First piece of advice is to know the application well- what is it you intend to do with the product using antennas? Make sure you understand the constraints that will be placed on your communications link- regulatory, environmental, kinematics (motion) involved with the two end-points of the communications link, etc.
Second, understand the receiver sensitivity, the transmit power, and the data rate expected to be used in the link. Understand the ability of the receiver to detect the symbols transmitted- what is the energy per bit transmitted, and received? Understand the practical aspects of connector and coaxial loss for both endpoints of the communications link.
Third, understand the propagation issues associated with the frequency to be used in the communications link- atmospheric absorption, rain absorption (fading), also, look into Fresnel effects of the proposed antenna installation- lower to the ground increases Fresnel effects, higher above ground decreases Fresnel effects. These effects are not modeled in free-space propagation models. Be ware!
Fourth, and finally, determine the gain required by the antennas in the link- overcome all the losses and margins under consideration above, and add a percentage to allow for slight misalignments of the antennas (especially if they are polarized). Now one can determine if there are commercial, off-the-shelf antennas that fit the application, or if custom antennas must be built. Remember- you don't get something for nothing. High-gain antennas come at the price of narrower beam widths, making mobile use more problematic. Omni-directional antennas also come with a price- less gain, and more susceptibility to interfering signals from directions other than that of the desired station.
~Shane J, Sr. Staff Systems Engineer, Mesa, America
For CB's you must realize you need a ground plane. If you have a non-reflective surface (fiberglass, aluminum, wood, or ungrounded reflective metal) you must utilize an antenna with built in counterpoise or ground plane in order to maintain a VSWR within safe operating range for your transceiver.
~Nate V., Technical Support, Appleton, WI
1dB difference may not seem a lot, but it could make a big difference in the functional system performance. 1dB antenna gain difference in a transceiver means 2dB difference in the path-loss budget. May come to a 6dB loss in SNR or picture quality when WBFM is used.
~Math P, Research & Development, Brunssum, The Netherlands