In December 1996 QEX I published a paper, and gave a simple DOS or unix
program, designed primarily for finding the impedance of anode lines,
taking into account the presence of the anode cavity - ie it is more
accurate than the usual microstrip formula, which assumes an isolated
microstrip. The anode enclosure will always lower the true impedance
from that you will get from the usual formula for a microstrip.
I've now covered the program to a Windows 95/NT based one, adding a
graphical user interface. It now works with two different dielectrics,
so can be used for finding the impedance of other lines too - ie PCB
microstrips etc. The picture in the code should make clearer what it
does, than I can explain easily. If anyone wants a copy, please ftp to
the address at the bottom of the email. Or I could attach to an email.
The program is based on the finite difference technique, which uses a
square grid. One needs to tell the program how grid squares to use for
all the dimensions of the striplines, anode cavity cross section, height
of stripline, thickness of stripline etc. The default dimensions (cavity
width (W) = 160 units, cavity height (H) = 60 units etc etc etc ) were
set up to model the K2RIW amplifier assembly, which uses a box 8" x 3".
Each grid square was chosen to represent 0.05" Hence W=8/0.05=160,
H=3/0.05=60 etc etc. The dimensions of the second dielectric are
immaterial if it has the same permittivity as the first. The parameters
'a' and 'd', which set the edges of the second dielectric, are chosen
pretty much randomly, but would not be if there was a second dielectic.
I would be interested in any feedback.
I should add that while the data has been carefully checked for air
dielectric, with the stripline placed centrally, I have not performed
such an analysis for multiple dielectrics, or with the stripline not in
the centre of the cavity. To the best of my knowledge, no theoretical
data exists, and I don't have access to suitable test equipment (vector
network analyser) to verify the results.
Dave Kirkby, G8WRB.
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