18' of coax??

Discussion in 'CB Radio Forum' started by ChixDiggit, Sep 18, 2009.

  1. Longshot379

    Longshot379 Light Load Member

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    I have been known to use just enough to reach the antenna and have a little slack to remove the radio usually about 11' and been able to consistently have a 1:1 to a 1.3:1 swr.
     
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  3. Gadfly

    Gadfly Medium Load Member

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    I am sorry, but this is one of the biggest myths ever to come from CB radio and that is mostly where it comes from and lives forever in CB legend and hearsay. Here's why:

    I have heard this "18 feet of coax" thing since the 70's. Prior to that I was setting up and installing both mobile and base stations, military, commercial and hobby as a kid in high school. Until somebody "corrected" me (or tried to), I had been setting up stations with great success and communication around the world with them with an average SWR (not "swr'sssssssssssssss plural) of 1.2 to 1.5. I was..............like, "SAY WHAT?" Except for some very special cases, this coax length thing is just simply born out of ignorance and lack of real knowledge of the subject. CAN you "tune" your antenna by trimming coax? Yes AND no. If you are getting differences and apparent low SWR simply "cutting" the coax to a certain length, you are merely fooling the meter into reading what you WANT it to see; hate to tell ya, but you STILL got a mis-matched antenna out there on the deck or trunk of your car. I am about to throw a huge monkey wrench into your theory right now!:biggrin_2559:

    First of all, you must accept and know that radio theory is radio theory, antenna and transmission line theory is antenna and transmission theory and it applies to ALL bands and frequencies the very same with very few exceptions. IOW, there's no special exclusion for CB radio that says that coax length applies JUST to CB ONLY. If it is true (and it definitely is NOT), then it must apply to EVERY radio frequency we know of.

    To demonstate this, I call your attention to a mobile HF antenna that is used by hams worldwide. It is a multiband antenna that, in its various forms, can cover from 1.8 MHZ up to 30 MHZ. Now this poster says that those of us who downplay coax length as a way to "tune" an antenna say that, "we don't know what we are talking about", so lets explore this. This antenna is called a "screwdriver" so named because it was originally powered by its inventor by a handheld, cordless screwdriver that moves a loading coil up and down inside a hollow mast. Metal fingers surround this coil and allows the coil to tune/resonate on any frequency within its designed range. By pressing a button in the dash, the operator can change frequencies from 1.8 Mhz up to 28 MHZ while on-the-go---all without stopping to change whips, taps,or leads. Some of these antennas are fully automatic and will set themselves to frequency simply by changing the radio's frequency itself. Almost fully automatic, hands-off operation! This involves around 10 individual bands of coverage from below 2 MHZ all the way up to 28 MHZ near the CB band (10 Meters). The coax length "gurus" say that we've gotta have a "certain" length of coax or the "swr'ssssssssssssssssss" will be off! Uh, there's a problem with your theory! To cover TEN bands I must have coaxes cut for EACH band, to wit:
    1.8 MHZ 130 feet of coax
    3.9 MHZ 60 feet " "
    7.2 MHZ 32.5 feet
    14.3 MHZ 16.4 feet
    18.1 MHZ 13.2 feet
    21 MHZ 11 feet
    24 MHZ 9.75 feet
    28 MHZ 8.4 Feet

    * Note: I have divided the figures by 2 to account for the fact that the vehicle itself is the other half of the antenna.

    Can you not SEE how rediculous this IS? ALL that coax coiled, rolled, stored or kept in a Malibu? BUT! It is simply NOT true! The amateur operators are working the WORLD with ONE antenna, on INFINITE numbers of frequencies, on MULTIPLE frequencies using ONE coax of random length and usually what it takes to get from the radio to the antenna! Mine is about 7 feet, and it does not correspond to ANY formula, any velocity factor (another bit of unnessary fluff and horse poop), or any particular length. Yet with that mobile antenna I have talked all over Europe, Russia, S. America, and Australia using 100 watts.

