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 2 of 32
The Sigma design uses elements that are about 62% mechanically full size and are made electrically full
size with end loading and center loading. The
Sigma-280 Magnum
elements are 76’ long. The elements are
made electrically longer by adding 26’ T-bars towards each tip for end loading. This type of loading is the
most effective and coupled with the huge center loading coils, the efficiency is 96%.
Using these efficient elements results in a 2 element Yagi with forward gain at least equal to that of 3
element Yagis utilizing mid-point coil loading and a boom of >60’. Coils with a Q as high as 600 still have
loss and the mid-point placement of the coils on the element creates a current distribution that is not as
favorable as the Sigma design. Additional disadvantages of using mid-point coils are: the potential for arc-
over at the coil (it is typically wound on a PVC core); the covering can become ineffective over time (due
to sunlight and weather); and, the structure itself (the coil form) is vulnerable to fatigue from the ever-
present wind, stress and vibration. The operating bandwidth (gain, F/B and VSWR) of the
Sigma-280
Magnum is also wider than that of a coil-loaded Yagi with the same element length. This is due to the
added electrical length of the T-bars. The equivalent length of a Sigma-280 Magnum element (without the
center loading coils) is 102’.
Both elements supplied with the
Sigma-280 Magnum are built the same. During assembly, one is selected
for use as the driver (the one connected to the feed line) and the other is selected as the reflector, the
parasitic element. The reflector will be tuned slightly lower in frequency than the driver and this is
accomplished by adjusting the center loading coils to the appropriate reflector frequency. The reflector is
set first, without the driver in the circuit, to the correct frequencies for phone and CW. The driver is then set
with the reflector active. Once set, the reflector is not re-adjusted, as it is pulled in frequency by the
presence of the driver. If measured by itself, the driver would resonate higher than in the 2 element Yagi,
because it is pulled down in the presence of the reflector.
The construction of the
Sigma-280 Magnum elements uses all tubing. The coils are in series with each side
of the dipole and are enclosed with a large diameter styrene cover. The elements are also trussed using non-
conductive line for added strength.
The power limit for this antenna is in excess of 8KW, with the limiting factor being the balun used to feed
the driver. The next limiting factor is the 30 amp relay contacts in the CW/SSB switch box. To date, these
relays have been used in many applications for more than 10 years with no failures except for a lightening
strike. The feed point is matched to 50 ohms using a hairpin (coil) match.
The
Sigma-280 Magnum is a large antenna, so care should be taken during installation. The T-bars add
another dimension, so be aware of their presence. Knowing it is possible that not everyone who has
acquired the
Sigma-280 Magnum is necessarily familiar with shortened elements, it is suggested that a
good book, such as the A.R.R.L. Antenna Book be utilized for further information. Of course, one can
always proceed directly and assemble the
Sigma-280 Magnum
!
Feed System and Antenna VSWR
The
Sigma-280 Magnum feed system is straightforward. The feed point is less than 50 ohms and is stepped
up using a simple hairpin coil across the feed point. The hairpin and balun are connected to the feed point
end of the relay box and the other end of the box is connected to the feed point terminals on the driver
element on the antenna. Since this is a balanced antenna, it should be fed through either a 1:1 balun (such
as the Force 12 B-1, B-1/C), or a suitable coaxial RF choke. The reflector element has a similar relay box in