| Author |
Message |
    
Apocolypse400
| | Posted on Monday, September 23, 2002 - 4:49 pm: | 
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OK,,,,
A good friend gave me a Cobra 146GTL brand new in a box from 1988, and a Eagle(Never Heard of them)150TX amp. Since I bought a new truck, I hadn't installed my old cobra 25, but couldnt resist with the big 146. I put the radio above the mirror on the roof, heres the question, 12ft piece of Belden RG8 down the pillar under the passengers seat to the amp. Then a 18ft piece of the same stuff out to the antenna, which I havent mounted yet. ANY PROBLEMS WITH THE JUMPER LENGHT?
Thanks Steve
Steve,
Always try and use the shortest length possible.
Lon
tech808
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Tech671
| | Posted on Monday, September 23, 2002 - 8:56 pm: |
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Your length should be fine. |
Apocolypse400
| | Posted on Friday, September 27, 2002 - 12:33 am: |
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Thanks guys, your right it works great! |
Marconi
| | Posted on Wednesday, October 09, 2002 - 11:49 am: |
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400, typically the guys are right, but the answer to your question will depend on the antenna and how well it matches. Personally, I find 12' jumpers to be a bit reactive in my setups. You will just have to try it though.
If you get a good antenna and a good match, then the lines will be fine. If it was just the radio in line you probably would not see any problem even if the antenna did not make a good match. The amp, however, can cause some problems if the match is not pretty good.
Just in case things don't go as expected.
Good luck,
Marconi |
Apocolypse400
| | Posted on Wednesday, October 09, 2002 - 11:47 pm: |
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Things seem to be going good. I got the swr way down, the needle barely moves on 27.385. It does however depend upon the antenna, the tram I had on inintially would never get that low. Maybe the coax lenght, or just a bad antenna. The radio dead keys 2 watts and swings to 11. Someone did a fine job on this radio. The amp is doing great. The bad thing is I have made my best contacts barefoot?
Thanks,
Steve |
Galileo
| | Posted on Monday, October 14, 2002 - 9:10 pm: |
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Question: What is the "correct" length of coax?
Answer: The shortest length that makes it from the radio to the antenna.
Question: Are there any exceptions to the above rule?
Answer: 75 Ohm harnesses for Co-phasing is the only exception.
Question: Why do most mobile antenna makers recommend 18 feet of coax?
Answer: You got me, they claim you should use 1/2 wavelength multiples of coax. 18 feet isn't even close to being a 1/2 wavelength in any 50 Ohm coax you will find. Check some commonly used coax using the above formulas. RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
Question: Ok, seriously nerd, when I trim my coax it changes my SWR. You can't tell me it's not good to lower my SWR from 1.5 to 1.2 by taking off a few extra feet of coax.
Answer: Hey, I'm not a nerd! Go ahead, change your coax length. If you change coax length and it affects your SWR in minute amounts, everything is working fine. If your SWR was 2.5:1 and putting in a 4 foot jumper brought it down to 1.3:1, this large change indicates you have real problems...i.e. common mode currents (see above). Really, you should be changing the antennas length to alter SWR. NO special length of 50 ohm coax is going to fix or lower your SWR signficantly and/or boost performance. Period. |
Apocolypse400
| | Posted on Monday, October 14, 2002 - 11:27 pm: |
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When I started I had all these questions and no answers, then I found coppers forum. Now, I have a great mobile install and am working on my base setup. People starting in CB and radio need to forget about the best radio, the big amps and concentrate on the antenna system, good coax, good grounds, and large power leads. These are the lessons I have learned.
Thanks again for all the help.
Steve |
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