Non-stop rejection

21 May 2012

An ultra-wideband (UWB) antenna for short-range communications over 3.1 – 10.6 GHz that can reject interference from WiMAX and WLAN without needing extra bandstop filters has been presented by a team at the Surrey Space Centre at the University of Surrey in the UK.

Narrow to wide

UWB communication systems are a very attractive prospect with their increased channel capacity and high data rate transmission over narrowband communications. UWB technology can also enable applications including high-resolution imaging such as UWB through-the-wall radar, UWB ground penetrating radar and UWB biomedical imaging.

With the antenna being one of the key components in these systems, an increasing number of researchers in both academia and industry are focusing on the design of UWB antennas. The challenge is to create an antenna that has good impedance matching across a wide frequency band, radiation stability, a compact size, a low profile and a low fabrication cost.

Interference from existing narrow frequency bands (WiMAX at 3.3–3.6 GHz, and WLAN at 5.15–5.35 GHZ and 5.725–5.825 GHz) also presents a challenge. Adding a bandstop filter can increase the cost and physical size, so UWB antennas with a good band-notched performance are needed.

Triple reduction

The UWB antenna presented by the University of Surrey team in this issue of Electronics Letters was developed as part of a UWB radar project for through-wall imaging applications, such as for homeland security, human rescue operations and security checking. Also contributing to this work were researchers from Northwestern Polytechnical University in China who provided insight into the theoretical analysis of the antennas.

The team’s CPW-fed double-layer antenna is made up of a circular radiating patch on one layer and a rectangular ground plane with a circular slot on the other layer. Unlike previous symmetrical designs, the radiator is offset from the centre of the structure to make the antenna asymmetric. This has the advantage of only one band-rejecting element being needed for each frequency band to be excluded, compared to an even multiple of elements needed in the symmetrical case owing to the symmetrical current distributions.

Another key feature is the use of three-quarter-wavelength strips connected to the circular slot to act as the band-rejection elements. These are smaller in size than the more commonly used half-wavelength strips or slits and, with a smaller quantity being needed as well, allows a more compact antenna to be made. Most triple-band antennas are over 30 x 30 mm in size, but the Surrey team’s antenna is 24 x 29 mm, and their approach may point the way to further size reductions.

The team’s antenna also demonstrates a good band-notched performance with VSWR ≥ 10 and a significant gain suppression of 15 dB at the required frequency bands. This is a significant improvement over most previous designs which were more limited with VSWR ≤ 10 and a gain suppression of ≤ 10 dB.

Directing the band

Further development for the researchers will include tackling problems with the stability and performance of the antenna at high frequencies. This is a general problem with the planar UWB monopole antenna, and the researchers believe that novel UWB elements or feeding techniques will need to be investigated.  They will also be looking to create a reconfigurable band-notched UWB antenna by integrating MEMS, varactors or pin diodes between the strip and the circular slot, and they will apply their approach to the design of directional UWB antennas such as tapered slot antennas.  

Alongside their UWB radar work, the Surrey Space Centre group also has many other projects underway. These include the development of next-generation satellite communication array antennas in the millimetre-wave band, low-cost small smart antennas for wireless communications, antennas and radio frequency/microwave front ends for global navigation satellite systems receivers, and spaceborne synthetic aperture radar for Earth observation.

The Letter presenting the results on which this article is based can be found on the IET Digital Library.

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Cover of Electronics Letters, volume 50, issue 08

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