[TowerTalk] Stub tuning

[Jim Lux]

At 06:45 PM 9/12/2005, Tom Rauch wrote:
> > Some are open and some are shorted.  They are bandpass
>stubs, as per K1TTT's
> > site, for a multi-transmitter setup I'm trying to get
>going.
>
>The optimum distance from the transmitter (amplifier) varies
>with the type of output network of the PA stage. Networks
>used in transmitters have greater or lesser (largely 2nd
>harmonic) suppression depending on the impedance presented
>to the PA at the harmonic by the stub system.
>
>The location of properly tuned resonant stubs in the
>transmission line significantly affects attenuation level of
>harmonics, even though it does not significantly affect pass
>SWR on the desired band.
>
>73 Tom

For those interested in a theoretical analysis of stubs as filters, I point 
you to:

B.A. Schiffman, G.L. Matthaei, "Exact Design of Band-Stop Microwave 
Filters", IEEE Transactions on Microwave Theory and Techniques, January 
1964, pp 6-15

You can probably get it at almost any college library, or request it on 
interlibrary loan.
They go through the whole cookbook procedure of turning standard filter 
design tables (Butterworth, Chebyshev, etc.) into transmission line stubs 
and sections.  While they're talking microwave filters, the equations work 
for anything.  A fine example of useful, practical design articles in a 
professional journal that's all too infrequent these days.  They also cover 
coupled parallel resonator filters (you  might have seen those in those 
"no-tune transverter" designs).

And, a decent modeling program like XLZIZL can do wonders for getting a 
decent conceptual understanding of what's going on.  SPICE (in it's many 
versions) can also do this kind of thing, but it's kind of clunky for RF 
applications.

Jim, W6RMK