Filter hacking for 2395 ISS TV

[G3YKI]

G4EML said
"The 10MHz BW design from the S53MV article is actually shorter which seems a bit strange. The pole spacing is shorter but they are using 8mm diameter resonators instead of 10mm in 40 x 30 tube so maybe that reduces the coupling."

I think the width of the tube is the main influence. Although you may have heard of "evanescent waveguide" as a particular class of filters, there is no firm dividing line between such classifications. One way to look at the filter you have would be to say the coupling is a result of evanescent waves in the rectangular tube. The reduced width increases the cut off frequency of the waveguide and hence the rate at which the 2.4 GHz field decays along the tube. Then it is easy to see that the coupling between resonators will be reduced.
The reduced diameter posts probably just keep them in proportion and the Z0 in the low loss region.

[g4eml]

OK, that makes sense now, thanks.

Colin.