We have found out the hard way that the protective VSWR monitor circuit on many of these RF driven lasers wants to see an exact multiple of half waves and a precise impedance in the cable. You can either 1. Pay for the commercial cable trimmed as needed, 2. Obtain a cable with a known velocity Factor coefficient, calculate the needed corrections for the frequency in use, cut the cable slightly long (1/8TH wave long) and install, reinstall the connectors till your trimmed correctly. 3. You can obtain or borrow a vector network analyzer and do it the right way.
Pasternak is generally viewed as expensive, but they might have the VNA to do the job.
First thing I would to is try a short cable, say less that two meters in length and 50 Ohms impedance and see if the system does not fault and does lase. If that is the case leave it be.
DX Engineering sells RG-8 and completed cables.
Steve
Qui habet Christos, habet Vitam!
I should have rented the space under my name for advertising.
When I still could have...