    IF your vehicle is METAL, the design does not REQUIRE that the coax is part of the counterpoise of the system, there is NO need to worry about coax length. TUNE THE ANTENNA, NOT the COAX. IF you are "tuning" the coax and not the antenna you are fooling the meter AND yourself! Having a resonant antenna is not the same as a "matched" one necessarily. You can install a 10 watt, 50 ohm resistor at the terminals of your radio, and the radio will be perfectly happy!!! You won't have must of a signal, but the radio will be happy because it is "matched"! :biggrin_2559: Coax length, except for certain narrow exceptions is hooey and faulty CB radio myth!!!!

    I'm telling you true! If you want to believe this, fine; it won't hurt a thing. You'll just waste a lot of time!!!!!

    Gadfly
     
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  4. WA4GCH

    WA4GCH Road Train Member

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    Seminole Florida
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    Does this meen on 1750 meters ( 170 Khz ) I can get rid of the 1200 foot of RG-178 and use the car frame ?
     
  5. Gadfly

    Gadfly Medium Load Member

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    :biggrin_2559::biggrin_2559: I thought about this and started to mention it, but then I thought................nah! But you ARE right. Eventually the frequency is going to get so high that, if you follow that CB radio coax length poop, the feedline will be disappear because it is shorter than what the guru's require. You'd have to eliminate the feedline totally. So now how do you gt from the radio to the antenna if the feedline is 1/8 of an inch long! ROTFLMAO!

    I have presented this problem MANY a time in response to the "18 feet of coax" "gurus", and NO ONE has ever been able to rebutt it! Oh, there's been a feeble attempt or two, but the responders only succeeding making themselves look a bit silly. Sure, you can play with coax if you want to and get your "swr'sssssssssssssssssssssssssssssssssss" to appear good on the meter, but you STILL got a MISMATCHED antenna hanging out there that is NOT resonant at the desired frequency. If coax is the way to "tune" antenna, then why not just use coax and leave the antenna OFF!!! :biggrin_25524:
     
  6. Gadfly

    Gadfly Medium Load Member

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    Welllll.............................................I see there's NO takers!!! :biggrin_2559::biggrin_2559:
    That's because coax length is irrelevent and faulty RF theory that born of CB radio, lives on CB radio and will live on in legend and CB myth! :biggrin_2559::biggrin_2559::biggrin_2559:
     
  7. WA4GCH

    WA4GCH Road Train Member

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    Seminole Florida
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    Now lets see If I take 6 inch hard line and run it into RG-174 (.1 inch coax ) would it conpress the RF by 60:1 making a 4 watt radio look like 240 watts ?

    Same logic :biggrin_2559:
     
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  8. 112racing

    112racing Road Train Member

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    pocono's, pa
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    i just put a w2000 9" shaft mirror mount and used a 9 foot coax and was between i.2 and 1.4 on all 40 channels

    thats good enough for me:biggrin_255:
     
  9. Xcis

    Xcis Medium Load Member

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    Bridgeport, Pa
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    Ladies and gentlemen, I hate to burst your bubble but the halfwave length of the CB frequency does not have anything to do with the length of coax in a mobile CB radio setup.
    .
    .First the halfwave length of the CB frequency is 18 feet ONLY, I say again ONLY, when traveling through air. You CB signal travels from the radio to the antenna through coaxial cable. The halfwave length of the CB frequency when traveling through coaxial cable IS NOT 18 feet; it is 11 feet 10 inches.
    But since 18 feet will work, that tells you that you can be off by more than 50% and it still works. Obviously, the 18 feet of coax is just a useless myth. Just ask a ham operator if they change the length of their coax everytime they change to a different frequency band.
    .Shorter lengths of coax are better because of their lower loss.
     
  10. Meteorgray

    Meteorgray Heavy Load Member

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    The mystery to me is that brands like Firestick and Wilson have repeated the myth. Maybe it's to sell more coax?
     
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