gregB wrote:
Roy, thanks for the reply.
So how do they do it?
Greg.

Making a decent antenna that works over the very wide bandwidth of the
TV UHF band isn't at all trivial. And there's a great big handful of
tradeoffs -- gain, front/back ratio, impedance match, physical size,
number of elements, sidelobe levels, and bandwidth among them. The UHF
TV antennas I've seen are mostly a combination of log periodic, Yagi,
and corner reflector types -- something like a few log periodic elements
with some parasitic (Yagi) elements and a corner reflector. There *are*
some formulas which can be used for pure log periodics and corner
reflectors -- you should be able to find them on the web, and a
serviceable if not optimum antenna could be made with either or both
techniques. You'll have to decide how to best make the tradeoffs for
your application. The main difficulty with a corner reflector is making
a very broadband driven element.

I imagine that beyond designing a basic log periodic or corner reflector
structure, most manufacturers do a bunch of cut and try. These days, I'm
sure they use modeling programs for it. I'm also sure that after some
years of designing the things, the engineers get a feel for the general
effect of certain techniques -- say, how to improve the front/back ratio
with the minimal number of added elements. Some of the bigger companies
might have developed some optimization software. If they have, it's
probably kept as a trade secret.

Roy Lewallen, W7EL