Blade type VHF antenna?

On Sat, 13 Dec 2003 10:58:17 +1100, smithxpj
wrote:

On Sun, 07 Dec 2003 11:01:29 -0800, Jim Weir wrote:

It depends ENTIRELY on how they are foreshortening the antenna. If they are
using super-wide conductors (the entire width of the blade) to get the
foreshortening, then it is a very efficient antenna. If they are using loading
coils inside with a relatively narrow antenna diameter, it is a very inefficient
antenna.

The only way to know for sure is to ask the manufacturer for a data/spec sheet
or cut the antenna open.

I tried that stunt back in the early 80's out of interest.

The couple of manufacturers that I asked were very coy about parting
with detailed information about what was inside their antennas. It was
very generic stuff like..."a printed circuit etched radiator with
double sided etched capacitors and inductors appropriately tapped to
the radiator to achieve optimum matching" !!!

I once managed to get access to an industrial X-ray machine and got a
picture of the innards of a Collins military VHF-UHF blade antenna.
Eye opening stuff, indeed...but trying to reproduce it? It would
hardly be worth the manhour effort compared with knocking up a 26"
piece of 1/8" brazing rod into a 1/4 wave radiator.

If it's a blade type, why not put in a flat piece of brass or copper
stock and trim for resonance, or use a matching network?
I'd think something that large (length to width) wouldn't take much
loading. OTOH I guess linear loading could be used. Double sided
circuit board with the back a thin ground....maybe widening out to
provide more capacitance near the end... It'd take some
experimenting, but I'd think it'd work.

I know...Lots of weasel words, but I haven't actually tried it so it's
more thinking out loud.


Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair?)
www.rogerhalstead.com
Return address modified due to dumb virus checkers

Jim




Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com