Few people remember today that CB radio once shared its spectrum with ham radio. The FCC removed hams from what was also known as the 11 meter band in the late 'fifties as the newer Citizens Radio Service gained popularity with farmers and business owners.
Nonetheless, CB would obviously never have become a pop culture icon if it weren't for the Arab oil embargo of the 'seventies. President Richard Nixon, desperate to distract people from Watergate, instituted a fifty-five mile an hour speed limit in order to conserve fuel. (At that time my father had a 1962 International Scout with a rather tired four-cylinder engine. The night President Nixon made this announcement, Dad turned to me with a grin and said, "Well, son, now the ol' Scout will go as fast as any of them!"). Truckers, with bills to pay and schedules to meet, couldn't be bothered to obey the new speed limit, and adopted the CB radio as a means to alert each other to the presence of law enforcement. Soon, other motorists picked up the practice. The era of the convoy, of "Breaker, breaker!" and of "Ten-four, good buddy!" had arrived.
Somehow, we survived the first oil embargo, the second Iranian embargo that coincided with the capture of the American Embassy in Tehran by Islamic militants, and the fifty-five mile an hour speed limit. Twenty-one cent per gallon gasoline was gone forever, but in the nineteen eighties the fifty-five mile an hour speed limit was repealed. Most motorists abandoned the CB radio, especially as the cellular phone came in and gave them a better way to communicate while mobile. Around interstates one still hears a little chatter on channel 19, but most of the other channels are dead as doornails these days.
Actually, this lack of activity is an advantage. CBs can still be had cheaply from many sources, and they are once again being used as the service was originally intended, that is, by individuals wishing to communicate for business or personal reasons. My wife and I, for example, use CB radio for base to mobile communications around our little farm, and for car-to-car communications when we are forced to use two cars for long road trips. Of course, if she would just get her ham license...
CB radio uses the upper end of the shortwave spectrum, around 27 MHz. This means that signals are only reflected off the ionosphere (and so propagated great distances) during the extreme peaks of the solar cycle. Most CB communications are therefore dependent on groundwave propagation, which decreases as frequency increases. Since we are at the upper end of the shortwave spectrum, groundwave coverage is actually quite poor at these frequencies. Hey, the FCC didn't want people to use the Citizen's Radio Service to work DX, and they chose these frequencies with that in mind...
Most mobile stations use quarter-wave whips, which work against the metal of the vehicle for a ground plane. Therefore, the more centered over the mass of the car the antenna is, the better the results will be. Many truckers like to use phased verticles; but these must be more than eight feet apart for them to project the signal forward and backward, as intended, so the signal goes further up and down the highway. If the antennas are closer together, the signal goes side to side, and reception up and down the road is actually impeded! For a long time users of quarter-wave phased verticals seemed blissfully ignorant of the spacing requirement, but one sees very few phased verticals these days that are not mounted on the side mirrors of eighteen wheelers, where the distance between them can approach the critical eight-foot seperation.
The nominal legal power limit for CB operations is about four watts; however, like the firing of a cannon every time the King of Denmark's health is drunk in Hamlet, this custom is held more in the breach than in the observation. Truckers routinely run 500 watts or more; base stations run similar levels of power, and in addition use beam antennas. Legal four-watt operation, such as my wife and I observe, is good for about four or five miles under favorable conditions; high-power operation can extend the range up to ten miles or more, even if the operator gets no help from the ionosphere.
The mass production of higher-power amateur radio gear capable of working on eleven meters after modification has allowed a great deal of out of band operation. It also allows greater use of single-sideband or SSB operation, with all its advantages, whereas regular CB operation is mostly AM, with only occasional use of SSB. 27.500 MHz is the calling frequency for this type of operation, often called freeband operation. This is where many of the people who used to DX on the CB channels are transmitting nowadays, which helps account for the lack of activity on the actual CB channels themselves. This kind of operation is highly illegal. It can be interesting to listen to; but you will also hear a great deal of offensive language, intentional jamming, pointless music broadcasting, etc., so be forewarned!