Now on my 4th rebuild of the 80 meter 4SQR aluminum 1/4 vertical array, I
have come across a very interesting means to increase the operating bandwidth
without major adjustments to the vertical antennas. I discovered this by
accident:
In my first design, each vertical was 44 feet tall with 19 foot top hats. My
usable bandwidth was only around 250KHz (usable bandwidth meaning that
bandwidth in which less then 10% of power is dissapaited into the power dump
port that bleeds off unused power due to higher SWR readings...at least this
is how an array using a comtek hybred coupler works...for more information on
this, look in TT August 99, TT Feb 2000, or Sept 2000 CQ Contest Magazine or
visit comteksystems.com) and my minimum power dump into the dummy load was
never below 6-7%.
My bandwidth was improved by going to longer elements with shorter top hats.
The 52 verticals with shorter tophats came down in an ice storm but what an
array......my minimum power dissipated into the dummy load was less then 2%
and it had over 400KHz of bandwidth.
After a rare September ice storm last year, I redid this array and went to
47 foot verticals this February but they too died in an April blizzard. (Only
one antenna remained standing after 2 inch ice and 80MPH winds took the other
three down) Bandwidth on this array was almost 400KHz and minimum power
dumped into the dummy load was less then 4%.
So back to the drawing board I went this summer and decided to just go with
the 44 foot verticals. I knew my bandwidth would suffer but at least these
proved to be survivors in Wyoming winters.
I got lazy in redoing this array. I was having to piece together elements
from some replacement aluminum from TexasTowers and still had one of the 47
foot verticals up in the air. So unlike the other arrays that each had
almost perfectly matched verticals, each of these were a bit different. One
tested at resonance at 3.600MHz, 2 were at 3.550MHz and the 4th, the
surviving 47 footer, was resonant at 3.500MHz.
So when I went to test the array yesterday, I was amazed to see it had 400KHz
of usable bandwidth, not the 250KHz I was expecting, and the minimum power
dump was at 4%, not the 6-10% I was expecting in using mismatched verticals.
The F/B was still 20-25dB.
So what does this prove? I'm not sure..all test equipment was the same on
all 4 arrays. My guess is that using the two verticals at the high and low
range of 3.600 down 3.500MHz, with the other two at the mid point of
3.550MHz, broadened the array, sort of like a log periodic reaction, without
seeing a noticeable decline in F/B.
If I had the right test equipment, and had a perfectly match 4SQR next to
this one, maybe I might notice better f/b on the perfect array even though I
know the usable bandwidth would be less and the minimum power dump
higher...but from a practical standpoint, I'm leaving this array alone-
Some other ideas I'd suggest in building an array from aluminum tubing:
If you live in an ice area, avoid parachute cord. While super strong, this
stuff soaks up water and can create ice sculptures in the wind. It also
stretches a lot compared to bailing twine....I'm now using orange bailing
twine that some have claimed rot's away after a couple of years. But here in
dry SE Wyoming, it has supported a lot of wire antennas I have with no
problem for the past three years. And while it builds up ice, it only builds
up about 1/2 of what the parachute cord has done in past storms.
Double and triple wall your verticals where you can. My larger verticals had
NCF (narrow channel failures) in-between the low and middle guys...the weight
of ice in blizzard winds caused the aluminum to buckle just at the point it
was not double walled...figures eh?
While from a current standpoint, making each vertical in the array as
identical as possible is a good idea, you might consider making the array
slightly more broad banded by centering two verticals and having each of the
other two tuned a bit above and below this center point. I never ever heard
about this effect but if anyone knows of this being done with other array's,
I like to learn about it-
Of note, each of my four arrays most resonant point, as measured by where the
minimum power was dissapaited into the dummy load port, jump around 170KHz
+-5KHz, from where each of the antennas were tuned and/or where their
collective SWR averaged before putting the array together.
73 Paul N0AH
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