Jeppe wrote:
"So my problem is really mostly that I am trying to guess how close the
tip of the 1/4 wavelength probe antenna rod can be to the opposite wall
of the wave guide - without "detuning" the probe / affecting the SWR in
a detrimental way."

I don`t know, so I`m glad you asked the question. This is a good
newsgroup and I`d wager that someone will provide the answer. It`s been
a long time since I worked with microwaves and then it was with
commercial equipment.

My first thought was that the open-circuit end of a probe has an energy
about-face at its end which produces a null. Then out of curiosity I
went to my 3rd edition of the RSGB VHF-UHF Manual where I found tuning
adjustments made by adjusting depth of insertion of probes into cavities
and waveguides. There is much practical information in the RSGB Manual
but the microwave is mostly centered on 10 GHz. That`s OK because most
of it scales to your frequency of interest.

Best regards, Richard Harrison, KB5WZI