Joel Kolstad wrote:

I'm curious... at microwave frequencies often the required capacitance for
some matching or tuning network gets down into the "picofaradish" range and
it's reasonable to use copper pours to provide that capacitance. Most often
these copper pours are in the shape of a fan (see, e.g., the photo in the
first page of http://www.downeastmicrowave.com/PDF/PDF10368.pdf -- that guy
makes nice stuff!), that is, a quarter-circle. Why that particular shape? My
own thoughts a

-- Well, I suppose that where you branch off of a microstrip you'd certainly
want a nice gradual transition in pour width, so that leaves you with at least
a "triangularish" shape
-- A "boxier" shape (either a rectangle or a triangle) probably makes a better
patch antenna than something with more curved edges?
-- To cover a given area (i.e., to obtain a specified capacitance), a circle
of course gives the smallest circumference, hence any resonant (antenna) modes
that are created have as high of a frequency as possible (e.g., a circle is
the best shape to use to make a copper pour electrically appear as "lumped" as
possible at a given frequency)

Opinions?

Thanks(ish),
---Joel Kolstad


I suspect, without knowing, that the 'fan' is because the wave
propagates from the entry to the capacitor at a constant rate, so you
want to have a constant-radius end for it to reflect from -- otherwise
you won't get a clean reflection.

Someone will tell me how I'm wrong, but in the mean time I'm going to go
drip my tea off my spoon into the exact center of the cup & watch the
pretty patterns.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/