Sometimes it's handy to have an antenna system that can be stuffed into a backpack and taken on a camping trip, or packed
into a brief case and carried to some special event where you'll be operating a temporary radio station. Equipment to build
an inverted-L or other wire antenna can easily be rolled up and packed into a container.
A mobile antenna is usually attached to some sort of vehicle -- a car, boat, or aircraft -- and used while the vehicle
is in motion. We will concentrate on antennas that can be used on cars, trucks, and vans, because this is the most common
application of mobile antennas.
Creating an antenna system for a car presents two big challenges: getting a good ground, and size limits. The ground
problem is obvious: you can't have a set of quarter-wavelength radials around the base of the antenna, nor can you use ground
rods hammered into the earth. Size is limited by the danger of snagging trees and overpass bridges, by the fact that you don't
want your antenna to look too bizarre, and by the risk of the wind tearing apart a large antenna structure.
Short vertical "whip" antennas, base-loaded or center-loaded, are the only type of transmitting antennas that have been
used by many people on frequencies near the AM broadcast band. There is a group of frequencies called "the 160 meter band"
(1800 to 2000 kHz), used by licensed radio amateurs for two-way communications. Most of the knowledge we have about mobile
transmitting antennas that might be useful in the upper end of the AM broadcast band comes from experiments done by amateur
radio operators in their 160 meter band.
Generally these mobile antennas use the vehicle itself as an artificial ground (counterpoise). Larger vehicles produce
better results than smaller vehicles, and of course all-metal vehicles produce better results than cars that are partly made
of composites (plastics). Mounting the base of the antenna at the top and center of the roof produces a measurably stronger
signal than mounting it on the bumper.
In a pinch, you could possibly use the largest mobile CB radio antenna you can find, with some additional inductance
between your transmitter and the antenna for better matching.
To learn more about building mobile antennas for frequencies below 2000 kHz, read articles about the 160 meter band in
ham radio books and web-sites. Visit a web search engine with a query such as "160 meters" and "mobile antenna" and you will
most likely find some articles or illustrations.