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Message |
    
Weatherman49
Junior Member
Username: Weatherman49
Post Number: 31
Registered: 2-2005
| | Posted on Wednesday, March 23, 2005 - 11:48 pm: | 
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This maybe another one of those mythes you hear about but my Q is does the coax length realy matter? I know that 102" (108 if you use a calculator) is quarter wave. (36.154) is full wave so convert that to inces and divide by 4 is the coax a factor in the actual resonent length of the antenna sytem I have heard thad your coax should be cut to match the freq your using (such as 3 6 9 18 feet for CB) or will it matter? If it does or doesn't tell me why or why not!
Thanks
Weatherman |
Tech808
Moderator
Username: Tech808
Post Number: 5329
Registered: 8-2002
| | Posted on Thursday, March 24, 2005 - 12:18 am: |
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Weatherman49,
From the Defpom Site:
COAX LENGTH
Hope this helps,
Lon
Tech808
CEF808
N9OSN |
Weatherman49
Junior Member
Username: Weatherman49
Post Number: 32
Registered: 2-2005
| | Posted on Thursday, March 24, 2005 - 12:30 pm: |
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thanx 808 maybe some more poeple will read that |
Marconi
Intermediate Member
Username: Marconi
Post Number: 413
Registered: 11-2001
| | Posted on Thursday, March 24, 2005 - 1:28 pm: |
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Hey James, after reading the article do you now understand why?
It may seem to be a simple issue, but an RF system is made up of very complex issues and perfection here is difficult for us to achieve. A few words will not explain it all either. Such things as resistance, balance/unbalance, ground affects, impedance, reactance, common mode currents, and other issues must be understood in order for us to really understand what is going on. Understanding how one thing affects the other is also important, thus it all gets very complicated due to the interactions of these factors.
Generally the article speaks the truth, but much is not explained and thus there is no answer as to why the line does or does not matter. It is sort of like explaining which came first the chicken or the egg. We are lucky that mother nature allows us such latitude in error and still operate.
The main point I think is that your 1/4 wave whip is not perfect and the best you can do is make sure the radio is happy with the match that it sees in operation and that the radiator is designed and installed in a known and workable fashion. As long as the SWR meter shows us a reasonable response when located at or near the TX'r you should be OK.
Sure there are improvements that could be made to just about any system, but that is difficult to put into words. Study and discussion is probably the best thing you could do if really interested.
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Racer_x
Intermediate Member
Username: Racer_x
Post Number: 205
Registered: 2-2005
| | Posted on Thursday, March 24, 2005 - 3:14 pm: |
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Hmmm. Has Lon changed his opinion about trimming coax? I seem to remember that he was opposed to the practice.
I disagree with the article in one respect, the coax IS part of the antenna system. Feedlines certainly do have an effect on the match as far as the radio is concerned. RF propogates through coax which is why trimming the length can make changes in the perceived match. |
Tech808
Moderator
Username: Tech808
Post Number: 5330
Registered: 8-2002
| | Posted on Thursday, March 24, 2005 - 6:20 pm: |
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Sorry,
But I still only use the amount of Coax I need to go from the antenna to the antenna switchbox.
Always have since 1963 and always will.
All you have to do is read my past posts to confirm this.
I posted a link and that was all.
Lon
Tech808
CEF808
N9OSN  |
Racer_x
Intermediate Member
Username: Racer_x
Post Number: 208
Registered: 2-2005
| | Posted on Friday, March 25, 2005 - 6:32 am: |
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Fair enough. I do the same. I acknowledge that there is a legitimate reason to trim your coax - I just think life's too short to bother. |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 316
Registered: 9-2004
| | Posted on Saturday, March 26, 2005 - 12:19 am: |
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i was reading a radio shack manual from the 70's yesterday, and it stated that the coax should be trimmed to exact multiples of 18 feet. i thought that was pretty funny. seems the myths have been around forever and probably will be.
matt |
Dx431
Advanced Member
Username: Dx431
Post Number: 994
Registered: 2-2002
| | Posted on Saturday, March 26, 2005 - 2:33 am: |
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I agree with Lon. I have been doin the same thing for yrs. Run enough coax to get you from ant. to transmitter. If runnning coax for a base, leave enough slack for drip loops and a little extra in the shack.
Enough said.....
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