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From: firstname.lastname@example.org (Dave Platt)
Subject: Re: Replacing two microstrip stubs with one
Date: Fri, 15 Nov 2002 20:13:21 -0000
Organization: Home for Obscure Mythical Beings
X-Newsreader: trn 4.0-test74 (May 26, 2000)
Originator: email@example.com (Dave Platt)
>I have a microstrip transmission line that has two short circuited stubs
>connected to it at the same point which radiate off in opposite
>directions to each other so that the interconnection point is a cross
>junction. Each stub has a different characteristic impedance and
>electrical length. I want to replace these two stubs with a single short
>circuited stub. How do I find the correct characteristic impedance and
>electrical length of the replacement stub ? Does a formula exist to
>calculate this ?
[Tried to email this, but your posting address refers to a domain
which appears not to exist]
My guess is that you aren't going to be able to find a single stub
which will have the desired effect.
Based on your description, I have a feeling that these stubs may be
acting as a bandpass filter. If one stub were shorter than a quarter-
wavelength (at some frequency of interest), and the other were longer
than a quarter-wavelength, then you'd have the equivalent of an LC
series resonant circuit inserted in the transmission line. This
circuit would have a very low impedance at the frequency of interest
and would pass the signal... at lower and higher frequencies, one stub
or the other would have a high (and reactive) impedance and would
block the signal. The stub lengths might also be tuned in order to
provide some amount of impedance matching for the final load.
If, in fact, this is what the original designer was doing, then you
won't be able to replace the two stubs with a single one, as a single
stub will give you either capacitive or inductive reactance, but not
both working in concert.
From what I've read recently, in my studying re amateur radio, the
traditional tool for working out stub matches and filters of this sort
is the classic Smith chart. Smith's book on the chart has sample
charts (with the standard layout) on translucent plastic, and
"overlays" in the book which are marked out for things like stub
matching. The overlays are designed for single-stub matches, though.
To do a double-stub analysis or synthesis you'd probably need to use
the standard chart, and draw out the impedance transformations
Maxwell's book on transmission lines ("Reflections II") is also very
valuable reading - he goes into series and shunt stub matching in a
good bit of detail.
There is a commercial program Serenade which does transmission-line
and stub matching... it can calculate, plot, and do some automatic
synthesis of such matching networks and stub filters. It's available
in a "student" version, which is freely downloadable... this version
is limited in the complexity of the circuit it can represent and has
some of the advanced functionality disabled. Our local ham club's
resident expert on T-lines recommends it for anyone who is playing
around with this sort of design work... he recently demonstrated its
use in designing a coaxial transmission-line filter which keeps a
nearby 152 MHz paging transmitter from overloading and intermodding
the front end of a 2-meter amateur transceiver.
See www.ansoft.com to download a copy. At the least, you could use it
to model your current stub arrangement and see what sort of behavior
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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