On 6/2/16 9:11 AM, Jim Thomson wrote:
Date: Wed, 1 Jun 2016 16:47:03 -0700
From: Bob K6UJ <k6uj@pacbell.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Change in Frequency As Antenna Height
Rises
You guys are way above my head :-)
I had no idea the resonant freq goes up and down as the antenna travels
on its way up.
## it only goes up in freq, as height increases.
Actually, no..
It goes up a bunch as you get away from "right next" (<1/10th
wavelength) to the ground, then it sort of oscillates around the "free
space" value until you are a "long way" (many wavelengths) from the ground.
So there are definitely places where raising the antenna a bit will
lower the resonant frequency and places where it will raise the resonant
frequency.
I thought it just went up on a steep curve till about 15 feet then
gradually after that.
## That’s exactly what happens. But beware, 20 ft above the ground doesn’t
mean 5 ft above a 15 ft high garage roof.
I can see why some head scratching as we pass thru the "decrease zone"
"what did I do now" hihi
## forget the ....’decrease zone’...it doesn’t decrease.
## and the simple full size dipole is not shifting 217 khz higher in
freq....that’s pure fantasy.
## ask anybody who has a motorized crankup, like the 89 ft or 106 ft
variety, how much the
resonant freq of a rotary dipole, full size, or loaded, or 2/3 el yagi (
this is all on 40m)
increases as tower is raised from min ...to max.
on 40 meters, a wavelength is 120 ft. To see the effect you need to go
from, say, 1/4 wavelength from ground to 2 wavelengths, and you get a
couple cycles of the wave.
you won't see that much change going from 10 to 30 meters from 1/4 to
3/4 wavelength.
you might see that much going from 3 ft to 30 ft.
I don't have time right now to run a model to find resonant frequency,
but I did calculate feedpoint Z at 1 meter steps for a 40m dipole
(dipole was 20m long) over moderate ground at 7.15 MHz
if the X component is positive that means that the resonant frequency is
LOWER than 7.15, and if X is negative, then the resonant frequency is
higher (e.g. short antennas are capacitive, long antennas are inductive).
From 1 to 18 meters the X component steadily drops (from 37 ohms at 1
meter to -19.4 at 18 m), then it heads up until local maximum of -1.4 at
29 meters, then back down negative again -14.2 at 39 meters above the
ground, then back up to a max of -4.5 at 50meters, then back down to -12
at 60m.
Neg peak at 18m (about 1/2 wavelength)
Pos peak at 29m (about 3/4 wavelength)
Neg peak at 39m (1 wavelength)
pos peak at 50m (1 1/4 wavelength)...
etc.
Running just a few heights, finding the resonant length, and then using
that to estimate fres
Compared to 7.15 MHz in free space 10.06m
1m 6.84 9.63m
10m 7.08 9.96
20m 7.21 10.15
30m 7.07 9.95
40m 7.19 10.11
So it looks like you get a fluctuation in resonant frequency, once you
get away from the ground of about 100-150 kHz, but that's over a pretty
big range of heights.
"resonant length" we're looking at a variation of 20cm (8") out of 33 feet
Given that the 1.5:1 VSWR bandwidth (remembering that at best, it's
1.3:1) is 7.15 to 7.35 (19.96 m dipole at 20 meter height above
"moderate" ground), I'm not sure it's real significant..
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