Size Matters

When we're talking about coax and how it relates to the performance of your radio system, length does indeed matter. Here's a technical article which will help you understand why length matters, and what you can do to get the most out of your RF sub-system. http://radioproshop.com/swr.htm Once you've used the correct length coax to achieve a flat SWR you could consider reducing the run to the shortest possible for efficiency reasons, but in a mobile application where the length is already short, you likely don't need to worry about it. Leaving it at the calculated length will allow you to realize constantly accurate SWR readings, whereas if you shorten the coax after matching you won't get accurate SWR readings after that.

In short, you can optimize your transmission line by building your coax to a key length based on your desired operating frequency. I just built a coax cable for my T2000 Kenworth yesterday, and improved its performance ten fold. Here's the materials and tools I used.

Platform: Kenworth T2000
Transceiver: Ranger 2970DX-150
COAX: RG-8X 50 Ohm Direct Bury
Connectors: PL-259 Silver Plated / RG-8X reducer for PL-259
Length Calculator: http://www.k5rmg.org/calc/length.html
Installation Time: 45 Minutes

First we mounted the "outback" antenna bracket on the rear grab handle, passenger side, at the back of the cab about halfway up. Then we attached a generic center-loaded antenna purchased at a truck stop for $30. Knowing we run channel 19 most of the time, we used the length calculator (see link above) and input the freq. of 27.185 mHz, the velocity factor percentage of the coax which is .82, and calculated the length. 22.27 feet was perfect for our installation route. Next we took the segment of measured cable, soldered on connectors to one end, and attached it to the antenna. Then using zip-ties we dressed the cable on the bracket, grab handle and under the cab, pulling it forward to the passenger side firewall under the hood. Then we removed the passenger side kick panel under the dashboard (the one with the red courtesy light) by removing the 4 screws, which gave us access to the firewall.

Below the wire harnesses on the firewall the T2000 (and other trucks) have extra holes filled with rubber plugs. We popped one out, poked a hole in it, and threaded it onto the cable. Once the cable was pulled through the plug and into the cab, we replaced the plug into the hole in the firewall and made sure it was seated. Now all we had to do was push the cable end up along the right side of the fuse panel, careful not to poke it into any hot circuits, and routed it around the fuse blocks and over to the hole in the dash behind the radio mount. The cable length was just right, and had about 1/2 foot left at the radio so we could solder the other connector easily, then push the extra back into the hole. Once that was done, the cable was tested with a multi-meter to make sure the connections were solid, then we connected it to the radio.

The Ranger has a built in SWR meter, which showed 2 bars. Then we hooked up a stand-alone swr meter which showed 2:1, a little higher than we wanted. Easily fixed with a mast adjustment; and after raising the mast stinger about 1.5" on the antenna, it brought the swr down to about 1.2:1 and the Ranger internal meter showed no bars at all.

Conclusion: For 45 minutes of work, this was an excellent upgrade. The receive side was markedly better as we were now picking up signals from that Mexican woman down south about 1500 miles away. Our range test for transmit was conducted before and after the upgrade, on AM channel 19 at 100 watts. My helper stopped driving at about 25 miles out, and while she could still clearly hear me, her transmit was only 5 watts and not making the trip back to me. Before the cable upgrade we were using an 18 foot pre-built coax from the truckstop (the kind with the clear plastic cable sheath) and it was not satisfactory at all. Lucky to receive 10 miles and the cable wouldn't take more than 50 watts without choking. Later that night I talked to a guy on channel 22 from Medicine Hat Canada, about 1000 miles away. He said I was hitting him with a strong clear signal to his 75' radio tower running a Yaesu base station.

Happy Trails!