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MARC Technical Notes  CB Antenna Tuning
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          CB Antenna Tuning

This is a document that was copied over from Wings on the Internet and forwarded by De Witt Morgan KM6UK.  

> 24 Jan 1997:  Special Internet Report   
Len Winder -  

There was quite a bit of discussion a while ago about instructions for tuning CB radios/antennas on the list. I'm not sure that the original request for instructions was answered and I finally found my instructions with some additional information. After a bit of typing, I finally have this in my computer. Hope some of you find it useful. The archives also has some good information.

Instructions for tuning a CB antenna with a Standing Wave Bridge.  SWR Measurement copied from original instruction sheet.

  1. ___ Turn the transmitter off. Disconnect the antenna coaxial cable at the transmitter output.
  2. ___ Connect the "TRANSM" connector to transmitter output, and "ANT" to antenna connector. A short cable equipped with male connectors on both ends will be required between the transmitter and the meter. (*Note: adapters will have to be bought or fabricated to tune Honda radio antennas.)
  3. ___ Set the center switch to "FWD" and rotate the adjusting knob to near minimum position (counter-clockwise).
  4. ___ Turn the transmitter on, rotate the adjusting knob for full meter swing.
  5. ___ Next, set the switch to "REF". Read the meter scale. The indication will give the SWR reading directly.
  6. ___ A perfect matching, viz., 1:1 ratio, in ideal from the theoretical point of view. Adjustments on the transmitter and antenna system should be made so that the SWR is as low as possible. An SWR of 1.5:1 is considered satisfactory, taking into account the line losses and slight mismatching. 

    * NOTE:  Always take readings with transmit lever [PTT switch] depressed. 

Additional checks and tips. Before tuning:

  • Clean and check all grounds. The ground plate in the trunk of the GL1500 must be cleaned and bolts tightened. A sure way to get a good ground to the m/c main frame is to run a wire from an antenna mounting bolt to the m/c frame under the seat. I have seen several GL1500s with a significant resistance between the trunk framework and the main frame.
  • Check antenna for continuity. Two checks, from the tip of the antenna to the end of the cable center plug, and from the base of the antenna mount to the outside portion of the cable end.
  • Check for infinite resistance between the antenna element and the base. If there is continuity, don't try to transmit. Possible cause may be a crushed antenna cable causing an electrical short.
  • Check cable (coax connections) clean, tight, with no moisture.
  • Vehicle should be outside and at least 5 meters from any buildings or large objects such as trees.
  • SWR meter hooked up to CB end of coax using short connecting cables.
  • Do not touch the antenna while transmitting.
  • Do not let anyone stand close to the antenna. It won't make them sterile but it will screw up your SWR reading.
  • Objective is to obtain an SWR of less than 2 to 1 (2.0:1) on all channels from 01 to 40. If you experience a high SWR on all channels, you are probably experiencing a ground plane deficiency. RE-CHECK your GROUNDS.
  • If the SWR on channel 40 is greater than that on channel 01, the antenna is considered to be 'long' and reduction of physical length is necessary. On m/c antennas with a center loading coil, adjustment is made after loosening the top allen screws. If it is required to cut the antenna, the amount that you cut off should be in increments of no longer than 1/8th of an inch. Remember, cutting is your last resort after all possible trouble areas have been eliminated.
  • If the SWR on channel 01 is greater than that on channel 40, your antenna is considered to be 'short'.
  • Follow the instructions for your SWR. You probably will have to 'recalibrate' the meter each time you make a major change in frequency (channel change) or adjustment in antenna length.
  • Coax for 50 ohm systems employing a single CB antenna should be RG-58A/U. Dual or co-phased systems require a 75 ohm cable, RG-59A/U. Cables supplied with CB antennas should not be cut or shortened as they are a predetermined length. Do not tightly coil excess cable as this will effect your SWR.

Len Winder
'91Gold Wing SE

Page:  [ 40-10.t1997-0124.r01p01 ]  08 Apr 2014 12:15

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