prof. Ian Hunter
e-mail: i.c.hunter@leeds.ac.uk

Education
Ian Hunter graduated from the University of Leeds , School of Electronic and Electrical Engineering with a first class honours degree in 1978 and a PhD in 1981.

Industrial Experience
After graduating he spent a number of years in UK and US industry, including periods at Aercom Industries Inc, Sunnyvale California, KW Engineering, San Diego California, where he led a team developing broadband microwave filters for EW applications, and Filtronic Components Ltd, UK where he worked as a principal engineer on a wide variety of defense related projects.
From 1995 to 2001 he was a Fellow Engineer at Filtronic Comtek, Shipley UK where he developed and patented new classes of Microwave Filters for Cellular-Radio Base-Stations.
He has also acted as a consultant for TDK Electronics, Ireland .

Academic Positions
From 1991 to 1997 he was a member of the academic staff in the School of Electronic and Electrical Engineering at the University of Bradford , where he was promoted to Senior Lecturer in 1995.
He joined the University of Leeds , School of Electronic and Electrical Engineering as a part-time Senior Research Fellow in 1999, and became a full-time Reader in 2001 and was promoted to Professor in 2003. He presently holds the Chair in Microwave Signal Processing at Leeds and is the Research Director for the School. Presently he is responsible for the RAE 2008 submission for the School. He teaches Circuit Theory, Microwave Engineering, Electromagnetism and Antennas..

Research
His main research interest is in passive and active microwave devices, and has supervised 24 PhD students working in this field. More recently he has been investigating applications of microwaves in agriculture. He also collaborates with physics colleagues on projects relating to THz propagation in low dimensional electron gas structures.
His research is funded by EPSRC, DTI , UK and US Industry and the UK Ministry of Defence.
He has published more than 100 papers in IEEE and IEE Journals and holds 3 patents. He is the author of three books, including the best-selling IEE book "Theory and Design of Microwave Filters".

Technical Societies and Professional Activities:
He is a Fellow of the IEE and was elected a Fellow of the IEEE in 2007.
He is a member of the IEEE MTT Society technical committee on Microwave Filters, a member of the Technical Program Committee for the IEEE MTT-S International Microwave Symposium, a member of the Adcom for the UK and Republic of Ireland section of the IEEE and a member of the editorial review boards of IEEE Transactions of Microwave Theory and Techniques and IEE Proceedings on Microwaves Antennas and Propagation.
He is Editor-in-Chief of the International Journal of Electronics, published by Taylor and Francis.
He was the workshop chairman for the 2006 European Microwave Week, Manchester 2006 and has been appointed as General Chair of European Microwave Week 2011.

Recent advances in Microwave Filters

Abstract

This paper will describe recent advances in techniques for the design and realisation of microwave filters for a variety of applications including mobile and satellite communications, and radar. The first area, lossy filters , is that of the synthesis of filters with finite dissipation loss. This enables the realisation of highly selective filters with lower resonator Q factor than conventional design approaches, consequently reducing the physical size of filters in receiver applications. The second approach, frequency selective limiters , is a non-linear filtering technique which reduces the dynamic range of signals across a wide frequency band, enabling simpler A-D conversion of radar signals. The third technique, reconfigurable filters , is a new method for constructing tunable bandpass filters with low loss and high intercept point. Finally we will present results on techniques for the design of multi-mode patch antennas with filtering characteristics, enabling wider band and multiband operation.

Lossy Filters

It is well known that the performance of narrow-band selective microwave filters depends strongly on having high resonator unloaded Q. The insertion loss of a bandpass filter is inversely proportional to both the fractional bandwidth and the resonator Q. Furthermore as the loss increases the filter characteristic becomes progressively more rounded, resulting in much poorer frequency selectivity. We will describe a technique which is applicable to receive filters where in-band interferers are of relatively low power level. One such application is for the input multiplexers for communications satellites. In the absence of large interfering signals it is possible to tolerate significant filter passband loss (e.g.6dB) by preceding the filter by a low-nose amplifier. In this case the problem becomes one of synthesising lossy passive networks with equivalent selectivity to lossless networks. The fundamental principle is to recognise that since insertion loss in a uniformly dissipative network is proportional to group then a lossy filter with flat passband loss must have unequal Q resonators. We have demonstrated that for a passband loss level of 6dB, a reduction in resonator Q by a factor of 5 or more is possible, resulting in much reduced filter size. (Refs 1, 2)

