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All specifications are subject to change without notice
or obligation. All rights reserved. Part No. 5022-3001-A
Problems of base station antenna design
What is so hard about designing a vertical base station antenna? It's not a trivial
question, because four important, interacting design objectives must be dealt with
simultaneously.
- Decoupling. The antenna must be so designed that the
coaxial cable and mounting structure will not become inadvertent parts of the antenna. A
properly designed vertical antenna is said to be "decoupled" from the feed-line
and supporting mast. Failure to achieve decoupling can ruin the radiation pattern of the
antenna, drastically lower the gain, cause the direction of maximum radiation to be either
raised or lowered from the horizon, and allow radio frequency energy to be guided down the
outside of the feedline to the transmitter, where it can be coupled into the electric
wiring, telephone wiring, and other electronic equipment.
- Radiation Pattern and Gain. The amplitudes and relative
phases of the currents excited on the radiating portion of the structure must be
controlled to produce the required radiation pattern and gain.
- Input Impedance. The input impedance of the antenna must
be well matched to the characteristic impedance of the coaxial feedline over the entire
band of frequencies for which the antenna is to be used. In other words, the standing wave
ratio (SWR) should be low (under 2 to 1) over the required frequency band.
- Mechanical Design. The antenna must be mechanically
designed to withstand severe environmental stresses, including high winds, high and low
temperatures, icing, etc. The mechanical capabilities of the antenna provide the highest
strength allowable, without significantly increasing the cost. Any impedance matching
network built into the antenna must be dimensionally stable and weather-protected in order
to preserve the tuning. The connector to which the feedline is attached should also be out
of the weather.
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