubts.

Glad we got this corrected before anyone bought the tower scrap.

-73-

LB, W4RNL



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