Force 12 C3 Triband Yagi
The C3 is a 2L reflector array on 20 and 15M.
Gain peaks at 0.15 WL (wavelength) falling off slightly
0.1 and 0.2 WL. This broad peak is why trapped 2L
Yagi's and 2L Quads often use a common spacing of
8 ft (.17 WL on 15M). Feed impedance rises with spacing.
Gain falls (slowly) with increasing frequency in the useful
range of reflector type 2L Yagi's. Personally, I would
emulate the C3S which uses reflectors on all 3 bands
and only requires a 12 ft boom. To my mind, this
produces the maximum gain per foot of boom length.
Reflector 2L arrays offer wide useful bandwidth and
a wide range of non-critical spacings and dimensions.
On 10M, the C3 uses a director. Peak gain for a
2L director array occurs at 0.08 WL and falls rapidly
at wider (or narrower) spacings. Gain peaks at
the HIGH end of the useful range and falls rapidly
on either side. The best thing to do with a C3 is
add a 10M reflector, making it a 3L Yagi on 10M (C3E).
This will improve the gain bandwidth of the antenna.
The C3 uses dual driven elements on 10M to
achieve wider SWR bandwidth. I'd be curious
to know the gain vs frequency response of the
C3 on 10M.
Director arrays are much more sensitive to both
dimensional and frequency variations. The only
real advantage offered by 2L director Yagi's is they can
be made smaller for narrow frequency ranges on 80M.
Otherwise, 2L reflector arrays are a better choice.
de Tom N4KG
On Sun, 12 Dec 1999 "Edward L. Bruns" <email@example.com> writes:
> Can some one give me the element spacing
> for the Force 12 C3? I have a pile of tubing, but
> as a retiree, little cash... think I can build one.
> (after I model it )
> Ed, W3EKT.
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