[OKDXA] Force-12 Sigma 80

[Kim Elmore]

Hi all,

I had a few requests to keep you updated on my Force-12 Sigma 80 vertical 
dipole antenna.  I received the antenna about two weeks ago, but have only 
just unpacked it.  It is made of very stout materials. The overall length 
of the antenna is 36 ft and it uses an off-center feed, placing the feed 
point about 8 ft off the ground.  A bead balun (sold separately) is used at 
the feed point. The vertical part of the antenna is made of 1 piece of 
2.25" diameter aluminum tubing and 5 pieces of 2 in diameter aluminum 
tubing, so it's quite heavy.  The antenna uses large (about 6 inch 
diameter) coils of 5/16" aluminum tubing (about a dozen turns each) as 
inductive loading at the center, and T-bars for capacitive end loading at 
the top and bottom.  The lower T-bar is quite a bit heftier than the upper 
T-bar, under the assumption that it's likely to see more abuse and to also 
keep the top of the antenna as light as possible.  This, along with a 
hairpin match at the center, is supposed to yield a 1:1 match at the 
resonant frequency with a 70 kHz 2:1 bandwidth.

The antenna requires guys, but nothing fancy for the base.  Force-12 
recommends some rebar driven about 12" into the ground with 4" sticking 
out.  The base of the antenna (2.5" diameter aluminum tubing) slips over 
this pin and sits on the ground.  I'll probably do something a tiny bit 
fancier, like use a couple of patio flagstones to keep the antenna off of 
the ground.  I thought that the the antenna required a single set of guys, 
but it looks like there are welded (heliarced) brackets for two sets.  The 
assembly instruction are a bit vague on this point, but moderately detailed 
on adjusting the match. The T-bars use riveted joints and the vertical part 
is held together with 1/4-20 bolts. The transmission line needs to run 
more-or-less perpendicular from the antenna for a ways: the company 
recommends anchoring the feed to a guy.

Force-12 claims 95% efficiency and given the size of the antenna, I don't 
doubt it.  It should have a very low radiation angle (the max will be at 
about 15 degrees), with a null in the vertical.  Hence, it probably won't 
be much good for short paths, but I expect it will *rock* for DX.  I have 
no idea how noisy it will be where I am: I'm in a rural suburban setting -- 
there are only 3 or four houses within a couple of hundred yards of the 
antenna. Things aren't too bad with my half sloper  right now. However, 
given the expected poor performance on high-angle signals, I expect that 
the stateside QRM should be noticeably reduced. We'll see...

73,

Kim Elmore, N5OP