[TowerTalk] Breaking up guy wires for nonresonance
alsopb
alsopb@gloryroad.net
Fri, 08 Jun 2001 10:32:37 +0000
What Guy is saying is:
There is a million ways to do it wrong and shoot yourself in the foot.
There are only a few ways to do it right.
Absent any concrete data or analysis proving you've done it right with
gobs of wires having multiple possible resonances, go for non-resonant
guys.
Unsupported testimonials just don't hack it.
Of course some people don't care and are happy no matter what they
have.....
For them, I prescribe G5RV's or verticals without radials.
73 de Brian/K3KO
"Guy Olinger, K2AV" wrote:
>
> 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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> >
> >
>
> List Sponsor: Are you thinking about installing a tower this summer? Call us
> for information on our fabulous Trylon Titan self-supporting towers - up to
> 96-feet for less than $2000! at 888-833-3104 <A HREF="http://www.ChampionRadio.com">
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>
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