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CB co-phased antennas

Discussion in 'CB and Export Equipment and Accessories' started by KF5FUR, Oct 4, 2011.

  1. KF5FUR

    KF5FUR Member

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    I may have a problem with the length of coax for my setup. I will be putting two posts mounted against the inside of the front bed panel to get the antennas clear of the roof. But I will be running the cable through the body plugs of the cab and bed so I wont have to worry every time I close the rear doors, (03' ext cab F-150 BTW). The radio will be mounted via a cobbled together bracket that allows the radio to be swung around a bit and also has 2 springs in the mix to help dampen bumps and vibrations, add flexibility as well as a bit of rigidity. My ideal location is near roof but getting in the way of the passenger sun shade, hence one reason it moves around. But the problem is I only have 12ft per run, and I need more like 19ft. The harness for the co-phase setup is a typical RG-59 A/U with the twin cable connector on one end. I read up on the differences between 50 and 75 Ohm cables for CB and I've reached my budget limit for this project. I was wondering if I could get away with hooking up a run of 50 Ohm to the harness and it not effect the radio. Or I could hook up my cheap Radio shack RG-58 U? My dual harness is about 12ft each run, the RG-58 is about 20ft. I also have a run of RG-8X that I could trim to exact length needed. But 7ft for RG-8X to a female coupler then connected to the dual 12ft run of RG-58. Would this change in the line impedance of the system negatively effect the SWR? Sorry so long winded, I like to be thorough.


     

  2. n8fgb

    n8fgb Well-Known Member

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    Just forget about duel antennas, to close together to be cophased. Mount one in the center of the front of the bed like you want the 2 mounted, will work much better.
    Rich
     
  3. Atlasta

    Atlasta Well-Known Member

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    Agree. As I understand it unless you can get the two antennas 9 feet apart (1/4 wavelength) co-phasing will do nothing for you.

    And don't use cheap coax or connectors!!
     
  4. KF5FUR

    KF5FUR Member

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    Thanks guys but you didn't answer my overall question. To quote a Firestik page "Some people believe that co-phased antennas must be separated by a minimum of nine (9) feet. We have successfully used co-phase antenna systems with spacing as little as four (4) feet. Space alters the pattern and not always negatively. Each vehicle will be different." Things Every CB'er Should Know -- ©1997 FIRESTIK® ANTENNA I do not expect to get the proper radiation pattern associated with these systems. However I do expect to possibly have a blind spot on either side for Tx directly perpendicular to the trucks length. But if I do go with a single antenna can I use either or both 50 and 75 Ohm cable?
     
  5. 5150

    5150 Guest

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    with a single antenna you want to use 50 ohm coax .

    co-phase use 70-72 ohm coax.

    if your mounting the radio above behind rear view mirror , then run coax above headliner and down rear c-pillars and out through vent holes behind rear seat panel as it will work with plenty of excess coaxial >

    mine is set up similar with radio mounted behind mirror , coax under headliner through vent to bed mount antenna nice clean install on a "02" f-350 crew cab, plus one can change lengths of coax to change phase (phase shifting) similar to running two hots on a burb and playing with spacing and phase for optimum gain
     
  6. KF5FUR

    KF5FUR Member

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  7. RickC.

    RickC. Hopeless antenna junkie

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    ARRL Antenna Book

    You'll thank me later, KF5FUR.

    73,

    Rick
     
  8. KF5FUR

    KF5FUR Member

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    I said screw it as for the co-phased setup on the truck, I'll save that for my 10 meter setup with rotator when I get my 10m rig in the house. I opted for a 102" S. Steel whip. Of course I am thinking of a horizontal full wave co-phased setup for the 10m base.
     
  9. RickC.

    RickC. Hopeless antenna junkie

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    Interesting! Post pics!


    Rick
     
  10. KF5FUR

    KF5FUR Member

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    10 & 11 meter full wave co-phased horizontal dipoles (revised size for WWRForums).jpg

    sorry for the weird resolution on the pic, I had to resize it for the forums liking, hope you can read it all.

    -Bill
     
  11. ElectronTubesRule

    ElectronTubesRule Active Member

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    You do understand that you will have less receive signal when talking to anyone mobile or base that has verticle polarity if you have horizontal polarity. If I could not have both horizontal and verticle polarity on a base I would do spiral polarity. This way you are compromised not only for either verticle or horizontal but for those signals that have bounced off the upper atmosphere they often have rotating polarity. That is one of the contributing factors to "skip" having those dancing needles as it rolls in.
     
