Perhaps this debate is really a measurement problem ---
I think the important thing to remember is that a SWR bridge is probably
NOT going to measure resonance when attached to some random length of
coax hanging in the air to an antenna.
What is needed is an analyzer that can be calibrated with the
measurement plane at the far end of the coax. A VNWA, AIM4170, SARK110,
etc. can be calibrated with S/O/L on the end of the cable so the
measurement plane is at the antenna. Then when jX = 0 you have
resonance. However, even then real R might not be 50 ohms, not 1:1
SWR. Otherwise some complex Z probably transforms to 1:1 SWR on an SWR
bridge at some other frequency than Fres depending on the length of coax.
Grant KZ1W
On 5/31/2016 7:01 AM, Jim Thomson wrote:
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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