From the factory, CB radios do not put out much power, perhaps around four watts. Unless the person you want to listen to is across the street or using an amplifier, you're going to want an outdoor antenna tuned for 11 meters (CB frequencies). The higher up it is and the longer/taller it is the better. This can be a wire, or a vertical antenna. You'll need some coaxial cable and more than likely an adapter to connect the cable to your scanner. The wire solution will be the most inexpensive. You can position it vertically, on a slope, horizontal or whatever, but each one of those positions will affect the lobe (RX direction). If you want to listen to people driving around and talking on the CB, then you should position it as vertical as you can. There are plenty of DIY wire antenna plans out there on the Internet.
As an example of antenna results, I just dialed up some local guys chatting. One of them is a little over eight miles away and he is using an outdoor antenna. Here are three outdoor antennas I used to receive:
- Using a discone antenna 25' AGL, I could barely tell he was there and I could not understand him.
- Using an OCF (Off Center Fed) inverted V dipole with an apex 33' AGL his signal was an S5 and clear audio.
- Using an Imax 2000 vertical antenna that is 24' long and mounted on 40' of mast, his signal was an S6. This antenna is tuned for 10 and 15 meters as that is where I use it. Still, it picks up 11 meter CB conversations well enough.
An indoor CB antenna is sort of unrealistic. The wavelength CB uses is 11 meters (36 feet). A halfwave wire dipole tuned for 11 meters may work for you. It is a compromise, but such is life. There are shorter antennas that are loaded with a wire coil in order to get closer to the length needed. Remember, most people using CB are already compromising on their antenna length and height. If you really want to hear them you will want to avoid compromising as much as possible.