I spent yesterday loading NE7D's tower on 160 meters and thought I
would share the results here as a data point.
His tower is about 75 feet of Rohn-25 with a SteppIR DB18E on top.
You can see a picture of it here:
http://www.kkn.net/~tree/misc_pictures/NE7D-DB18E.jpg
Rocky put down about 15 radials and I spent the afternoon trying
different gamma match attach points. We had a 600 pf variable cap at
the base. My initial attach point was around 55 feet - which in the
end - might have been just fine - but was not producing any good
results initially. I moved it down to around 45 feet and still had no
luck.
My next step was to put a loading wire on top of the tower. I made
one about 55 feet long and this did change things. I did find I had a
very nice 80 meter antenna now - being able to move the resonant point
most anywhere on 80 I wanted. I then increased the length of the
loading wire to about 65 feet and put a second one up going the
opposite direction. I also put a gamma attach point up around 65 feet
and tried that. The results were better - as I was able to get a good
match around 2.1 MHz - but not lower. I decided to increase the
loading wires out another 12 feet or so - probably bringing them close
to 75 feet each - but things were still not very good.
Just for fun (or out of desperation) - I hooked up the 45 foot gamma
match wire again - and VOILA!! I was able to get a 1.2:1 SWR anywhere
I wanted at the bottom of the band. The 1.5:1 SWR points were about
20 or 25 kHz away from the sweet spot.
So - my conclusion - even with the SteppIR stretched out to the 20
meter position - it is not offering much in the way of top loading.
Perhaps there is a way to modify it to connect the parasitic elements
to ground to help with this - but this might be tricky as this model
allows any of the 3 elements to be driven.
Our gold standard was a wire Rocky had up in the trees that he was
using on 160 meters previously. It was actually a pretty good looking
antenna - with about 70 feet vertical and then some similar length
horizontal.. The new antenna seems to be a couple of S-units better
so far - so I think it is working as it should.
I hope this information proves to be useful for others. Keep an ear
out for an improved signal from NE7D on topband.
Tree N6TR
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