Filters designed for lower frequencies such as VHF are usually large due to wavelength requirements. But a practical technique for achieving miniature microstrip filters at lower frequencies can yield designs for application over wide bandwidths. The technique has been demonstrated with a filter designed for use at 600 MHz, with simulated performance and measured results agreeing quite closely. The compact VHF filter design demonstrates good selectivity and is small in size and ideally suited for use in communications systems such as satellites and mobile communications equipment.

Next-generation mobile communications and satellite communications systems require microwave filters that are small in size but provide high selectivity with low insertion loss. Microstrip filters are inherently small in size compared to other filter technologies, such as waveguide filters, although limited in the degree of miniaturization possible due to physical wavelengths at lower operating frequencies and compromises in electrical performance. Nevertheless, for some applications where small size is of primary importance, miniature microstrip filters are desirable, even though reducing the size of the filter generally leads to increased dissipation losses for a given circuit substrate material and, hence, reduced performance.

Microstrip filters can be shrunk in size by using lumped-element approaches or substrate materials with high dielectric constant. But for a given substrate material, a change in filter geometry is usually required, calling for different filter configurations when switching from one substrate material to another. For a given substrate material, a dual-mode-resonator- type filter configuration has been shown to yield good electrical performance in a miniature footprint.

A number of filter types and configurations are available when designing in microstrip. But for a filter with good selectivity and other favorable electrical characteristics that can be applied at VHF, a lumped-element approach must be ruled out because of its low quality factor (Q) and resulting poor selectivity. A surfaceacoustic- wave (SAW) filter can provide high selectivity at VHF, but it also suffers high insertion loss in that frequency range. A waveguide filter can provide excellent electrical performance, but is too large for many practical applications at VHF. A proposed dual-mode resonator filter provides good selectivity with acceptable insertion-loss performance at VHF, and can be fabricated in a relatively compact size.

Filters based on dual-mode resonators find application where small size, low mass, and low insertion loss are important, such as in many terrestrial and space-based communications systems. In a filter based on a dual-mode resonator, generally the two degenerate modes at f0 will generate a bandstop response. By adding a perturbation to the structure, however, the degenerate modes will be detuned so that a bandpass response is created. For a filter based on dual-mode resonator operation, the perturbation is introduced to couple the degenerate modes. By simply adjusting the size of the perturbation, the response of the dual-mode filter can be changed. The appearance of the transmission zeros in the response is due to the presence of parasitic coupling between the input and output ports.

A variety of different symmetrical dual-mode resonator circuits have been developed for microstrip circuits, with a typical schematic diagram of a dual-mode resonator design shown in Fig. 1. As Fig. 1 shows, the use of the meander line in the dual-mode resonator results in small circuit size.

To illustrate the use of the dualmode resonator approach in practical bandpass filters for VHF applications, a design goal was set for a dual-mode resonator bandpass filter centered at 605 MHz that is small in size and exhibits relatively low insertion loss. The filter is also required to exhibit good selectivity. The dual-mode resonator approach should be able to provide the desired filter response at that frequency when fabricated in microstrip circuitry, with a meander loop resonator design selected for its compactness. The total length of the resonator is g, which is the guided wavelength of the resonant frequency (Fig. 2). Because of the relatively low operating frequency, the size of the bandpass filter would be large if a basic dual-mode resonator approach was used. In order to miniaturize the filter, a geometric structure was created from a repetitive pattern of the basic dual-mode resonator structure. The modified structure turns out to be a fractal form of the basic dual-mode resonator, a very reduced-size form of meander dual-mode resonator structure. Square patches are added at the corners of the microstrip design, with perturbations introduced at a location that is assumed to be 45 deg. offset from the two orthogonal models. The square patch is added to couple the two degenerate modes.

In order to optimize the performance of the dual-mode resonator filter, another patch was added. The feed structure was optimized in order to achieve minimum insertion loss for the filter structure. An alumina substrate with 50-mil height and relative dielectric constant ( r) of 9.8 was used for the microstrip filter. A computeraided- engineering (CAE) simulation of the design was performed using the Momentum planar electromagnetic (EM) simulator in the Advanced Design System (ADS) suite of CAE design tools from Agilent Technologies. Once fabricated, the performance of the filter was measured using a PNA series vector network analyzer (VNA), also from Agilent. The results of the forward transmission (S21) measurements are shown in Fig. 3 and Fig. 4 with center frequency tuned around 605 MHz.

The measured results agree closely with the simulated values from Momentum. The filter's measured passband of 27 MHz features insertion loss of only 1.6 dB while the filter also achieves selectivity of at least 23 dB. Some of the additional insertion loss within the passband is due to connector losses and radiation losses.

The filter achieves excellent out-ofband rejection as well as better than 35 dB rejection at lower frequencies. The return-loss performance exceeds 25 dB. The filter, designed for a center frequency of 605 MHz, achieves its goal of miniaturization in microstrip by measuring just 35 x 35 mm.

This article has demonstrated the feasibility of using a dual-mode resonator approach with meandering circuitry to achieve true miniaturization at lower frequencies, including at VHF for the example filter that was simulated and fabricated. The design approach achieves low insertion loss with high selectivity in a compact footprint, making it ideal for size-andweight- sensitive applications, such as in mobile communications devices and satellite communications systems. In addition, this approach can be extended to frequencies well beyond VHF without compromising the basic filter electrical performance. The design approach was validated by achieving a close match between simulated performance and measured results of fabricated microstrip filters.

Acknowledgment
The authors are grateful to Dr. V. K. Lakshmeesha (Group Director) and Dr. S. Pal (Deputy Director) for their kind support and encouragement. Also, sincere thanks to the staff of the communications systems group for their cooperation in realizing the filter structures in the article.