Accessories in antennas are devices for radiating or receiving electromagnetic waves. Their main purpose is to create means for the perfect operation of antennas. The accessories interconnect these antennas to the transmission and reception systems efficiently and safely. An electromagnetic coupling generally exists between the antennas and the accessories, and one can explore them to improve antenna performance. In this way, the accessories are as fundamental as the antenna itself, whose behavior depends on appropriate connections.
Among several kinds of accessories, the following are most commonly used: attenuators, baluns, chokes, coax lines, insulators, lightning arresters, planar structures, phase shifters, twin-lead. These components are described in the next sections.
Design of an Attenuator
The designer has two concerns in designing an attenuator:
1. attenuation in dB
2. input and output impedances
The attenuation is given by Eq. (1):
ATT = 10log10 N (1)
where ATT is attenuation in decibels and N is the ratio between the power absorbed by the circuit (from the generator) and the power delivered to the load.
The design starts from the desired input and output impedances and from the value of the ATT obtained in Eq. (1). Equations (2), (3), and (4) give the resistances of the “T” circuit:
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Figure 2. Resistor configuration for the ‘V’ attenuator. Zj and Zo are resistive impedances presented to the attenuator by external circuits. |
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(2) (3) (4) |