Some answers to this question contain more bulldust than the Simpson Desert ! Let’s try to make it simple.
Consider two people, each using a handheld radio with the maximum allowed 5-watts ouput (It does not matter which brand of radio, 5 watts is 5 watts ) The receivers in the radios are working just fine.
Range using UHF CB is limited to line of sight. That’s pretty well a hard and fast law of physics and practicality. (There are some cases where it’s possible to get extra range with reflections off large objects &/or freak weather conditions, however you can’t rely on that happening).
Put the two people, each with the handheld radio antenna 1.5 metres above the ground on a flat beach. Radio range without any dunes or hills in the way will be about 10km assuming the antenna on the radio will cut the mustard. With the FDP range extender antenna fitted to the radios the range will be 10km for sure.
Reduce the power of the radios to 1-watts and range will probably drop to somewhere in the region of 3km.
Put one person on top of a 6 metre dune, assuming no higher dunes or hills are in between blocking the signal, range will increase to 15km but using the small antenna which is supplied with the radio it may be less. With the FDP range extender antenna fitted to both radios 15km would be very achievable.
Now put one person up on a 70 metre hill and the line of sight should be about 39km…but you probably will not get anywhere near that range because the little antennas on the handheld are just not up to the job. Connect 6dB gain mobile antennas fitted to cars via adapters to the handheld radios’ antenna sockets and the range should be up to 39km.
3dB or 4.5dB gain antennas seem fine out to about 15km vehicle to vehicle and further to repeaters. More than 15km and you might need to start looking at 6dB gain antennas.
If you want to play around with this a bit more CLICK HERE for a good line of sight calculator.
If you require greater range you can use whats called a repeater station, these are usually situated on high ground & will relay your signal to other CB users, Range can be between 50 – 150km or more.
Out of interest here in Hervey Bay I can access a repeater over 70km away with a FDP radio using just the supplied antenna !!!, However the repeater is situated on a hill which is over 300 metres above sea level, as we say in the communications trade “Height is Might”
Repeaters are situated all around Australia – Google “UHF CB Repeaters” for more details
Get the idea?