1. Field of the Invention
The present invention relates generally to antennas, and more specifically to a boot for spring mount in a mobile antenna.
2. Description of the Related Art
The physical size of an antenna largely depends upon the purpose for which it is to be employed. For example, an antenna for receiving a particular frequency range must have an electrical length capable of resonating within that range to achieve optimum reception. Generally, lower frequencies require longer lengths because the wavelengths at lower frequencies are longer, but limitations in use often demand design modifications to achieve appropriate electrical length in a smaller space. It is known for antennas in some applications on mobile vehicles to be 10 feet or more in length.
Such antennas sometimes have a thin, dielectric, flexible core that carries the electrical radiator and they are mounted to a vehicle by way of a spring. These types of antennas are known as “whip” antennas because the flexible core and spring together absorb energy from forces acting on the antenna, such as impacts. If a whip antenna were to impact an object while the vehicle is in motion, the flexible dielectric core and/or the spring can absorb the force of the impact, preventing damage to the antenna or its mounting.
Some antenna applications, however, are complex, requiring multiple frequency bands, electrical lengths, and other devices that make the use of whip antennas impractical. Such antennas may require diameters of 1 in. or more at a length of 10 feet. The less flexible an antenna is, the more the spring must absorb the energy of an impact against the antenna. It has been observed that an antenna having a molded or extruded fiberglass piece 1¼ in. in diameter and 10 feet long will fail when the antenna is impacted at its midpoint on a vehicle traveling 25 miles per hour. Failures occur either in the spring or in the dielectric piece, or both. These failures can occur both at initial impact and upon the antenna's recoil from the impact where the antenna's mass causes excessive extension of the spring and unnatural forces acting on the spring mounting.
A spring which is too limp will allow over rotation when the antenna hits an obstruction. A spring with larger wire has less elasticity and absorbs less energy when the antenna hits an obstruction, causing the antenna to absorb more of load. Simply changing the spring does not offer a satisfactory solution.