Fellow Tower Talkers:
To make sure the development record is straight, the comment from Mr. Cox
relating to the basic design of the C-3 open sleeve system is not quite
The original open sleeve was developed in about 1945, maybe 1946 by Dr. J.T.
Bolljahn. It was both a broadbanding technique and also a two-band feed system
that was used as a two-band dipole.
In 1982, Mr. Cox added traps to the center driver of a basic open sleeve to
achieve excitation on 3 bands. The center driver operates on 20 and 15 mtrs and
the adjacent elements operate on 10 mtrs. This trapped 3-band system was
used in the Explorer 14.
The Force 12 enhanced open sleeve system does not use traps to achieve more
than two band operation. We went back to the efficient, original open sleeve
system and improved the invention to excite more than two bands (without
traps). This covered about 2 1/2 years of effort. During that time, the
two-band system was utilized on a wide variety of antennas.
Some of these duo-band antennas were stacked horizontally on the same boom
to achieve 4 band operation with two feedlines. Side benefits from this
configuration are that the rear antenna (lower in frequency) is enhanced
slightly from the presence of all the (higher frequency) elements ahead of
it. The main 4 bander is the Force 12 4BA, which is a 24' boom and covers
17-15-12-10 with true monoband antennas (and gain). We aded a 3 element 20 mtr
Yagi behind it and made the 5BA, So, on a 32' boom, one has all 5 bands.
One of the duo-banders that was unique was the N1217, first produced in 1992.
was originally on a about a 15' boom and is actually a pair of 3 element Yagis,
one for each band. They overlay each other, with the 3 element 12 mtr Yagi
inside the 3 element 17 mtr Yagi. It has a 3 element open sleeve (two-band)
driver in the center. The unique aspect of the N1217 is that normally the
director(s) of the lower frequency antenna cannot be placed in front of the
common driver. In this case, however, they co-exist just fine. Another
for being able to have director(s) for the lower frequency band out in front is
to also "sleeve" them. For example, a 17 mtr director could be "sleeved" with a
pair of 12 mtr directors, which would render the 17 mtr director "invisible" to
the 12 mtr director portion of the Yagi. We have done this on several test
antennas for vaious band combinations. To conclude the N1217 development story,
the boom was expanded to 18' in 1994. This was done primarily to improve the
natural balance point, as there was an internal counter weight at one end of
boom and by expanding the boom and re-arranging the elements, the balance was
Force 12 continues to work on more designs using the efficient open sleeve.
If you have any favorite combinations, let us know!! They are probably already
73, Tom, N6BT
Force 12 Antennas and Systems
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