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Advanced Antenna Theory


Q: Can you explain antenna propagation and the proper grounding planes for the different types of antennas available?
A: Ground planes are required to properly impedance match quarter-wave or collinear antennas launched directly from a conductive surface. The first element relies on image theory, which is described as a virtual antenna of equivalent length and spacing below the ground plane performing cooperatively with the physical antenna to provide impedance matching and far field radiation. Additional half-wave or 5/8-wave radiators can be stacked vertically to achieve higher gain. The proper ground plane dimensions are defined by a ground plane which is much larger (in both directions) than the antenna length.
Q: Is the antenna length detrimental to reception?
A: Antenna length is critical to maximize performance and provide the proper radiation characteristics.
Q: Explain the difference between unity gain, 3 dB gain, and 5 dB gain.
A: Unity (0 dBd), 3 and 5 dBd designs differ by the number of elements incorporated to achieve increased gain. This gain increase is achievable by stacking multiple elements in a collinear manner to compress the vertical plane pattern and direct more energy along the horizon, toward the cell site.
Q: How much loss is induced with each connector/fitting that is installed on the cable?
A: Connector insertion loss is negligible with respect to the overall cable loss. Some manufacturers suggest adding 0.1 dB for every 2 connectors within a cable system for planning purposes.
Q: If you cut a 3 dB whip to match a 1/4 wave antenna, are there any concerns?
A: The only concern is that the technician has the appropriate RF equipment to verify the VSWR or impedance match of the cut antenna. Actual physical length of the antenna is dependent on the mount type and radiator diameter, in order to optimally match the antenna.
Q: Will dual band antennas work as well as single band or do they match the antennas in the middle of the frequency range?
A: Properly designed dual band antennas will provide a good impedance match (i.e. 2:1 VSWR) over both bands. Dual band designs should be designed for resonance and proper phasing in each band and not centered between the bands. However, compromises must be made with respect to the radiation efficiencies in each band. Single band antennas are optimized for single band performance and typically perform better than dual band antennas, in their respective band of operation. Reference "Dual-Band Vehicular Glass Mount Antennas", Antenna Specialists bulletin SD-1301, April 1999.
Q: Explain the theory behind portable antenna design and their length with respect to operational wavelength.
A: Traditional portable antennas are generally less efficient, short radiators that are impedance matched to the radio circuitry and case. At higher frequencies (shorter wavelengths) such as 800/1900 MHz, end fed designs can accommodate true -wave radiators that are independent of the case and user, providing efficient dipole radiation characteristics.
Q: Can you explain how a coupling box works?
A: The coupling box is an RF circuit that acts as an impedance transformer and coupler to the antenna whip through the glass. Optimally designed coupling boxes also provide a balanced impedance transformation between the coaxial cable and the antenna. This balance effectively eliminates unwanted RF currents on the outside of the coaxial cable and was specifically developed and patented by Antenna Specialists for the original 800 MHz "On-Glass" antenna.
Q: What's the difference between elevated feed and flat surface roof mounts?
A: The traditional roof mount antenna relies on the ground plane of installation to properly impedance match the first element as a 1/4-wave radiator. Subsequent stacked and properly phased radiators can be added as 1/2- or 5/8-wave sections for increased gain. Roof mount antennas are known as ground plane dependent.

The elevated feed antenna raises the feed point above the ground plane surface by either center feeding or end feeding the first element as a dipole or
1/2-wave radiator. Radiators can then be serially stacked in a collinear fashion to achieve higher gain. The elevated feed design is defined as ground plane independent.




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