Page 1 of 4 1234 LastLast
Results 1 to 10 of 39

Thread: Coherent GEM CO2 laser RF cable lengths.

  1. #1
    Join Date
    Apr 2012
    Location
    Beaverton, OR
    Posts
    292

    Default Coherent GEM CO2 laser RF cable lengths.

    I picked up a Coherent GEM-50 from a friend. Right know it is missing the RF cable and I know the length of these things are multiples of the operating frequency of the laser (supposedly 100mhz) The manual lists the model numbers and the lengths bit they just don't work out. At best the lengths seem to be around 90mhz figuring a teflon dielectric cable.

    Can someone with a Gem tell me the length of their cable and what kind of coax they are using? I really don't want to spend $300 for a cable for a laser I don't even know if it works.

    -Jerry

    Click image for larger version. 

Name:	Screen Shot 2021-01-13 at 10.31.24 PM.png 
Views:	5 
Size:	389.6 KB 
ID:	57550

  2. #2
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,890

    Default

    I have an OEM GEM RF PSU setting in my garage, I believe it is 76 Mhz on mine ; I do not have a laser head for it.

    ~
    You'll have to factor in the velocity factor of the coax when you cut it. ; I'm stuck at work till Zero Dark Thirty, but I will look when I get home to make sure what the frequency is. It does not have to be perfectly a half wave, not darn even close, but for best performance you want the half wave if that is what they tuned/designed their amplifier output stage for.

    ~
    Most of the rest of the RF world has disposed of that old requirement, it is odd for me to see it again.


    ~
    I'd get a really good grade of cable from a known vendor such as RF Parts, Trimming the coax for a half wavelength multiple sounds like some one has bought into the theory of an old CB myth, but I'm sure their RF engineer has his/her reasons

    Wavelength Calculator K7TJO (wxtofly.net)


    Edited for math error I made.
    Last edited by mixedgas; 01-14-2021 at 12:13.
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  3. #3
    Join Date
    May 2009
    Location
    UCSB
    Posts
    715

    Default

    I think you are pretty close, the sticker on a 100W gem says 100MHz so it seems likely the 50W is the same. The cable lengths seem right for a velocity factor of 0.76 which seems about right.
    Last edited by krazer; 01-14-2021 at 21:28.

  4. #4
    Join Date
    Apr 2012
    Location
    Beaverton, OR
    Posts
    292

    Default

    Yeah, the power supply on this unit says 600w @ 100MHz. I was talking to one of the hams at work and he pretty much agreed with not worrying too much about length. Ill just make up a cable close to one of those lengths and give it a try.

    Thanks!

    -Jerry

  5. #5
    Join Date
    Apr 2012
    Location
    Beaverton, OR
    Posts
    292

    Default

    I found a chunk of Belden 8267 which crosses to RG-213. Has a N on one side and one of those giant BNCs on the other. Its about 3.54m which should be about right, just need a N connector to put on the other end and I should be good to go.

  6. #6
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,890

    Default

    I stand corrected, mine is 100 Mhz.

    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  7. #7
    Join Date
    Apr 2012
    Location
    Beaverton, OR
    Posts
    292

    Default

    So I made up a cable and got the laser all plumbed and powered. No luck. It gives a VSWR light meaning the tube is not lighting. Tried a couple different cables and exactly the same thing. The manual says that if one of the test voltages (forward power) is below 2.5v the power supply is bad. It is, but when you try to fire the laser it pulses above that.

    So I guess the next step is to try and locate a real cable I can use to verify it is not the cable. Not sure where to go past that.

  8. #8
    Join Date
    May 2009
    Location
    UCSB
    Posts
    715

    Default

    My gem 100 (with integral power supply) exhibits a similar failure -- I concluded that it needs a regas. I haven't got up the courage to send it out yet, but my plan was to try one of the sketchy regassers who claim they will refurb the tube for $1000.

  9. #9
    Join Date
    Apr 2012
    Location
    Beaverton, OR
    Posts
    292

    Default

    I have seen as low as $850 for a pump and fill.

    I just need to first find out if it is the cable, power supply, or head for sure though. I wonder if I can hook the power supply to a 50 ohm load and if I can find someone with one that big around here.

  10. #10
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,890

    Default

    Photovac outside Columbus Ohio is the real deal.. Multiple plants, multiple employees. I visited on business. I was suitably impressed.

    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •