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Thread: coax length

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    coax length

    Time and time again I have made a comment about how coax length does matter.
    Booty had very often argued the issue. But in this post he sucked up to the guy...
    I disagree with fact 4 though. 1-3 shows why fact 4 is bull shit

    this was posted by some other tool bag

    The electrical half wave length of LMR-400 at 27.205Mhz (middle of the CB band) is:

    15.365 feet (15' 4 5/8")

    6 electrical half wave lengths would be 92.2' (92' 2 1/2")
    7 electrical half wave lengths would be 107.5' (107' 6")

    FACT #1:
    This takes into consideration that the LMR-400 has a perfect velocity factor of 85%. It may, or it may not. In reality, the velocity factor of a piece of coax can vary across the same exact reel of cable! Coax is not perfect ! This is why phasing cable manufacturers will use a network analyzer and trim the coax down to the perfect length for that piece of coax.

    FACT#2:
    That electrical half wave length of coax is tuned specifically for the frequency you choose. It will be too short for channel 1 and too long for channel 40. The length you have will work fine for any frequency just as a 100' piece of coax works fine for any frequency. After all, 100' is an electrical half wave length of some frequency, and people use that all the time, right?!

    FACT#3:
    The ONLY advantages of using electrical half wavelengths of coax are twofold;

    * Your reading at one end of the coax is IDENTICAL as the reading at the other end of the coax, but only on the frequency that cable was cut for. If you build your antenna for channel 20 and have it in the air, its easier to stand on the ground to take your SWR reading then hanging from under the antenna 60' up.
    * For phasing two or more antennas.

    I have an electrical half wave length of coax I use when building I-10K antennas cut for 27.205MHz. This allows me to stand on the ground, away from the antenna, and test the feedpoint for R-50 and X-0 on 27.205. As the analyzer teaches us, as we move away from that frequency, the readings will change. But it will be dead nuts on 27.205! I've tuned a couple up at 27.025 and 26.915, but only used a very short jumper for those. Again, the reality is that its close enough.

    FACT#4:
    The correct length of coax you should run from your antenna to you shack is; The shortest length that will reach! Again, look at the two examples in FACT 3 above for using an electrical half wave length of cable. If these are not relevant to your operations, don't worry about them! Why do I say, "the shortest"? The longer the cable the more the loss!

    I use electrical half wave lengths for testing purposes and phasing antennas. I use what ever I have on hand for everything else. I hope this helps.
    by BOOTYMONSTER » Tue Nov 11, 2008 2:07 pm
    how about an option to delete a thread we make ?
    ill admit im at least a part time idiot . sometimes i do double post when editing a post .
    some may find this hard to believe , but ....... sometimes i even get pissed and make a post i regret later . hard to believe huh?? lol

  2. The Following User Says Thank You to coolbreeze For This Useful Post:

    High Voltage Mobile NJ (02-11-2013)

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    LOL , still making yourself look stupid ..............
    some things never change .

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    Beercanman (07-30-2012)

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    Quote Originally Posted by BOOTY MONSTER View Post
    LOL , still making yourself look stupid ..............
    some things never change .
    yep sure is ! you cant handle the truth!

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    so you say ..
    and we all know how much value what you say has ...................

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    yea like you can do better ?





















    on









    your own , with out macaroni &cheeze

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    Some people say multiples of 3 are better like 25,50 or 75. But not how sure this really is? I've known alot that didn't use this rule of thumb and still got good swr's. Same with jumpers supposed to be in mulitiple of 3's. I use 2-4 footers and notice no change.
    RCI-2950DX, 1978 Cobra 148 GTL Taiwan, SHURE526T MIC, Tug-8 With Astatic 10 DA Head, Astatic 575M6, ASTRON 70/20 AMP POWER SUPPLIES, IMAX 2000 W GROUND PLANE KIT. TIMES MICROWAVE LMR-400 COAX, RF CHOKES. DIAMOND SWR/POWER INLINE METER. 250 of Sea Level Under Me. Recent World Radio Member.

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    Quote Originally Posted by NY714 View Post
    Some people say multiples of 3 are better like 25,50 or 75. But not how sure this really is? I've known alot that didn't use this rule of thumb and still got good swr's. Same with jumpers supposed to be in mulitiple of 3's. I use 2-4 footers and notice no change.

    you better look at you radio !!!!! i bet the swr between your radio and your driver box is a 2 swr or higher . alot of you guys are clueless about tuning period . capacitance plays a very important role in your feeline. do some experiments and you will find out what we are talking about .

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    I am afraid not my swr is flat barefoot and at times 1.3 at highest, come on that is just common sense. My height makes up for all the bs you talk about! It's all about location and height, you should even have the common sense to know that. Lol !!!
    RCI-2950DX, 1978 Cobra 148 GTL Taiwan, SHURE526T MIC, Tug-8 With Astatic 10 DA Head, Astatic 575M6, ASTRON 70/20 AMP POWER SUPPLIES, IMAX 2000 W GROUND PLANE KIT. TIMES MICROWAVE LMR-400 COAX, RF CHOKES. DIAMOND SWR/POWER INLINE METER. 250 of Sea Level Under Me. Recent World Radio Member.

  11. #9
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    Remember i am a base with no tvi to neighbors !!!!
    RCI-2950DX, 1978 Cobra 148 GTL Taiwan, SHURE526T MIC, Tug-8 With Astatic 10 DA Head, Astatic 575M6, ASTRON 70/20 AMP POWER SUPPLIES, IMAX 2000 W GROUND PLANE KIT. TIMES MICROWAVE LMR-400 COAX, RF CHOKES. DIAMOND SWR/POWER INLINE METER. 250 of Sea Level Under Me. Recent World Radio Member.

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    i will not use anything smaller than a 6 foot jumper between the driver and the big box . from the radio to the first driver box is at least a 12 footer , 18 works a tab better .

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