Well, hope I can get something in the air for this coming weekend. It
will likely be the antenna described below, but I may go with a V with
the top at 100'.
Thought the info might be usefull as I have seen a couple of posts
from guys looking for capacitors for inverted Ls.
OK, I come off the top of my crank up ( 90', but with 10 feet of
Molydenum mast out the top. Go out a little ways from the tower and
here is the setup.
It's an inverted L. Dimensions are 82 feet up and 86' out on the top.
ZO of the antenna with these dimentions is 49.7 +j276, and of course
that makes the feed inductive. However, inserting a series capacitance
of -276 ohms cancels it entirely and we wind up with a feed point
impedence of 49.7 -j0. Lowest SWR, and the antenna is cut for 1.840,
is 1.06:1 with these values.
Feed point is one side to the ground rod and the other to the vertical
part through a series capacitor with a reactance of 276 ohms, and of
course that is - j276.
With the dimentions shown, the take off angle is 27 degrees, and the
elevation beamwidth is about 45 degrees, so useful radiation at very
low angles is also present.
Gain of the thing is 1.49 dbi.
Now, for the capacitors, RG8 has a breakdown voltage around 4000 if I
remember correctly
and a capacitance of about 30 pf per foot. So, all I'll do is figure
out how much
I need for a -276 ohms, and the feed will be to the shield with the
center conductor going to the vertical part. Check any source for
exact figures.
Bandwith of the antenna is 1.95:1 at 1.8 and 2:1 at 1.89, which gives
me a pretty good spread.
Hope this helps a little
73
Ed
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