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307
| | Posted on Friday, November 30, 2001 - 8:48 pm: | 
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Antennas on Trucks
Here is some useful information about putting a mirror mount antenna on the mirror of a tractor/trailer truck . In my shop I have had many trucks that come in with the same old problem of SWR trouble . Most of them drive into a truck stop and have a mirror mount popped onto the truck , screw an antenna on and away they go . I have seen the SWR change just by the way the door was "slammed" shut . I have seen the old "attach a wire to the bracket and run it to the body ground" trick . Most people do not know that this trick will sometimes fix the problem but a lot of the power out of the radio is re-routed to the ground of the truck because the wire will act as an inductor and very well could be resonant at the same frequency as the CB . You can not read the ground with an ohm meter because the meter just says it is shorted at the frequency of the meter . In most cases the meter is a Simpson 260 or equivalent and is in the 20 Khz. range . This is a long way from 27 Mhz . In most cases that same wire is "OPEN or RESONANT" while power at 27 Mhz . is applied using the CB . I have seen antennas that were "cut" down in length so far that they lost all their performance .And here is why there are so many problems .
Truck mirrors , especially the big trucks all have massive vibration when traveling down the highway . So what the truck manufacture does is several things and I will mention some of them . If you look at the mount that the mirror uses to stay on the truck , in most cases you will see that there are rubber washers between the door and the mirror and that the bolts that hold the mirror mount on go through the rubber not touching the metal . This is automatically a problem , loss of ground . One way that I have fixed many of the SWR problems is to remove the one bolt closest to the antenna bracket , remove the rubber insert and remove the paint from around the inside of the door where the nut is attached to the bolt . After doing this you will notice several things....You do not get shocked when touching your "box" while transmitting . Your SWR will be FLAT and in most cases you know that squeal you get through talkback??? Well it will be gone . Ever key up the mike and unkey and your box stays keyed up??? Well that to will be gone . The last thing to check when installing a mirror mount is to look at the door latch . Most of the time the latch has rubber around it to keep the door from vibrating . This to will cause problems if not carefully removing it and readjusting the doors. And last but least the hinges are usually grease fitted and sometimes a heavy braided wire less than four inches long has to be added across the hinge (bolt to bolt) on only one hinge . Hope this helps.... |
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