Date: Tue, 31 May 2016 08:27:27 +0000 (UTC)
From: Dan Maguire <djm2150@yahoo.com>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
Jim VE7RF wrote:
>>> There is NO way in hell that the 66 ft long dipole, made from 1 inch OD
>>> tubing, is gonna shift 210 khz upwards in freq, when the tubing dipole is
>>> elevated from 30 ft to 60 ft.
I sure don't see anything in the model that would tax the capabilities of NEC
so I guess we'll have to agree to disagree on that. However, in playing around
a bit more I did stumble on something that may be of interest to those folks
who still put at least a little faith in antenna modeling.
I started with model 340-48H.YW from the ARRL Antenna Book CD, a 3L 40m Yagi
with a 48 ft boom, and used the Scale.exe DOS program to convert the elements
to monotaper. I also changed the units from inches to feet. Here's the result:
http://s33.postimg.org/hhm5hxebj/resonate4.png
Then I calculated the frequency of resonance (jX=0 at feedpoint, not
necessarily minimum SWR), first for the 3L Yagi and then just for the
stand-alone driven element (a dipole of length ~64.8 ft and diameter ~1.5
inches), at heights above Average ground from 2.5 to 100 ft.
http://s33.postimg.org/4yn6idnsv/resonate5.png
>From a height of 30 ft to 60 ft the Fres of the *Yagi* changed from 7.278 to
>7.258 MHz (-20 KHz). Over the same range the Fres of the *Dipole* changed
>from 7.132 to 7.349 MHz (+217 KHz). Ground reflection works in mysterious
>ways when there is also mutual coupling between multiple elements.
Bob K6UJ wrote:
>>> With my 2EL Force 12 40M yagi, I followed Tom Schiller's guidelines and set
>>> it up initially at 15 feet above the ground. When I raised it up to 70 feet
>>> the resonance frequency increased by 20khz.
This is a 3L Yagi, not a 2L, but the model shows similar results. From a
height of 15 ft to 70 ft Fres changed from 7.244 to 7.255 MHz (+11 KHz). Since
a *Yagi* model seems to be in the same ballpark compared to actual measurements
I don't see why a *Dipole* would be any different.
Dan, AC6LA
## heres the problem. f12 sez to check the resonance of each ele,
individually. IE: each of the 3 els has a center insulator. The REF +
DIR normally have a shorting strap across their center insulators. The DE
normally has a helical hairpin
+ balun across it. When tweaking say the REF for resonance at 6900 khz, the
shorting strap on the DIR is removed. The helical hairpin + balun is also
removed from the DE. Each ele is tweaked one at a time to whatever freq the
manual sez to use.
Then the shorting straps re-installed on REF + DIR. Hairpin + balun
re-installed on DE. A temp balun + MFJ used to tweak each ele individually.
The F12 N style 40m eles are not full size. They range from 59 ft for the
REF...down
to aprx 52 ft for the DIR.
## If the 40m dipole alone shifts upwards 217 khz.... then how come it
doesnt shift 217 khz... when a motorized crank up is changed from 30 ft to
60 ft ?? It shifts very little.
A shortened 80m rotary dipole shifts very little from 40 ft to 100 ft.
## I say something is amiss in the model. Anybody who has a 40m full
sized wire dipole strung between trees..at 30 ft.... then decides to raise it
to 60 ft a year later.... never saw a 217 khz increase in resonant freq.
Jim VE7RF
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