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just don't get it

Discussion in 'Communication (CB | GPS | HAM)' started by dubl_t, Apr 23, 2003.

  1. dubl_t

    dubl_t 1/2 ton status

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    just don\'t get it

    Ok, at 1st radio was recieving but not sending. Took it to shop where tech said radio works fine, not enough coax (9'). So installed 18' coax and now all's I get is white noise with faint voices. Could it be the antenea? K40 mounted on fender oposite of am/fm radio antenea. Thanks for replies, Tom.
     
  2. Dale fan

    Dale fan 1/2 ton status

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    Re: just don\'t get it

    Does it have a RF gain adjustment knob? You might need to adjust it. I would look into the antenna as not having enough coax is a bunch of bull. They will work with any length.
     
  3. Judd

    Judd 1/2 ton status

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    Re: just don\'t get it

    What he said, the 18ft of coax "Golden Rule" is bull.
     
  4. 4GUNZ4X4Z

    4GUNZ4X4Z 1/2 ton status

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    Re: just don\'t get it

    The coil in the K40 could be busted, you got another antenna? A solid ground on your antenna?
     
  5. 88Silverado

    88Silverado 1/2 ton status

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    Re: just don\'t get it

    Try a different antenna and listen for the difference.
    As mentioned, you could have a bad antenna or connection.
    Check the coax and make sure its not shorted (disconnect the antenna from its base and use an Ohm meter to check between the center conductor and shield for shorts then check end-to-end for good continuity)
    The 18' coax issue is BS. proper place to tune the antenna for resonance is at the antenna base. Since most people cant do this they check it at the radio end of the coax with a VSWR meter. The reading here will not be accurate depending on the length of the cable attached to it. The antenna impedance will repeate itself every 1/2 wavelength down the cable. If a quarter wave antenna is 9' (standard whip antenna) an additional quarter wave is another 9' or total of 18' which is a half wave. However, you need to take into account the velocity factor of the coax (about .66 for standard teflon type) so the actual length comes out to around 22-23 feet. With a 22' length of coax between the antenna and radio would allow you to reasonably tune the antenna from the radio end.
     
  6. 88Silverado

    88Silverado 1/2 ton status

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    Re: just don\'t get it

    OOps math error, formula for full wavelength of coax. (984\freq*Vfact) So 984\27=36.4 36.4*.66=24' 24\2=12

    12' is a halfwave for standard coax, therefore any multiple of 12 should repeate the antenna impedance at the radio for tuning.
     
  7. Z3PR

    Z3PR Banned

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    Re: just don\'t get it

    Damn, I thought I knew alittle bit about radio's, but you blow me out of the water. I bow down.
     
  8. bryguy00b

    bryguy00b 3/4 ton status

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    Re: just don\'t get it

    uhhh run that by me again!!?? /forums/images/graemlins/eek.gif /forums/images/graemlins/ears.gif
     
  9. 88Silverado

    88Silverado 1/2 ton status

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    Re: just don\'t get it

    OK...RebelK5 can jump in here at any time and bail me out /forums/images/graemlins/thumb.gif

    The radio, coax and antenna are designed to work into the same Impedance (similar to DC resistance but calculated at RF frequencies) Basically they're 50 Ohms. The radio has a 50 Ohm output, the coax is 50 Ohms and the Antenna needs to be "Tuned" to 50 Ohms. When the system is matched you have a 1.1 VSWR or perfect match. If one of the components is mis-matched you start increasing the VSWR. For instance the radio is 50 Ohms and the antenna is 100 Ohms it works out to a 2:1 VSWR.

    Now...VSWR or (Voltage Standing Wave Ratio) of a high enough level (greater than 2:1) can cause some problems for "Transmit" not receive. Thats because the radio sends power to the antenna, the antenna is not tuned to accept or absorb that power and due to the mis-match energy is reflected back to the radio where it is again sent to the antenna and back and fourth. Some radios dont like high SWRs and they can cause damage to the outputs. The reflected signal that bounces back and fourth can be lost by RF losses in the cable. Smaller lossy cable deplete this energy as heat faster. A lossy cable can make a bad the SWR look good because the signal travels down the line and it looses some energy then reflects back loosing more energy making the reflected power level look lower. Dont get caught up in a low SWR means I have a good radiating system. A 50 Ohm resister will have a 1:1 match but radiate nothing /forums/images/graemlins/eek.gif

    A simple way to think of this Reflection issue is to think of yourself in the center of a round swimming pool. Now jump up and down and you will start waves that propagate out (similar to a radio transmitting a signal) now when the wave gets to the edge of the pool it "reflects" back towards you (this is the reflected wave from the mis-match of the antenna) now you will notice there are peaks and nulls when the outgoing and reflected wave intermix (this is what happens in the coax) at the peaks (where the phase of the outgoing and reflected add up) the SWR is high, at the nulls (where the phase of the outgoing and reflected cancel) the swr is low. Also, because of this reflection your power output can look Higher or Lower than normal. You cant have more power out of the radio than the radio can output, even though the meter may look like it.

    Now...depending on the length of the coax you can be anywhere between a peak and a null and by trimming the coax you can make the SWR look good at the radio by hitting a length where the reflected wave cancels the transmitted wave at the transmitter and the SWR "looks" low even though there is a mismatch. Thats why "Trim the coax until the SWR is low" is BS.

    OK...that said, a signal or better yet, the impedance of the radio or antenna "repeats" itself every half wave or every 180 degrees. With that, you can actually tune the coax to be a even multiple of half wavelengths thereby having it repeat the impedance from the antenna back down the coax towards the radio so the radio end (where your SWR meter is connected) will see the actual match (minus small losses in the cable) of the antenna allowing you to tune the antenna. So if you had a half wavelength, or multiple of half wavelength the impedance of the antenna (shown in VSWR) will repeat itself.

    I have heard it said "make the coax 18 feet long and everything will work" The belief is a quarter wave antenna (102" whip), is 9 feet. If you double that to a half wave we have 18 feet. See how that works. The problem is, the "electrical" length is different when your using air as the propagation medium or another medium like teflon which is inside a coax. RF energy will travel slower in a dielectric other than air thats why you see things like VF or Velocity Factor associated with coax. You have to calculate the length by taking into account the Velocity Factor of the dielectric medium. Standard Teflon is approximately .66VF Foam filled types are around .88. Thats why a half wave length of coax is longer than a half wave antenna because the singal is slowed down due to the dielectric.

    I dont know if I have completly confused everyone with this explanation and maybe RebelK5 can state it better. We'll go from here. /forums/images/graemlins/thumb.gif
     
  10. rebelk5frk

    rebelk5frk 1/2 ton status

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    Re: just don\'t get it

    [ QUOTE ]
    OK...RebelK5 can jump in here at any time and bail me out

    [/ QUOTE ]

    I think you did a pretty good job by yourself /forums/images/graemlins/thumb.gif

    If anyone is still confused I might be able to clarify.. /forums/images/graemlins/ooo.gif
     

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