26.1.2.4 Antenna Q
As an antenna becomes smaller in relation to the wavelength, the stored energy increases,
representing an increase in the reactive portion of the antenna impedance as seen at the
feedpoint. The antenna quality factor, Q, can be loosely considered as the ratio of the
reactive term of the impedance to the resistive or dissipative portion.
Q ~ (reactance/resistance) ~f/??†f (10)
A higher Q implies a narrower bandwidth. So, as the antenna becomes smaller the
reactive portion increases in relation to the resistive portion, increasing the Q and reducing
the bandwidth. This behavior is readily observed in the Chu-Harrington curve in Figure 2.
In the Chu-Harrington space, the best antennas achieve radiation efficiency approaching
100% and fall just above the 100% efficiency curve indicating the reactance to resistance
ratio (i.e., Q) is minimal. That means that the antenna exhibits the broadest instantaneous
bandwidth without using an additional tuning mechanism.
26.1.3 Fundamental Limits of Electrically Small Antennas
It is well known among antenna engineers that the design of a small antenna will be a
tradeoff between its dimensions and its electrical performance, and furthermore that
physical laws determine the ultimate limitations inherent in any design.
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