Ken blends a number of things together in his post. Let's separate them
out:
Proportions are at issue. Most of the time I have seen a tribander and a
two meter 10 element or some such up together, the two meter job has
been five, six, ten feet above the tribander. This is like 50 or 100
feet separation on HF. The elements near the 2m driven element are
something like ten, twelve inches apart. The interactions between the
2m elements have the time (space) to form before the tribander below
comes into play. To keep it analogous or proportional with the HF
problem, IF you put that 2m beam inline with the tribander boom, only
five inches above it, you WOULD get notable interaction, because one of
the elements of the tribander is JUST AS TIGHTLY COUPLED into the 2m
beam as its other elements.
In the case of an 8JK, both elements are driven and create PRIMARY areas
of partial cancellation IN THE NEAR FIELD. None such happens with a
yagi. The primary radiation from the driven element is like a dipole.
Only hole is off the ends.
In the case of an HF triband or monoband yagi (what most people on this
reflector are talking about) at the top of a tower with guy wires to
within a few feet of top, the guy wire is right smack in the dense part
of the near field, CAN have significant current on it, CAN distort the
pattern. Put it in the model. It doesn't radiate only when it
serendipitously just happens to balance out by some happy aspect of the
instant orientation of the yagi elements and the length and orientation
of the guy wire. From there rotate the yagi a bit and you're back into
interaction. Or don't rotate it and see how much is radiating off the
other two guys.
On HF that upper guy is just like a snake in your pant leg. Plenty close
enough to cause trouble. Best not ignore it.
73
-----------------
Guy Olinger
Apex, NC, USA
----- Original Message -----
From: <K7GCO@aol.com>
To: <towertalk@contesting.com>; <k2av@contesting.com>
Sent: Thursday, June 07, 2001 7:00 PM
Subject: Re: [TowerTalk] Breaking up guy wires for nonresonance
> In a message dated 6/7/01 12:49:46 PM Pacific Daylight Time,
> k2av@contesting.com writes:
> >> Further more beams have a vertical directive pattern that points
> >> straight ahead. Therefore the pattern component pointing down at
> guy wires is greatly attenuated and any reflected RF is
attenuated
> again back
> >>toward the beam. The higher the gain of the beam the more isolated
the
> beam
> is from surrounding objects to the side/back and below. It just
doesn't
> see
> >> them.
> >OK so far.
> Actually, NOT ok.
>
> The typical image we form in our mind of a beams pattern is a FAR
FIELD
> pattern, that is to say when we are far enough away from the beam
that
> the sum of the radiations from the beam to the observer has
> significantly converged toward a point source in appearance.
>
> The really destructive interactions are very much in the NEAR FIELD.
The
> intuitive pattern image does not apply. In order to figure out what
is
> going on, you have to consider such things as guy wires or any close
> metal AS PART OF THE ANTENNA. Rather simple modeling applies well
here,
> just make sure that the model contains all the metal. None of the
issues
> that make it tricky are in force. Just a bunch of wires interacting
with
> each other.
>
> For each individual wire, the model will figure out the interaction
with
> EVERY OTHER wire and then add up the resultant vectors. Most of the
> antenna modelers will either display or report the resultant currents
on
> each of the wires in the model. That's when you find out a bunch of
> NON-INTUITIVE aspects about guy wires underneath a beam.
>
> There is an intuitive way to remember that near field stuff is
> different. Ken's idea above incorrectly depended on all of the
> interactions between driven element, reflectors and directors having
> ALREADY been formed before the guy wire came into play. This
obviously
> is true in the far field. HOWEVER, energy from the yagi DE is going
to
> reach the guy wire in roughly the same or less time as it does the
other
> elements, and CERTAINLY before RE-radiation from the other elements
has
> reached the guy wire. The guy wire has PRIMARY radiation from the DE
and
> must be considered as part of the antenna.
>
> Far field considerations are a SUBSET (remnant) of the rules in the
near
> field. The published ARRL non-resonant lengths were invented without
> benefit of computer based complete near field analysis, and have
> perpetuated a myth ever since.
> 73
> Guy Olinger
> Apex, NC, USA
> >>
> Guy: You have some good points but as I suggested in my post -- take
a 3
> element and a 11 element 2M beam connected to a MFJ SWR Anlyizer and
hold it
> next (really near field) to a variety of metal objects and you will
see the
> "extra isolation" the 11 element beam has on the SWR and therefore the
free
> space pattern. Put in a W8JK in Eznec and see how little affect other
> parasitic elements have on it's pattern compared to regular yagis.
Those 2
> elements are "so tightly coupled to each other'' they see far less of
other
> parasitic elements-even in it's major lobes. It literally doesn't see
> anything above or below it. What I said was not only "intuitive" but
based
> on "actual live beam tests and in Eznec". I suggest you get yourself
a
> Palomar RF Current Meter and run some tests. I assume you have the
MFJ.
> "One test is Worth 1000 Opinions" (Old K7GCO Axiom)
>
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