Vivaldi steps up a notch

A reconfigurable antenna that can switch its operation between three different modes is showing promise for future cognitive radio systems.

The first multifunction antenna to combine wideband, notch and narrowband modes has been demonstrated. Motivated by the requirements of future cognitive radio systems, researchers at the University of Birmingham (UoB) in the UK and the Universiti Teknologi Malaysia (UTM) have created a frequency reconfigurable Vivaldi antenna with a high degree of flexibility. “In our antenna, we have introduced a capability to switch from broadband operation to narrower-band operations with five different subbands, and included stopbands so that interference from other communications systems can be eliminated,” said Mohamad Rijal Hamid, one of the researchers in the team. “We believe that this reconfiguration concept can also be used in other types of wideband travelling wave antennas.”

One antenna to fit all

The biggest challenge for antenna designers in ultra-wideband (UWB) and multimode systems is to control the antenna performance over a very wide frequency range that covers all the different standards and modes. With UWB-type signals, the antenna needs to have a similar radiation pattern and gain for minimal distortion over the entire operating band. In multimode systems, the multiband antenna design usually involves a challenging tradeoff between size, gain and bandwidth. Most antennas in current use are specifically designed for fixed performance in terms of frequency and bandwidth.

To support many wireless applications, the ideal antenna would be reconfigurable, allowing passbands with an adjustable centre frequency and bandwidth, along with low phase noise and amplitude distortions, and eliminating interference through multiple rejection notches in frequency and pattern. Such an antenna is necessary for future cognitive radio systems in which the aim is to actively and dynamically seek frequency ranges for operation without causing or suffering interference from other systems. Reconfigurable antennas have appeared in the literature but they are limited to one service at a time and have additional loss resulting from the switches. Wideband antennas can support all of the services but require additional filtering which increases the front end complexity. Filtering is a big issue that needs to be addressed in the design of a single antenna for future cognitive radio systems.

Ringing the changes

In their design, the UoB/UTM researchers use the well-known Vivaldi antenna, which radiates and receives over an extremely wide range of frequencies with moderate directivity. It achieves this with a gradual taper in a slot transmission line that forms a transition between a guided wave and free-space radiation. The team created their reconfigurable antenna with additional flexibility by means of two types of resonant filter in the radiating element that can be switched in or out to interrupt the flow of currents in the edges of the slot transmission line. The notch band reconfiguration is realised by a microstrip line resonator across the tapered slot on the opposite side of the substrate. Their antenna demonstrated a wideband performance at 2-8 GHz, tunable narrowband rejection between 5.2 and 5.7 GHz, and five narrow passband modes that can be selected at 3.6, 3.9, 4.8, 5.5 and 6.5 GHz.

Switching tactics

Now that they have shown these three operational modes are possible in a single antenna, the researchers are working to widen the tuning range of the band-notching capability to 2-7 GHz. They also believe that a tunable narrowband is possible rather than switched narrowband and they plan to develop their design further by using varactors in the ring resonator to replace the switches, and also examine efficiency and linearity issues arising from the switches. “Most reconfigurable antennas have moderate performance owing to additional loss arising from switches,” explained Hamid. “With future advances in switch technology, we hope that a low-loss and low-cost switch will make the reconfigurable antenna performance comparable to that of a non-switched antenna.” The UoB/UTM team is also currently working on a combination of frequency and space domain reconfigurable RF front ends consisting of wideband antennas and wideband reconfigurable band stop filters (UoB) and wideband Butler matrices (UTM).

The Letter presenting the results on which this article is based can be found on the IET Digital Library.
For further reading, please visit http://www.aael.co.uk/ and http://www.utm.my/fke/