looking to add linear

Discussion in 'CB Radio Forum' started by Mud Dog, Nov 14, 2012.

  1. twolane

    twolane Medium Load Member

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    Wow!
    Thanks so much for the explination. I don't understand the math or some of the symbols, but now do have a better grip on what he meant when he said the antenna wasn't resonating. This is very interesting to me and I'll have to think on it awhile to relate it to my little system.
    Thanks again!!!
    928
     
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  3. twolane

    twolane Medium Load Member

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    Dec 16, 2010
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    Ok now I'm starting to get it...the swr meter is not just a glorified ohmeter afterall, and theres more to the antenna system than overall resistance.
    I'm slow but I'm getting there.
    Thanks...928
     
  4. twolane

    twolane Medium Load Member

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    MsJamie,,, heres the next quest. to help me understand.

    When that guy with that big expensive meter said my swr was high measured at the antenna mounting lug, (2.3-1) why did that same meter give me/him a 1.1-1 or 1,2 at the radio?
    Whats up with that? Didn't the coax involved change what the radio sees? I'd think this would be (@ the radio) the most imporntant reading?

    Thanks for taking the time to help me understand better, whats actually going on here.
    I expect that others here are interested as well.
    928
     
  5. MsJamie

    MsJamie Road Train Member

    Two things are happening here.

    First, there is loss in the coax. A 21' piece of RG8X loses about 8% of its signal, so when it gets to the antenna (where the 2.1:1 match is), there's only ~92W there. The 2.1:1 match reflects back ~12.6% of the 92W, or ~11.6W. That 11.6W also loses 8% on its trip back thru the coax, so when it gets to the radio, ~10.66W make it.

    OK, so an SWR meter at the radio will see ~10.5% reflected instead of ~12.6%, but that will only drop it a little below 2:1. Maybe if you coil up 100' or so of cable under the dash, you'll get the SWR under 1.5:1. (You'll also have a foot warmer.)

    Obviously, something else is happening here. Envision a cup of coffee sitting in the cupholder of your truck, and the truck is idling. At a certain frequency (engine RPM), the ripples in the coffee turn into a set of concentric rings that simply bounce up and down without moving. Those waves are "standing waves", and they are dependent on both the vibration frequency and diameter of the cup. Change the cup diameter, and you'll have to find a different frequency to make the rings form. Also, you can change the frequency, and eventually you will find another speed where the rings form, but there will be a different number of them.

    Much the same thing happens in a feedline with a mismatched antenna. For certain frequency (engine RPM) and coax length (cup diameter) combinations, the voltage on the line will bounce up and down, but not move down the coax. If the transmitter is at one of these voltage nodes, the voltage is zero, so the reflected power is zero (P=(V^2)/Z), and SWR=1:1. (What happened to the reflected power? It added itself to the power leaving the transmitter and headed back down the line. So yes, there is 110W coming out of your 100W radio. You still are getting only ~90W out of the antenna.)

    Another problem you have with a mismatch is that RF currents may be flowing on the shield of the coax. This is Bad, with a capital B. This coax is radiating your precious RF, but it's INSIDE your vehicle, not outside where it's useful. Inside the vehicle, it gets into other electronics, such as your stereo, TV, engine computer, Qualcomm...

    I made some significant simplifications in my description of what happens with a mismatch. My description is what my physics teacher used to call a "500' overview of the problem". The math is rather complex, and going over it has given me a headache. :p

    Bottom line: The antenna SWR is still 2.1:1. There is still a good bit of loss from RF bouncing back and forth in the antenna cable. It's just that the coax is acting as a matching stub and keeping the radio from seeing the reflected power.
     
    twolane Thanks this.
  6. twolane

    twolane Medium Load Member

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    Dec 16, 2010
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    Thank you again for taking the time...will have to think about this as before. GOOD STUFF! Keep it coming.

    BTW,,,thanks for the simplification. I bet speakin' "truckdriver" is tough but I'm payin' attention.
     
  7. twolane

    twolane Medium Load Member

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    Dec 16, 2010
    Arizona
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    MsJamie, I've put alota thought into all you've said, so believe me you haven't wasted you time with me.

    I guess my last quest would be...

    Assuming the actual antenna swr is 2.1,,,with it being "sorta corrected" thru the coax,,,is there an issue for the radio? -OR- am I just loosing 10-12% of my "power"?
    IOW, Is the radio seeing the 1.1- 1.2 match the meter shows at the radio? (If so,,,I can live with that.)
    My outfit ain't much but works really good...would really hate to hurt this radio. I've had several great radios ruined by so-called CB experts (peek-n-tune guys) that now, I run stock outa the box only. This is why I ask these questions & why the antenna "system" for me, is so critical. I only turm the amp on when I'm DXing, or when there is some kinda emergency.
    Anyway thanks again!

    edit; just re-read your last post for the about the 5th time and finally found the answer in your last sentance.
     
    Last edited: Nov 19, 2012
  8. Mud Dog

    Mud Dog Tattooed & Insane - Forum Sparkler

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    Dec 8, 2009
    Devil's Land, kentucky
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    Thanks for all the input think I'll just stay with what I have...but since the swr talk is goin on when I 1st hooked up using a meter not the pos on the cb I got 1.5 now I have 1.7 coax is 9' . I have tip of antenna low as it'll go how can I get swr <1.5 again
     
  9. MsJamie

    MsJamie Road Train Member

    Do an SWR check throughout the band; channels 1/10/20/30/40 and see where the SWR is. My bet is that the SWR is higher at channel 1 than at 40. If so, then you need to lengthen your antenna a bit.
     
  10. Mud Dog

    Mud Dog Tattooed & Insane - Forum Sparkler

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    Dec 8, 2009
    Devil's Land, kentucky
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    0.2 (1) 1.7/1.8(19/20) 2.8(40)
     
  11. jesus curiel

    jesus curiel Bobtail Member

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    Sep 15, 2017
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    Your right in california that is a walkie talkie. Big radios 500 watts and up TS, BBI, xforce, and Dave Made!!!!!
     
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