Force 12 Sigma-280 Magnum, Assembly Instructions Manual

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DW-Manual-Sigma-280 Magnum-001 FORCE 12 SIGMA-280Magnum Assembly Instructions Page 3 of 32
series at the center. The far side of the box (where the balun is attached on the driver) is jumpered across
with a short wire. The relays (i.e. relay #1 in each box) in both boxes are switched simultaneously (in
parallel).
The VSWR curves for the
Sigma-280 Magnum allow coverage of about 100 kHz at the 2:1 points. Adding
the flexibility of the relay box, this gives the antenna 4 segments of about this bandwidth. Two (2)
segments are normally on CW and two (2) are normally on phone; however, all four (4) segments can be
set for phone. As a short note, a low or high VSWR does not necessarily mean that an antenna works or
not. A fine book addressing this is Reflections, published by the A.R.R.L.
Each element is set to the correct operating frequency by adjusting the spacing between turns on the two,
large coils located at the center of the element. The hairpin match is adjusted for the best match (usually
1:1) on the driver at the desired frequency. The input impedance at the feed point is much less than 50
ohms (about 30 ohms) and is stepped up to 50 ohms by using the hairpin match mounted across the feed
point. This hairpin is in the form of a "helical hairpin", or coil, as a typical hairpin would be physically too
large. The match to 50 ohms is adjusted by making the spacing on the coil wider or narrower. Adjustment
at the low phone position is normally sufficient for all operating segments.
Please consult FORCE 12, Inc. for any questions.
Mechanical Overview
The mechanical design of the Sigma-280 Magnum is a pair of Sigma-180 magnum dipoles attached to a
mounting plate (weldment) on each end of the boom. The mountings are bolted to the boom using several
stainless bolts. The bolts are inserted from inside the boom, through the boom and through the mount. The
non-conductive element truss is attached to the element and to the center riser post on the element mount.
The element tubing segments are both bolted and riveted. The bolted joints are on the larger tubing and the
riveted joints are on the tips and the T-bars. The riveted element is a Force 12 “first” and identifies the
product. The antenna can be fully assembled, tuned at a reasonable height (20-25’ minimum) and then
mounted in place. Antennas always shift upwards in frequency as they are raised, so checking it at as it is
raised to a reasonable height will ensure the shift is known. If one thinks about this, having an 80/75 meter
Yagi at 25’ is like having a 20 meter Yagi at 6’ above the ground. The higher the antenna is raised to check
and verify the frequencies, the more optimum the settings. Remember that the reflector frequency is going
to be set only about 35 kHz lower than the driver. It is important to do this right!
The elements are insulated from the plates and the mast. Connection to each element is through a pair of
10-24 stainless machine screws.
As previously mentioned, since this is a balanced antenna and coaxial
line is unbalanced, a means to choke off antenna current from the outside of the coax feeding the
driver element should be used.
Two devices are fine: one is an RF choke, made by winding several turns
of the coax in a circle close to the feed point; or, a 1:1 balun, such as the Force 12 B-1, can be used.
Most of the hardware is stainless steel. It is type 304, not 18-8, which is only rust resistant. The plated
hardware is used for the element-to-boom bracket installation, with stainless lock washers and nuts to
enable removal. Stainless U-bolts are not necessary, except in extreme environments and a preferred
method is to paint these parts. Stainless hardware is easy to gall, meaning to freeze the nut on the shaft,
rendering the bolt useless. If all stainless is required, please contact the factory. The entire antenna can be
painted to eliminate any glint in the sun, although methods have been employed to limit glint already. For
example, the tubing is all 6061-T6, with a matte finish.