As is well known, the radiation properties of antennas depend on the relationship between some characteristic length in their structure and the frequency being radiated. For example, dipole antennas are nearly resonant at a length close to one-half the wavelength of the excitation signal . It is logical, then, that in order to increase the flexibility and usability of antennas, one would look into ways of reconfiguring their structure and dimensions.
Tunable Dipole Antennas:
In the tunable dipole antenna  (Figure 4.21), a set of symmetrically located center-fed and segmented dipoles are networked via a twodimensional array of MEM switches.
Then, by closing and opening the MEM switches in an intelligent manner, the shape and length of the antenna are reconfigured and, consequently, its radiation pattern. The concept can easily be extended to a variety of antennas, such as Yagi-Uda antennas, log periodic antennas, helical antennas, and spiral plate and spiral slot antennas . An extension of the tunable dipole antenna is the reconfigurable multiband microstrip resonator antenna, as shown in Figure 4.22 . In this antenna, a microstrip resonator 16, on substrate 20, designed to radiate at the highest frequency (band) of interest, is excited by a signal traveling down the microstrip line, printed on substrate, through a coupling slot, defined on ground plane. The lengths of both the resonator and the coupling slot are chosen to be approximately one-half of the wavelength  corresponding to the highest frequency of interest. The antenna is made reconfigurable by MEM switch, which, when in the on state, changes the resonator length to include the additional piece of transmission line. The physically longer resonator resonates and radiates most efficiently at a lower frequency (band).
To maximize the power coupling from microstrip through slot to the resonator, the microstrip is terminated by quarter-wave open-end tuning stubs, which induce short-circuits at the junction of the highest and lowest frequencies of interest with the microstrip. Then, by exploiting the high isolation of the MEM switch in the off state, and its low insertion loss in the on state, the resonator length can be switched between its high-frequency configuration, main resonator element alone, and its low-frequency configuration, which includes the extra tuning line segment. This realizes a multifrequency tunable antenna.