"Fan" capacitors for microwave RF design
 

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

"Fan" capacitors for microwave RF design
 

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/

"Fan" capacitors for microwave RF design
 

Joel:

Don't think of these things as capacitors. They are really a version
of an open-ended quarter-wave transmission line stub. The transmission
line section (which has varying impedance along its length due to the
varying width) transforms the open circuit at the big end to an
effective short at the small end.

I believe (though I am not sure) that this shape produces a wider
bandwidth short circuit than does a more traditional constant-width
quarter wave microstrip stub with the same line width as the small end
of the quarter-circle.

A search on "radial stub" is likely to turn up more info (though I
haven't tried it).

73,
Steve VE3SMA

"Fan" capacitors for microwave RF design
 

Hi Steve,

wrote in message
oups.com...
Don't think of these things as capacitors. They are really a version
of an open-ended quarter-wave transmission line stub.

Ah, thanks, I didn't realize people really were treating them as being
distributed instead of lump! (Since using copper pours as lumped capacitors
is certainly reasonable at lower frequencies.)

A search on "radial stub" is likely to turn up more info (though I
haven't tried it).

The first hit is pretty good (now that you're given me the correct search
term) -- the Microwaves 101 web site,
http://www.microwaves101.com/encyclo...uarterwave.cfm.

---Joel

"Fan" capacitors for microwave RF design
 

Call him. The phone number is right there. There are most likely 10GHz
bypasses. It's hard to tell from the pix. They are all in the center of
another line going to two places and all the same size.

73, Steve, K9DCI


"Joel Kolstad" wrote in message
...
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

"Fan" capacitors for microwave RF design
 

"Steve Nosko" wrote in message
...
Call him. The phone number is right there.

I kinda prefer the approach on Usenet here where's it's completely voluntary
for people to read your question and provide answers (and there have been a
couple good ones).

I met Steve (of Down East Microwave) a couple years ago at Microwave Update in
Seattle... great guy!