I'm taking a new look at the old G3JVL iris coupled
filters in waveguide as used in his transverter. I
have all the original design software, and a copy of MYJ
where the design theory originally came from. But I
want to do it with a difference; in that the original
used WG in and out, but I want SMA in/out
So, for a three cavity design, originally we had four
irises; two outer ones with large holes setting the Q
or input/output transformation and two inner small
irises defining the coupling, or K values. I want to
replace the outer irises by coax probes, so only two
iris plates have to be installed, and the rest is
drilling and tapping (about the only sort of metalwork
I'm happy doing !)
So, how to design the probes for adjustable coupling.
Coax to WG transitions are a complete field in their
own right, and there is no one correct answer; there are
more variables than there are needed to do a complex
match, so all sorts of responses can be produced.
However, for a filter, particularly a narrowband one,
a reduced coupling is going to be needed over that for
a transition, so a shorter probe nearer to the
backplane. Which where it gets complicated if I don't
want to make false starts in cutting metal and have to
throw away bits of prepared and soldered guide.
IF I insert an SMA probe into the broad face of WG, I
get a simple transition. Now, the $24M question: If I
place a tuning screw on the opposite face of the WG so
it would meet the probe, does anyone know how much it
changes the coupling, or, indeed, in which direction.
Ie could I use a very short probe at a decent spacing
from the backplane (say 20% of a wavelength) and rely on
adjustment screws to get it right
Is there a better place for a coupling adjustment
Or if I use an offset probe, not on the centreline of
the broad face that will have reduced coupling, where
then is the best place for a tuning screw ?
Anyone with experience in this area?
The G3JVL transverter RF input filter, designed for
60MHz bandwidth, was originally targeted with a 28MHz IF
in mind. It would be nice to be able to do a 10GHz
system that could feed directly into a high performance
SDR radio without having to go via 144/43/1296 etc.
Even a 50MHz IF would suit many modern SDRs.
With just two iris plates to make and install instead
of four, the JVL filter becomes an awful lot easier to
build for those of us who aren't all that happy with