微带--波导转换Waveguide-to-Microstrip

Narrow Band Ridge Waveguide-to-Microstrip Transition for Low Noise Amplifier at Ku-Band

Zahid Yaqoob Malik, Abdul Mueed, Muhammad Imran Nawaz

Centre for Wireless Communication

National Engineering and Scientific Commision

Abstract- A compact Ku-band waveguide-to-microstrip transition integrated with low noise amplifier is designed. It acts as an interconnect between waveguide antenna and RF receiver modules. The transition design consists of standard waveguide WR62, a cavity for the low noise amplifier and a solid transformer section in the form of a staircase called ridge. The ridge is fixed in the bottom wall of a waveguide with the help of a screw. The centre conductor of a coaxial connector is brought near this transformer but doesn’t touch the transformer; these elements together with the back of the staircase and an adjacent portion of the bottom wall define a magnetic field coupling loop. This design methodology gives us narrow bandwidth of 500MHz at Ku-band and hence eliminates the need for a filter in receiver section for specific applications.

I.I NTRODUCTION

Lower loss of waveguide at higher frequencies above X band is advantageous as compared to the coaxial line. At higher frequencies, waveguide-to-microstrip transitions replace waveguide to coaxial transitions to act as interconnects between modules and antennas. These transitions can be also be made to operate at Millimeter wave bands. Waveguide is made from a single conductor which usually propagates a dominant TE mode, having a cutoff frequency below which the waveguide is highly attenuative.Most of the transitions are designed to operate within the frequency band of dominant mode propagation only. As compared with coaxial line, waveguide modes have impedance characteristics that tend to make transition design more challenging. The impedance of each of waveguide modes changes with frequency. In addition, the impedances of standard waveguides are much greater than 50 ohms, typically a few hundred ohms. Consequently, the bandwidth for most waveguide-to- microstrip transitions rarely reaches the full dominant mode bandwidth [1].

Microstrip-to-waveguide transitions have been widely used in testing and evaluating millimeter-wave hybrid and monolithic integrated circuits and combining integrated circuits with waveguide components [2]. The present transition relates to a ridge waveguide-to-microstrip line transition for an amplifier which uses a field effect transistor (FET) or the like. Generally, a waveguide-to-coaxial line transition or a waveguide-to-microstrip line transition is employed to supply an FET with a microwave signal coming in through antenna [5]. The transition apparatus may off-course utilize the magnetic field associated with the electromagnetic wave energy propagating in the waveguide. If the inner conductor of the coaxial transmission line is utilized as a probe to couple to this magnetic field, then the longitudinal axis of the coaxial line may be aligned with the propagating axis of the waveguide. With such an orientation of axes, the overall structure requires less space than those depending upon electric field coupling [6].

This transition provides a simplified and compact structure for waveguide-to-coaxial transmission line. This transition consists of three main subassemblies. The first part is a standard Ku-band waveguide WR62. The second part is impedance transforming section which is mounted in the WR62 waveguide with the help of a screw, the third part is the low noise amplifier cavity having the centre pin of coaxial transmission line. This pin is brought close to the staircase transformer to a side with the waveguide on one end and other end is connected to the alumina substrate used for the low noise amplifier in the cavity. Rest of the paper is organized as follows. The design of the ridge is discussed in section II. Section III discusses simulation work. In section IV, manufacturing details and test results are presented. The work is concluded in section V.

II.D ESIGN OF THE R IDGE

Impedance Matching Section is designed to match the higher impedance of a waveguide section to a coaxial line, the general practice is to decrease the narrow dimension of the waveguide, that is, the distance between the broadwalls of a rectangular waveguide in a series of steps so as to arrive at an internal dimension that achieves an acceptable impedance match with a satisfactory voltage standing wave ratio (VSWR). The impedance matching transformer (ridge) consists of five quarter wave sections as shown in figure 1. These sections take the form of a staircase of individual steps. The heights of the steps which are generally unequal are chosen in accordance with a set of numerical coefficients referred to as Techbyscheff coefficients [3]. The distance AB between the cavity wall and the end face of the first step is between 0.01

λ and 0.1 λLNA cavity is approximately one quarter of a wavelength. The width of each step is generally between one third and one

the first transformer section. This impedance level is dependent