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Thread: Tuning a CB with a built-in SWR meter

  1. #1
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    Question Tuning a CB with a built-in SWR meter

    Does anyone out there know how to tune a Cobra 29 XLR cb? I have everything powered, set up, antenna hooked up all that jazz. I can hear SOME voices, but they are real fuzzy. After looking at other CB's, someone made me realize that this model has a SWR meter built into it, so I have all the necessary equipment, just not the know-how. Its a 70's model Cobra, but anyone with information on how to tune with a SWR meter built in is welcome to post.

    Stu, I saw your site and read you instructions on how to tune, but what is a full reading? I cant get the dial to move hardly at all when adjusting the SWR calibrate switch...

    And yes, I hit search. Only 1 post from me when I was asking about mounting the thing when I first bought it...
    97 Gunmetal TJ, Dual Tops, 5 speed, Dana 44, Tuffy Security, 70's Cobra 29XLR/Firestick II, Herculined, 2.5" OME, Currie MM, JKS BL, BFG 33X12.5X15 on 15X8 AR Baja II rims, Wet Okole's, BP rear swingout, TC SideBars, Steering Skid, Recovery Gear, Fire extinguisher, and tools. Some vain stainless hood jewlery and stereo upgrades. (Kenwood CD, 400 watt Pioneer 12")

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    First CB's range in options and features so I will walk you through a SWR cal the way mine is set up. You may have to change things for yourself.
    Set SWR switch to cal. Set CB to mid channel.(20 for 40 ch. CB or 10 for 19) Hit the Mic button and adjust SWR knob so that needle and SWR mark on scale align. Then move SWR switch to SWR and hit mic button again. SWR should be below 1 for optimum performance. If not check connections, cable condition, ETC....
    Then set SWR switch to RF for normal reception.
    Almost forgot, if you do not have a SWR Knob for adjustment you have to adjust the antenna. Most have some sort of fine tuning height adjustment somewhere.

    [This message has been edited by gunrunner (edited 07-20-2001).]
    84 CJ-7 Heavily Modified

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    Red face

    Wow! If you can get an SWR reading BELOW 1 then I want your secret...

    ------------------
    James
    99TJ-Sport Desert Sand, 4L I6, 5spd, D30, D35 w/trac-loc, 2" TrailMaster, Invader SS, 31" BFG A/T KO's, Rubicon Express Discos, Cobra 75WXST, Wilson FGT 4', recovery strap, first-aid, extinguisher, etc... jprior.jeepz.com
    James
    99TJ-Sport Desert Sand, 4L I6, 5spd, D30, D35 w/trac-loc, 2" TrailMaster, Invader SS, 31" BFG A/T KO's, Rubicon Express Discos, 4xDr D30 Guard, Cobra 75WXST, Wilson FGT 4', recovery strap, first-aid, extinguisher, etc... jprior.jeepz.com

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    Yeah, guess I should have been more descriptive. gunrunner... little luck with that...

    My CB has 4 buttons that read (l to r)

    Button 1: SWR/CAL/RF
    Button 2: NB/ANL/OFF
    Button 3: CB/PA (I got this one covered)
    Button 4: HF/OFF

    I also have 6 knobs (l to r)
    Knob 1: Mic Volume (duh)
    Knob 2: Volume (again, duh)
    Knob 3: Squelch
    Knob 4: SWR CAL
    Knob 5: RF Gain
    Knob 6: Delta Tune

    Okay, besides the 3 I know what they do, anyone want to fill me in on the others, and what I might want to tweek for tuning? THanks again.

    gunrunner...where in Kentucky?
    97 Gunmetal TJ, Dual Tops, 5 speed, Dana 44, Tuffy Security, 70's Cobra 29XLR/Firestick II, Herculined, 2.5" OME, Currie MM, JKS BL, BFG 33X12.5X15 on 15X8 AR Baja II rims, Wet Okole's, BP rear swingout, TC SideBars, Steering Skid, Recovery Gear, Fire extinguisher, and tools. Some vain stainless hood jewlery and stereo upgrades. (Kenwood CD, 400 watt Pioneer 12")

    Rater: 261
    Not too shabby

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    Talking

    Looks like your set up will do fine with my instructions. We have about the same buttons.
    Hebron KY, just across the river from Cincy.
    84 CJ-7 Heavily Modified

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    I think one of the most important things in tuning for SWR is understanding what it is, and what causes it.

    SWR= standing wave ratio. Best SWR possible is 1:1 (happens only in theory).

    SWR in a transmission line is defined as:

    SWR =( 1 + P ) / (1 - P ) where P = the reflection coefficient.

