On Feb 6, 3:56 pm, K7ITM wrote:
Dave

I really like the coaxial collinear design for relatively narrowband
work. The coaxial connecting stubs (whose outside surfaces are also
the radiating elements) keep the phasing locked down tightly.
Unfortunately for your application, that very advantage for narrowband
designs is a killer for broadband. That is, the pattern will change
from a "flat pancake" at the nominal design center frequency to a cone
up or down, above or below the design center frequency.

You can mitigate that to some extent by feeding the coaxial collinear
antenna in the center (with the feedline balanced and perpendicular to
the antenna axis for some distance) instead of at an end; in that
case, you can think of the pattern as a cone going one way for the
section above the feedpoint, and by symmetry, a cone going the
opposite direction for the section below, and the sum of the two
results in just a lowering of the gain--not so flat a pancake--when
operating off the design center frequency.

But a better way to do a broadband vertical collinear is to feed
several dipoles, stacked end-to-end (with some gap from one to the
next), each fed with the same electrical length of feedline, with the
far ends of all the feedlines paralleled. If the gap from one dipole
to the next is enough that the mutual impedances among the dipoles are
all small, then each dipole will have current very nearly in phase
with the others and the radiation pattern will be perpendicular to the
axis of the dipoles. It's a messier feed arrangement, but it's much
better for keeping the antenna currents in phase along the whole
antenna across a relatively wide frequency range.

Cheers,
Tom


OK, thanks... The stacked dipoles was actually my first idea when I
started thinking about it a while back. But then I wondered if the
collinear would work. I guess there's a reason why nobody has done
that before. I do believe I've seen some fat stacked dipoles before
though.

Dave