  12. KF5FUR

    KF5FUR Member

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    Another thought

    After reading this thread http://www.worldwidedx.com/cb-antennas/100792-truth-according-firestik-get-drink-snack-lol.html
    I kinda feel like a jackass for posting this earlier quote from that link.
    Which goes to show you that not all information on the Internet is exactly accurate. Now I don't know if that particular quote is right about the co-phased antennas, but I have seen a lot of folks on both sides of the fence about this one. You won't believe the number of oil field company pick ups around here with co-phased setups on them or sometimes they have a single 2' (less common) to 3' (more common) wire wound antennas mounted to the toolbox behind the cab, some even within a foot of the headache rack! I think they just throw it in their trucks and don't give crap, "if it f*cks up the company will buy another one for me" -one guy told me once. That's just willfully ignorant there.
     
  13. KF5FUR

    KF5FUR Member

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    I see what your talking about, that totally didn't come to mind when I sketched that up. I have read about that before in a CB book I had years ago. It had good plans for a circular polarity Yagi in it. I just like the big colossal antennas that all the non-Hams will look at driving by wondering "what is that for, is that legal?". Or to scare the ignorant neighbors into wearing foil hats :LOL:
     
  14. BlueArmidillo

    BlueArmidillo New Member

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    KF5FUR i am a avid 102 fan, i have ran 102 singles 102 cophased doubles, and it is my prefered antenna! there is not alot of topic on co phasing . my expierence has been on 11 meter band width. and i know theres rules for optimim gain/ ect ect... so beings your question is relaitive to coax length and a coax system of different impedences i will state i have never ran anything other than a t connector directily to my radio , or a combined harness (either made by me , or store bought) all i have expierence is working rg 59 (which in my oppinion has a very small inside single strand conductor).. i have added small patch cords (less than 6 feet) and swr dose not seem to change a whole lot (assuming your running legal 4 watts)..i have never once had swr readings (above 1.3:1) within legal power range (4 watts.
    Here is the 3 setups i have co phased with 102" sst whips.
    1) small car 8.5' (home made harness rg 59) pl259-2x (8.5') front mount 102s 48" spacing, swr- 1:1 barefoot, 1.3:1 200w. signal was well well worth the added expence.,
    and seeming to be (not fact or proven) front to rear directional!
    2)suburban Again same setup (8.5) feet rg 59 harness home made . far front mount antenna base at bottom of hood line. Spaced 65" center to center),,,but with a new twist...(i added a coupling and ran another 8 feet of rg 213) .
    So radio out 8 feet 213+coupling+home made cophase harness (8.5) feet, total length (18.5').
    once again swr near perfect! i will attest that that setup ran a 1.1:1 with 4 watts, and maybe a 1.3:1 200 watts.
    again signal was awesome! and that setup rocked !
    heres what i would suggest,,.
    Due to manufacturing tollerences, ect it is stated there is a velocity factor due to coax/ bla bla bla...
    Even if its 100% correct . you will find a variance due to manufacturing tollerances, and sample size.
    bottom line (i allways make my final coax cut on a 50 ohm feed line 1 foot longer than 1/4 wavelength of desired resonant frequency) for 11 meters.
    so that being said, make your cophase harness 8.5', ,,forget its length.....or make it 12. (whatever feet). or make it 18.( ) feet....and forget its length!
    cut the 50 ohm line that feeds this to achieve a total length of 1/2 , 5/8 or 3/4 wave length +/- (velocity factor), at your frequency (?)......_+ 1 foot (if a lower frequency.
    ^ however me doing all the math time and time again has proven= im wasting my time.......after i check swr way above and way below my desired resonant frequency. I conclude this>
    I am wasting my time doing math, and getting exact lengths down to the nats ass.

    Play around with a few swr meters, play with some different 50 ohm final patch cord lengths and you will conclude the same thing.

    (now to those of you who are more advanced than a swr meter and reflected energy).
    yes i know it dose matter and yes i realize if you have fancy tools you can further tune a system, and what not.
    But the average guy dose not have thousands of dollars to spend to find the exact resonant freq, and alot of time to spend dialing it.
    I will say this much (2 102s on 11 meters booms! royally!) and there is no setup i have found better, despite "optimum gain being spaced a half wave length apart".
     
  15. 15minigrass

    15minigrass Active Member

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    or, http://www.skater229.com/55antennamagic.html One in the front, one on the raer. Minimum of 6 ft apart. Get um up above the roof line. and a metal plate or square tubing connecting both antennas. One Hot(Front) one grounded(Rear). Hereʻs what I had. Hot antenna puck mounted through the roof, Reflector on camper shell. Ran flat bars under the shell the some chicken wires then grounded them to the truck bed via the widow frames. [​IMG]
     

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