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Message |
    
Mauka
New member
Username: Mauka
Post Number: 4
Registered: 1-2007
| | Posted on Friday, January 04, 2008 - 11:46 pm: | 
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What length of coax should I use? I am using a 108" whip on a davemade mount? |
Captian_radio
Intermediate Member
Username: Captian_radio
Post Number: 313
Registered: 9-2004
| | Posted on Saturday, January 05, 2008 - 8:27 am: |
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Long enough to reach from the radio to the antenna.
Bob CEF451/VE1CZ
Robert L. Spicer The days of radio are just beginning!
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2ir473
Intermediate Member
Username: 2ir473
Post Number: 132
Registered: 7-2002
| | Posted on Saturday, January 05, 2008 - 4:12 pm: |
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You don't need to have a particular length of coax, other than that required to reach, like Capt. Radio stated.
You don't tune your antenna by using a particular length of coax, so you may use whatever length is appropriate.
The length of coax can fool your SWR meter, which is why some say to use a half wavelength equivalent, so as to reduce the "influence" of the coax on your meter. In the case of 27 mhz, that is around 18 feet long.
But as long as your antenna is properly tuned for the band you are using it on, the coax length is not a factor.
Paul
2IR473
CEF-218/ham #107
CDX8412
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Mauka
New member
Username: Mauka
Post Number: 5
Registered: 1-2007
| | Posted on Monday, January 07, 2008 - 2:10 am: |
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Does the velocity factor come into play? For instance the LMR-400 has a .85 VF. Some have said to multiply that number by 18ft which should be the coax length? Any comments on that? |
Bruce
Senior Member
Username: Bruce
Post Number: 4575
Registered: 9-2003
| | Posted on Monday, January 07, 2008 - 8:09 am: |
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GARBARGE .......
Coax length is not a factor in making a antenna work. If your antenna is working coax doesn’t care how long or short it is. If you have to cut coax you have a bad antenna ....
On 6 since 66
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2ir473
Intermediate Member
Username: 2ir473
Post Number: 135
Registered: 7-2002
| | Posted on Monday, January 07, 2008 - 8:59 am: |
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The length of coax and the velocity factor, have to do with trying to get an accurate swr reading on your meter. Coax cable length won't tune your antenna swr, but it can affect the reading your meter sees, so by taking into account the frequency you will be using, the velocity factor of the coax, you can help eliminate false readings on your swr meter. In the case of LMR400, operating at 27.205 mHz, your coax would be 15.3 feet long. With this length, you should eliminate any "influence" coax may have on your meter. (492 x VF/frequency)
However, you should use a 15.3' length from your radio to your meter, and then again from meter to antenna. If that won't reach, then you have to go to a 30.6' length, and so one.
It is simpler to measure the swr at the antenna feedpoint (if possible), then you know the true swr of your antenna, and use any length of coax that reaches.
Paul
2IR473
CEF-218/ham #107
CDX8412
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Tech237
Moderator
Username: Tech237
Post Number: 941
Registered: 4-2004
| | Posted on Monday, January 07, 2008 - 11:01 am: |
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Read the article titled: Coax Cables – Facts and Fantasy for a better understanding of what effect coax can have on antennas..
CLICK HERE >Coax Cables – Facts and Fantasy
Simon
Tech237
N7AUS
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Mauka
New member
Username: Mauka
Post Number: 6
Registered: 1-2007
| | Posted on Tuesday, January 08, 2008 - 3:56 am: |
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Thank you very much Paul and Simon. Very good knowledge and information. I appreciate your professionalism when corresponding back to people like me who is trying to learn and understand.
Also thank you to "On 6 since 66" although your comment's are well stated, I'll have have gotten many responses locally, similar to what Paul and Simon said. |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 4103
Registered: 7-2004
| | Posted on Tuesday, January 08, 2008 - 9:03 pm: |
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i always use 'an odd multiple of 1/2 wavelength times the velocity factor of coax in use', both base AND mobile. does it work? i guess so. does it help? don't know, but it can't hurt since so many copper forum members have heard me or spoken to me, base & mobile....including bruce & captian radio  |
