Well, as usual, a number of opinions are offered/stated/given as
fact/experienced in TT and direct email messages I've read so far. Most
agree that, at least for wavelengths below at least 1/2, the effect will
probably be a decrease.
Some feel it could vary either way depending on conditions, but generally .
. . .
I don't have EZNEC nor have I ever done any programmed antenna modeling. At
this point in my 60 years of hamming and 80 years of life I'm just trying to
stay on the air amid all the things my wife has in that 50-gal job drum to
do, but do appreciate all the input.
My situation is I have an old TH7DXS on a 70 foot crankup. When rarely
necessary I've worked on the antenna with the tower was fully lowered (or
course). However, my doc is not real happy about my hanging around on the
top of any tower, even with the proper harness and equipment (son was an
expert rock climber so learned a lot and have a lot from him). I acquired a
layover fixture a few years back which I've now used for the first time. Now
I can get to the antenna with a ladder (it is easier!). Doc didn't say
anything about ladders! With the layover my TH7 15/20 M reflector is about a
foot from the ground (no beam tiltover saddle, out of my budget) so the
antenna is pointed well skyward. The SWR low point for 10 and 15 in the
latter configuration indicates the antenna, based on TT input, when raised,
would probably be at a slightly lower freq than my antenna analyzer is
showing at the lowest SWR. That's good!
Unfortunately, on 20 the SWR low point for any freq in the band is 3:1
(ugly, obviously I have a 20M trap problem to look for, assume it is on the
primary DE).
So, now can someone remind me of solutions to help loosen element tubing
should I run into some frozen condx (a tubing to tubing problem). This
antenna was bouht used and put together 'inland' by someoen else where there
is no salt air. It has performed well for 13 years (this is my second TH7, I
had one a my Portland home but this one was put it up before I tore the one
in Portland down).
Appreciate the responses, all of you are a big help. That is what is
challenging about amateur radio operations. So much compromise resulting in
and contributing to what we use for antennas/where we live etc.
Again thanks.
Don W7WLL
-----Original Message-----
From: Jim Brown
Sent: Thursday, May 26, 2016 8:37 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Change in Frequency with Height above Ground
On Thu,5/26/2016 8:24 PM, Don W7WLL wrote:
Lowering the height of an antenna above ground, such as a beam, will cause
the resonant freq to decrease, increase?
Decrease. But not always -- I have a 10M beam at about 15 ft that goes
up! The variable here is mutual coupling between elements and between
elements and ground.
Another error in using SWR as the indicator of resonance is that
feedpoint Z changes with height, and in a very complicated way (and
because of mutual coupling to ground). In general, low dipoles (as a
fraction of a wavelength) are lower Z, high dipoles are higher. AND --
feedpoint Z is also affected by the quality of the ground.
SO -- we may THINK the resonance is affected by the height as indicated
by the low value for SWR, but it's actually the feedpoint Z changing!
73, Jim K9YC
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