[TowerTalk] Stub tuning

[Tom Rauch]

> 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

The whole thing in a nutshell is this:

You want to space the stubs so low shunt impedances at the
undesired frequencies are reflected as very high load
impedances to the next low shunt impedance along the line as
you move towards the source.

That means you need 1/4 wl spacing at the second harmonic
between stubs at the second harmonic when you use shunt
shorted stubs.

If the PA has a low output Z at the output port, like a pi
does, you want the first shunt stub 1/4 wl (at the harmonic)
away from the amp. This will actually increase the tank
circuits harmonic suppression.

If the PA has a high output Z at the harmonic (like a pi L
might) you want the stub close to the PA, the optimum
distance depending a bit on the PA output reactance at the
harmonic frequency. Pi-L's have better harmonic suppression
when the output is a very low impedance at the harmonic.

73 Tom