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Oclocker
New member
Username: Oclocker
Post Number: 3
Registered: 1-2005
| | Posted on Friday, November 11, 2005 - 12:02 pm: | 
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I am new to tunning with an mfj metter i have the mfj-269 just like the 259. what my question is i am tunning an 102" ss whip with a spring. stud mount on a tool box. my coax is cut a 1/2 have 14.1 inch. my swr are 1.0 r= 50 to on 40 and r=53 on 1, x stays at x=0 the whole time. this is for 11 meter cb freq. ok that is fine. when i put the amp in line with a 3 foot jumper it changes my ohms to around 54 on 1 and 46 on 40. my x=about 3 with an swr of 1.2. with a 3 foot jumper (amp still off of course) so i change the jumper to a 12 foot and everything is flat again. just wondering why the longer peice of coax after the amp to tune everything flat again. with the bird meter at 400 watts there is 0 reflect. what i dont know is why would the 3 foot not work and it took 12. i know the change is small but i will be running a max of a 16 pill and want it right |
Marconi
Intermediate Member
Username: Marconi
Post Number: 484
Registered: 11-2001
| | Posted on Friday, November 11, 2005 - 6:27 pm: |
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Can't understand this, "my coax is cut a 1/2 have 14.1 inch."
Ok on the following you post, "when i put the amp in line with a 3 foot jumper it changes my ohms to around 54 on 1 and 46 on 40. my x=about 3 with an swr of 1.2. with a 3 foot jumper (amp still off of course) so i change the jumper to a 12 foot and everything is flat again."
I would think the two jumpers are making different transformer affects to the system and it is showing up at the TX'r end of the system. I would think this would be OK unless a different length made things look even better. I guess that could happen if you made the jumper more of an electrical 1/2 wavelenght line like I think you were trying to indicate you did with the feed line from the amp to the antenna, at the top of your comments.
Try something else and let us know.
Eddie
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Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 894
Registered: 6-2004
| | Posted on Friday, November 11, 2005 - 7:05 pm: |
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It has to do with how RF propogates through coax. If the match is not absolutely perfect, different lengths of coax will show different impedances at the radio end.
You should reconsider running that much power mobile. The RF fields would be dangerously strong. |
