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852
Advanced Member
Username: 852
Post Number: 578
Registered: 3-2006
| | Posted on Monday, June 11, 2012 - 12:34 pm: | 
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Cobra 29 LTD, no frills with a Wilson 2000 mirror mount..What would be the proper length of coax to run? I have heard stories from 18 feet, to just whatever it takes to get from the radio to the antenna...Can anyone clear this up? My mind is smoking!!!!! |
Tech237
Moderator
Username: Tech237
Post Number: 1591
Registered: 4-2004
| | Posted on Tuesday, June 12, 2012 - 8:39 am: |
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This is like the "how long is a piece of string" question. Yes long enough to reach is good enough with a couple of caveats.
1) Any SWR reading taken at the radio end of the coax MAY NOT be accurate. Random legnths of coax can affect the apparent SWR due to the coax's reactance.
2)Some antennas require a specified length of type X coax as part of it's mathcing system.
Try to work with a multiple of an electrical half-wave length of coax (less than 18ft depending on the type of coax) for accurate SWR readings at the radio.
Try to run coax the shortest distance possible, without running close to senitive auto circuits.
There two article in the "Subscribers" section that may help - one on Facts and Fiction of Coax, and a second one which is the first two parts of installing radios in cars. Part t3 is under construction.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Tech833
Moderator
Username: Tech833
Post Number: 2176
Registered: 8-2002
| | Posted on Monday, June 18, 2012 - 2:06 pm: |
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Excellent info from Simon.
May I add:
To his answer #1- The reading at the radio end of the cable will be different from the antenna end if the reactance at the feedpoint is not zero. If the antenna is perfectly matched, you will get the same SWR reading at the radio end regardless of how long the cable is. Using "tuned lengths" of cable is a neat way to trick a radio into seeing a better match into a load with capacitive or inductive reactance at the feedpoint.
To his answer #2- This is most common with "co-phased" antennas or special matching instances like a wire quad antenna. If you're running a standard everyday A99 or Imax antenna, using anything other than 50 ohm cable will negatively alter your system efficiency. The percentage of efficiency lost depends on cable length.
Your radio 'Mythbuster' since 1998
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