## 3 Multiple Reconfigurable Features in Antenna
### Frequency and Bandwidth Reconfigurable Antennas  
There is an increasing demand in multi-service radios conforming to different spectrum standards for antennas switchable to single-band, multi-band, or wideband operation. This multi-function characteristic has become an important research area [47; 48; 49; 50; 51; 52; 53; 54; 55]. There are different antenna design methods which achieve this multimode function:  
* to use a reconfigurable band-pass filter on the antenna feed-line to switch from narrow-band to wide-band operation [47; 48; 49; 50]. * to use a modified ground plane with embedded parasitic and slit structures and also slots along with active elements such as PIN diodes [51; 52; 53; 54]. * to combine different antennas which can give narrowband or wideband operation and could be used in cognitive-ratio (CR) or MIMO mobile communications [55]. Some examples are described below. Figure 4: (**a**) Schematic and (**b**) standing wave ratio (SWR), gain, and axial ratio characteristics of the antenna [44]. Reproduced with permission from [44], Copyright IEEE, 2017. In [47; 48; 49; 50], the main idea proposed was to integrate a switchable filtering element on the feed-line of a broadband antenna. In [47], a circular disc microstrip-fed monopole antenna with wideband to narrowband frequency reconfiguration was introduced. As shown in Figure 5a, the antenna was integrated with a reconfigurable band-pass filter in the feed-line. The impedance bandwidth could be reconfigured from wideband to narrowband using an active element, as illustrated in Figure 5b. For the narrowband state, a pair of varactor diodes has been used to tune the antenna response from 3.9 to 4.7 GHz with constant isolation. In [48], an elliptical monopole antenna with a rectangular slot in the partial ground plane was developed. The configuration of the antenna is composed of four resonators coupled to the split transmission line at the center along with the integrated reconfigurable band-pass filter (BPF) element onto the 50 \(\Omega\) feed-line. The frequency performance of the antenna can be switched to operate at 1.8 GHz, 2.4 GHz, 3.5 GHz (WiMAX/5G), and 5.2 GHz (WLAN), using four PIN diodes to switch between bands. With all diodes in their OFF state, the antenna provides wide bandwidth cover of all the four bands. In [49], a new narrow-band antenna into a large ultra-wideband antenna was proposed. Its configurationis the integration of an UWB monopole antenna printed on the top layer and a reconfigurable feeding structure printed on the bottom layer of the substrate. The antenna not only exhibits UWB impedance bandwidth but also could be tuned between six narrow bands, thus the slot antenna with its reconfigurable feeding structure makes the antenna suitable for CR applications. Another similar design was proposed in [50]. It is a microstrip-fed antenna including two resonators coupled to the feed-line. The basic antenna has UWB performance while each resonator can provide one narrowband, with the bi-bands mode achieved by activating both resonators at the same time. In [51; 52], two designs of integrated reconfigurable MIMO antennas for 3G/4G/5G mobile terminals was presented: an UWB sensing antenna are integrated with reconfigurable MIMO antennas  
Figure 5: (**a**) Configuration and (**b**) performance of the frequency and bandwidth reconfigurable antenna in [47]. Reproduced with permission from [47], Copyright IEEE, 2015.  
on the same substrate. In the antenna cited in [51], a pair of reconfigurable antennas with meander-line configuration, is employed on the top layer of the board. The UWB antenna on the reverse of the substrate acts as a ground plane for the reconfigurable elements. The UWB antenna exhibits a broad bandwidth of 720-3440 MHz, while the reconfigurable MIMO element can switch between different frequencies in the 573-2550 MHz range. The reconfigurable MIMO element in [52] contained modified printed inverted-F antenna (PIFA) antennas each with their digital biasing circuitry.