Electrically Small Antennas: Canonical Types
The term electrically small antennas (ESA) applies to antennas whose dimensions are small compared to the wavelength. As will be shown, these are mostly dipoles or loops, or minor modifications of these. Various techniques for improving performance are discussed below.
Both short dipoles and small loops are superdirective; the directivity remains at 1.5 as the dimensions decrease, but the radiation resistance decreases and the reactance increases. Thus, the bandwidth is narrow, the efficiency may be low, and the necessary tolerances will be small. See Chapter 7 for more discussion of superdirectivity.
3.2 Dipole Basic Characteristics
3.2.1 Dipole Impedance and Bandwidth
First take the case of dipole (or monopole) that is transmitting. A typical equivalent circuit is shown in Figure 3.1. The dipole radiation resistance, loss resistance, and reactance are , , and . The source and matching circuit has loss resistance and reactance and and load resistance . The important parameters are the radiated power efficiency ...