FACTORY PRE-WIRED 18-WHEELERS

Compliments of Firestik® Antenna Company Technical Support Team

Copyright © 1999 Firestik® Antenna Company

INTRODUCTION:
During the last year or two, several 18-wheeler truck manufactures started pre-wiring their tractors with coax cable for CB radio antennas. Soon thereafter, Firestik started receiving poor reception complaints from drivers. Their SWR readings were not the best, but still under 2.0:1. In many cases, the radio and antenna(s) came off of previously owned vehicles where they performed as expected. Knowing that, we knew we had to look elsewhere for the problem. Since we didn't know that the manufactures were pre-wiring the vehicles, we started looking for changes in the way the vehicle was constructed (e.g. the amount of composite materials used during manufacturing). Eventually, after eliminating the obvious, we came to the coax and soon thereafter became aware of the factory pre-wiring.

HISTORY:
As near as we can tell, one or more of the companies redesigned their side view mirrors and decided to make the CB antenna mount an integral part of the mirror assembly. To make the installation easier in the production setting it must have made sense to them to have the mirror assembly pre-wired with the coax. Instead of using 18 feet of good quality coax and routing it from the mirror to the radio, they choose to go modular. To do that, they couldn't use PL-259 connectors due to their relatively large diameter. Instead, they used Motorola pin-plugs. Making choices based upon cost and production efficiency versus choices based upon quality and performance has created a communications disaster. It makes no sense. First of all, the cost between basic 70% shielded mil-spec cable and hi-quality 95% shielded coax is less than three cents per foot. Secondly, instead of sticking to what has been a proven trouble free coax length (18-feet) they used just enough to get from the antenna bracket to the radio. Thirdly, Motorola pin-plug connectors were made for AM/FM radio antennas, not for 2-way radio communications. And finally, additional connectors in the line adds resistance to the system and creates additional points for corrosion, shorts and broken wires. Let' summarize the pre-wired cable program:

  1. Cheap quality coax.
  2. The wrong length of coax.
  3. The wrong type of connectors.
  4. Seven connections instead of three.

It is no wonder that poor communications are the results.

FLAWED APPROACH:

Mechanical and production engineers do not understand the critical points of communications. In this case, it is obvious that little or no thought was given to the technical aspects of CB communications. The truck manufacturers totally failed to understand how the over-the-road trucker uses and depends on his radio. If they had, they would have made sure that the energy delivery line (coax) from the radio to the antenna was as perfect as the fuel line from the pump to the injector. Today's big rig drivers push their radio equipment requirements to the edge. They use power microphones, modulation kits, wideband radios and power amplification. From what we've seen of the pre-wired trucks they don't even work very well with a basic CB radio. It is no wonder that they fail even worse when challenged with "enhanced performance" products.

A communications engineer would never use connectors that were not suited for 2-way radio communications. And, they would avoid making in-line connections strictly for the convenience of production. Insofar as coax is concerned, to sacrifice any quality for the sake of cost is a very poor decision. Furthermore, it has been shown time and time again that eighteen feet of coax fixes coax resonance problems created by mismatches between the antenna and available counterpoise. Every practical rule of coaxial lead lines are being broken or abused by the pre-wiring truck manufacturers.

SOLUTION:

The only way to fix the problem is to rewire the vehicle with better coax. At a minimum, you should use stranded center conductor coax with a minimum of 95% shielding. If you use cheap coax you'll get cheap performance. Single antenna installations should use either 18 feet of RG-58A/U (for installs using up to 750 watts of power) or 18 feet of mini RG-8U (commonly referred to as RG-8X) if power exceeds 750 watts. Dual antenna set-ups should use 18-foot leads of RG-59A/U going to each antenna. Any additional components inserted in the coaxial line between the radio and antenna (SWR meters, amplifiers) should be connected with either RG-58A/U or RG-8X regardless of whether you are running single or dual antennas. In our experience we have found that jumpers between the radio and any other in-line equipment should be as short as possible.

It would have been nice if the truck manufactures used good coax with the proper connectors. But, they didn't! In our opinion, it would have been better for them not to pre-wire the trucks than it was to do it so poorly. Most drivers assumed that there was something wrong with their antennas and started spending money trying to fix the problem with new antennas. All along it was the coax.

From what we have heard, it is a real pain getting to the coax in the new trucks. Across the country radio shop installers are secretly thanking the truck manufacturers because they are getting all kinds of re-wiring work and those of us who have quality coax have seen an upsurge in the demand for co-phase harnesses. If you don't have the time and/or desire to change the coax yourself, you will need to find a radio shop to do it for you. Then after you've spent the time or paid the bill, write to the manufacturer and tell them to quit using garbage coax in their trucks. The additional cost the manufacturer would need to add to the price of the truck for good coax and connectors would be far less than what it is going to cost you to re-do it.

Several times a year we get tech calls from installers who place dual antennas on their vehicle and run RG-58 or RG-8X from each antenna to a T-connector at the back of their radio only to find that the system "doesn't get out". You should not use 50 ohm coax on a ground plane dependent dual antenna installation ... it MUST be 72 ohm RG-59 type coax. These flawed installs can be misleading because SWR tests can show exceptionally low SWR, making the installer think that all is well. However, the impedence of the antenna system does not match the requirements of the radio and therefore, the output power of the radio is greatly reduced. In several tests, we found that a 4 watt radio would only generate 1.75 watts of output power which is the equivalent of having an SWR reading that exceeds 6.0:1.


Firestik Antenna Company - 2614 E Adams St - Phoenix, AZ 85034 - (Tel) 602-273-7151 - www.firestik.com

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