Frequency Selective Limiters

In wide-open radar warning receivers the problem is often one of detecting a large number of signals across a wide dynamic range (e.g. 60dB) across at least an octave bandwidth. Furthermore the frequency of large unwanted signals may be unknown, thus making it impossible to filter them out via conventional filtering techniques. The frequency selective limiter takes a different approach using non-linear bandstop filters. The basic element in these devices is a bandstop resonator incorporating a PIN diode. If a large signal close to the resonant frequency is incident upon the resonator then the diode conducts and the resonator produces a bandstop characteristic thus attenuating the large signal. Furthermore, a small signal would fail to make the diode conduct and the resonator would act as an all-pass network, producing no signal attenuation. By correct design it is possible to construct multi-element networks, composed of these resonators which produce a true frequency selective limiting effect. To achieve wideband operation then a cascade of N multi-element devices can cover a prescribed bandwidth. Our results to date indicate that a multiple frequency signal with a dynamic range of 40 dB can be compressed to a dynamic range of less than 20 dB. Furthermore the spurious response performance of the device and its rise time are excellent, making it a valid method for dynamic range compression prior to A-D conversion. (ref.3)

Reconfigurable Filters

Over the past few decades numerous papers have been presented on methods for electronically tuning the centre frequency and/or bandwidth of microwave filters.
Most of these approaches rely on using some form of variable impedance device which is incorporated within the filter resonators. The types of devices include both semiconductor and MEMS varactors and switches. Plus several other mechanisms including active approaches and YIG spheres. The problem with this type of approach is that the filter losses tend to be high, as the losses inherent in the variable impedance device directly affect the resonator Q. Furthermore, any non-linearity in the device is directly coupled into high fields within the narrowband resonant circuits and consequently the intermodulation performance of these filters is usually poor. Indeed, often these filters produce spurious signals as large as the unwanted signals for which they are designed to remove. Our approach is to use a new type of reconfigurable resonator which consists of a ring transmission line, both ends of which are fed in-phase via a power divider. The output of the resonator is via an SPNT switch which can be toggled to different positions along the ring. Thus a passband occurs when the two parallel paths are in-phase, and transmission zeros occur when they are in anti-phase. The switch is no longer within the resonator, but forms part of the inter-resonator coupling network. Consequently the loss and non-linear performance associated with the switch is that of a wideband switch rather than a narrow band lossy non-linear resonator. Experimental results indicate that cascaded devices can achieve very wide tuning bandwidth combined with low loss and excellent intermodulation performance. (ref.4)

Filter Techniques Applied to Antenna Design

One of the fundamental constraints on antennas is that the smaller the size of an antenna relative to the wavelength at its operating frequency, then the narrower is its bandwidth and the worse its efficiency. This can be a particular problem for the design of mobile wireless devices operating in the low microwave range. Broadband matching techniques can improve the situation to a certain extent, but only at the price of adding more circuit complexity. We will demonstrate a different approach, where a microstrip patch antenna may be considered as a basic dual-mode resonator within a dual mode filter network. In this case, each mode of the multipole filter is coupled to the load, which represents free space. Thus each resonator may be considered to be lossy. The multi-mode antenna can then be considered in terms of its lossy coupling matrix. All the techniques available in lossy filter design, plus dual-mode matrix rotation techniques may be applied to the design. By designing antenna systems consisting of cascaded dual mode patches it is possible to greatly improve antenna bandwidth. Indeed, we have demonstrated a four-mode patch antenna, consisting of a stack of two circular patches, which experimentally achieved a 6dB return loss bandwidth of more than four times the bandwidth of a single-mode design. Furthermore, we will show that these techniques may be extended to multi-band devices and to antenna diplexers. (ref.5)

References
1. Passive Microwave Receive Filter Networks Using Low-Q Resonators. Ian Hunter, Andrew Guyette, Roger Pollard. IEEE Microwave Magazine, Vol.6, No.3, Sept. 2005.
2. The Design of Microwave Bandpass Filters Using Resonators with Nonuniform Q A.C. Guyette, Ian C.Hunter, Roger D. Pollard, IEEE Trans, MTT, Vol.54, No.11, Nov 2006.
3. Frequency Selective Limiting Using Nonlinear Bandstop Resonators P.Phudpong, I.C. Hunter, paper being reviewed by IEEE Trans.MTT.
4. A New Class of Low Loss High Linearity Electronically Reconfigurable Microwave Filter P.W. Wong, I.C. Hunter, IEEE Trans.MTT, Vol.56, No.8, August 2008.
5. A Circuit Theoretic Approach to the Design of Quadruple-Mode Broadband Microstrip Patch Antennas A.Abunjaileh, I.C.Hunter, A.H.Kemp, IEEE Trans.MTT, Vol.56, No.4, April 2008.  

[Downlod]