Doug and the rest of the Top Band gang,
Here is what I have observed on a 40 foot tower with half slopers, and I look
at the half sloper mounted a small percentage of a wavelength from ground in
a much different way then probably anyone else you will talk with.
I made antenna input impedance measurements at the following mounting heights
for both my 160 meter and 80 meter half slopers. The 160 meter antenna was
68 feet long with a coil at the feedpoint to make it look like an electrical
1/4 length element. The 80 meter half sloper was a full 1/4 wavelength long
(no inductive loading).
Antenna input resistance at resonance (no reactance).
Mounting height 160 mtr ant. 80 mtr ant.
35 feet 68 ohms 179 ohms
25 feet 49 ohms 73 ohms
15 feet 40 ohms 39 ohms
1 foot 42 ohms 32 ohms
(At 1 foot the ant. is like a ground mounted vertical but laying almost
horizontal to the ground)
(All impedance measurements were made at the end of the feedline, and then
transformed to the antenna input so the actual antenna input impedance could
be plotted)
You might wonder what in the heck my antenna visually looked like at low
mounting heights. I kept the far end of the antenna about 5 feet off the
ground as I lowed the feedpoint. Not a great set up, but consistent.
I wound up getting close to what I predicted using a theory I had regarding
the distance of the feedpoint from the bottom of the tower (ground). During
my college days in my Transmission And Propagation Of Energy class we always
had a rough rule for deciding when we needed to apply traveling wave theory
in place of classical AC circuit analysis (it was just a general rule, but it
made us think about the importance of wavelength to how we approached
problems). A general rule was if we were dealing with a distance around 10%
or more of a wavelength then traveling wave theory was in order if not before
then.
I had always had trouble getting my 80 meter half sloper to work without
having a high SWR. On the other hand my 160 meter half sloper had a very
good SWR (not that a low SWR is always a good thing). I then wondered how
far up the tower do I have to go before it does not look like I am connected
to ground with the shield side of my feedline (the 10 percent rule concept)?
Turns out the 160 meter antenna at 35 feet is only 6% of a wavelength from
the base of the tower / ground, while the 80 meter antenna at 35 feet is 12%
of a wavelength from ground. I therefore decided to lower the 80 meter
antenna to a level where the feedpoint was 6% of a wavelength from ground to
see if I could get a low SWR and sure enough the antenna had a low SWR just
like the 160 meter antenna.
That is what then prompted me to actually measure both antennas at the
various tower heights to see how the antenna input impedance and resonant
frequency changed.
What I saw with my 80 meter half sloper at 35 feet explained why I could
never find resonance with my SWR meter. The SWR plot was always above 3 to 1
and not well defined. As you can see the SWR calculates out to 3.6 to 1 at
the 35 foot level for the 80 meter antenna.
Note : When I disconnected my shield at the 35 foot level from the tower the
160 meter antenna would not work (high SWR and low noise level / signals),
but if I replaced the tower with just a wire to a ground rod I had similar
results as if using the tower. (My tower had a Mosley TA33 mounted at 42
feet, and I had an 8 foot ground rod tied to the base of the tower. No
ground radials so probably lots of loss heating up the earth, but still
worked lots of DX.)
I also plotted frequency of resonance versus feedpoint height, and to my
great surprise it did not change as much as you would expect (only 50 KHZ
total for the 160 meter antenna from 1 foot all the way up to 35 foot
mounting level). Again supporting the fact that the length of tower between
the feedpoint and ground was not changing the electrical length of the system
that much.
My basic conclusion at this time is that as long as the half sloper is not
mounted very high up from ground (maybe 8% or less of a wavelength), then the
feedpoint impedance will provide a reasonable match to 50 ohm line, and no
great antenna circuit analysis should be required. I have no idea how top
loading of the tower impacts the analysis (especially on 80 meters), but I
know of another ham who erected a 40 foot pop up mast to be used only with my
160 meter reduced size half sloper and he had similar results as I did with
my 40 foot tower.
I am making no great claims here. My antennas are probably horribly
inefficient, but they have allowed me to get on the air from a city lot on
160 meters. Just thought this information might be interesting to others,
and found your information very interesting. I sure wore myself out climbing
up and down the tower. Most likely everyone reading this now thinks I am a
crack pot, so I best sit back and crawl into a deep dark hole.
73's
Don Kirk (wd8dsb)
(P.S. You can find my reduced size half sloper in March 1998 QST and also the
ARRL's wire antenna classics book if you are curious about my actual antenna.)

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