Both the TB5EM and its predecessor, the TC-99C, had
18 ft booms--and they were 2 inch dia with 2.5 inch dia
element reinforcements. From the prints I have it is very
difficult to tell what the differences between the two models
There was also a "Dew Line Special" Model XCTBS-101520
that had a 3.5 inch boom center and 3 inch tips. BUT IT
WAS ONLY a 4 element, somewhat-curios, design with a
16 ft boom. The curios part is that all three elements were
tri-band trapped in the classical 20/15/10 reflector, driven
element, director configuration--but there was also a second,
separate,10 meter reflector. At least according to the old
catalog that I have from 1977. This catalog is geared towards
the military and commercial customers rather than the ham
I really don't have any more information--but wonder if
the antenna you have is somehow a mil-spec version of
the TB5EM with the 3.0/3.5 inch boom--but wonder why
it is only 17 ft. (How accurate are your +/- dimensions??)
rather than either 18 ft or 16 ft.
BTW The center sections of the elements of the Dew Line
Special were 2-5/16" diameter whereas the center sections
of the regular ham antennas such as the TB5EM were 1-1/8"
Not sure if this helps or adds to the mystery.
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