Sunday, January 25, 2009

Twist & shout

My college roommate taught me almost everything I know about installing electrical accessories on cars, from sound systems to two-way radios to auxiliary lights. One of his lessons was that a CB radio isn't a "plug'n'play" proposition -- each combination of radio, antenna and vehicle is different, electrically and physically.

The quality of the antenna, and especially where it's mounted on the vehicle, can have a huge effect on something called signal-to-wave ratio (SWR) and, as a consequence, performance. If a particular setup has a high SWR (3.0-to-1 or greater) when transmitting, it can limit the radio's range, interfere with the vehicle's other electronics or even damage the transceiver. Despite that, most CB users (and some installers) don't bother to check SWR before keying the mic and yakking away.

I learned long ago how to use an SWR meter, a simple and inexpensive device connected temporarily between radio and antenna, to measure a system's SWR when transmitting on various channels. With those measurements in-hand, the setup then can be "tuned" by adjusting the effective length of the antenna.

The FireStik II antenna mounted on my TrailBlazer makes adjustments easy -- turn the knurled tip clockwise to make the antenna longer, counterclockwise to shorten it. A rubber cap slips over the tip to hold the setting.

This morning it took just ten minutes to put my new system in nearly perfect tune. Before adjustments, the SWR meter showed:
  • Channel 1: 3.5-to-1 (transmitting 67% of the radio's power)
  • Channel 40: 1.5-to-1 (97%)
  • Channel 19: 2.5-to-1 (80%)
Because the SWR on channel 1 was higher than the reading on channel 40, the antenna needed to be lengthened. A few left-hand twists later, the numbers were just what I was looking for:

  • Channel 1: 1.5-to-1 (97%)
  • Channel 40: 1.5-to-1 (97%)
  • Channel 19: 1.1-to-1 (99%)
It's impossible to get ideal (1-to-1) SWR throughout the band -- 40 different frequencies, 40 different signal-to-wave characteristics -- so my goal always has been to match channels 1 and 40 and come as close as possible to 1-to-1 on channel 19.

After disconnecting the SWR meter, I placed a cell-phone call to my wife and asked her to tune our desktop scanner to CB channel 19. Under-less-than-optimal environmental conditions -- hills, trees, a run of high-tension lines and heavy snowfall -- transmission range was more than two miles on channel 19.

That'll do just fine.