Quarter wave stub
Lance Collister, W7GJ
Just using an open quarter wave stub worked great with my tube amps, but that was
probably because I could adjust the tuning and loading to match into it. I hooked up
a simple open quarter wave stub to the output of one of the Harris amps, and it shut
down with a high SWR error code at about 600w output with about 2.5:1 SWR. I think
to use such a filter with these solid state amps you would have to go through the
complete impedance matching process by adding additional stubs for various harmonics
at a specific electrical length between the stubs.
G4SWX offered the following solution, but I have not tried it here (yet). The basic
idea behind John's Coaxial Stub Filters is to keep the impedance (at the pass
frequency) of the elements at the output port of the filter the same as at the input
- hence you can match the whole filter by transforming the input impedance to its
conjugate at the output. The simplest filter of this type uses a quarter wave open
circuit @2f on the input and open circuit quarter waves @3f and 4f on the output.
This is a 'magic' combination because @f the impedance of the 2f stub is the same as
the parallel combination of the 3f and 4f stubs. All that is needed is to transform
the impedance at one end into its conjugate at the other.
John suggests that the easiest way to do this at high power is to use a coax line
rather than an inductor to do this transformation. To construct a high power filter
using the above coaxial elements, John recommends a quarter wave open circuit
@100MHz on the input and quarter waves open circuit @150MHz and 200MHz on the output.
Join the input and output sections of the filter together with a low loss coaxial
line (0.265 x velocity factor) wavelengths long. This will give you >20dB return
loss ( <1.2:1 vswr) at the pass frequency.
VY 73, Lance
--
Lance Collister, W7GJ
(ex WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8, E51SIX, 3D2LR, 5W0GJ)
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