    If P=0 then you are golden, but it will never happen SWR of 1.1 is very good

    So the only thing you can change to lower the SWR is the minimize the reflection. What's that? and how do you minimize it?

    The transmitted power from the radio travels down the antenna cable as waves of energy and are burped ( technical term ) out the antenna into the airwaves as a larger wave.

    If the wave can roll out clean then the reflection coefficeint is near zero and you get a very low SWR ratio.

    But if the wave cannot roll out clean then some "splashes back" into the cable and 1) results in a smaller wave to the airwaves and 2) can cancel some of the energy from the next wave.

    In seriously bad cases, the reflected energy is so great that it can damage the transmitter.

    So, how to you minimize the reflection

    1) make very good connections
    2) match the antenna length to the transmitted frequencey (wavelenght varies by frequency) but most antennas are pretty close from one end of the CB channels to the other
    3) make sure the cable isn't too short.

    1) pretty obvious, but consider both antenna ground and the center. a good ground is often overlooked, both at the CB side connector and at the antenna connector

    2) Most good CB antennas are properly sized and wired to have a greater apparent length, and the better ones can be tuned by adjusting a screw on the top.

    3) 18 ft minimum cable length is another helpful tip as this reduces reflections from coming back all the way into the CB.

    So, assuming you have good connections, a good quality antenna and 18' of cable, the meter is measuring transmitted vs reflected power and showing it on the gauge.

    by optimizing channel 20, you then adjust the antenna to provide the lowest SWR. This is then a comprimise in the middle of the frequency range of the radio.

    But, if you spend time on lower channels, tune for them. or if you spend most of your time on upper channels tune there.


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    01 TJ sport with some stuff

  7. #7
    www.stu-offroad.com Stu Olson's Avatar
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    Dan-H,

    Very nice description of SWR. Don't spoil it with the 18' coax junk. It is 1970's vintage urban legend BS and can be easily proven as such when one considers the velocity factor of coaxial cable used for CB radios. I assume you are aware of velocity factor since you did a great job of explaing SWR and how it works.


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  8. #8
    www.stu-offroad.com Stu Olson's Avatar
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    RoRo,

    Button 2: NB/ANL/OFF

    NB = Noise Blanker
    ANL = Automatic Noise Limiter
    OFF = well, you can figure that one out.

    Noise blankers are designed to help reduce pulse type noise, such as from your spark plugs, lightning, etc. The drawback to most noise blanker circuits is that they can be overloaded by a strong local radio signal and cause severe receiver distortion. If this is noticed, just turn it off.

    Automatic Noise Limiter is most likely to help reduce "static"....ie, atmospheric noise that just comes into the reciever. It can sometimes cause very weak signals to be missed. Turn it on or off as the situation requires.

    Knob 5: RF Gain - in simple terms, it controls how much signal from your antenna is applied to the RF amplifier section of your radio. The RF amplifier takes the very small signal from the antenna and boosts it in size. Once boosted, it can be properly detected and then amplified again in the audio amp and sent to the speaker. If you are hearing a lot of signals from a long ways away (ie., the "skip" is coming in), you can try turning down the RF gain. This will then require a much stronger signal (hopefully from just the local guys in your group) to hit your antenna before you can hear it.

    Knob 6: Delta Tune - If you had a single sideband (SSB) radio, I would say this is for fine tuning the SSB signal so you can clearly hear it. In your case, it might be a simple IF center frequency shift, which can also help reduce interference. What ever its design, adjust it for the best sounding receive signal. If it does not see to make a differnce, turn it to the middle of its adjustment and let it be.


    The run down you were given, on how to calibrate and use the built in SWR meter, sounds correct.


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    Stu's '98TJ Sahara
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    www.stu-offroad.com

    [This message has been edited by Stu Olson (edited 07-21-2001).]
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    Stu, thanks for the comment on the SWR, but I won't rollover on the cable length issue.

    I should have stated 12' minimum, but I've never been able to route just 12' and have it be long enough.

    My calcs (done by the way back in the 70s) for 11 meter, worked out that an effective length of coax for 1/4 wave of 11m is just under 6 fett. so any cable in a multiple of 6' is a good thing for impedance matching for a bunch of reasons I've forgotton.

    So, 18' is 3/4 of a wavelenght and usually long enough to go from dash to rear of any rig so I suggest 18'.

    Anyway, its late, my phyics books are in the garage (and have been for 20 years) memory is fading, so I'll leave at at that and say, you can use whatever length cable works for you. I'll just xmit and rcv a tad better with mine

    Cheers,

    - Dan, who usually wires over kill with 18' of RG8U foam shielded coax to his 5' firestick, but lately has been running an el-cheepo mag mount because he hasn't wired his new rig up yet...



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    01 TJ sport with some stuff

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