New-Tech Europe Magazine | March 2018

attenuator involves slits that run the length of a circular waveguide. Microwave energy is attenuated as the position of the slit is rotated by means of resistance to currents in the walls of the waveguide. The energy is then dissipated via a parallel plate transmission line near the slot. The lossy parallel plate transmission lines can then be mounted to a thermally conductive surface for simpler thermal management. Moreover, convective cooling can be provided with liquid or air distribution in the enclosure [10]. High Power Variable Attenuator High power variable attenuators often have three ports where one allows for the microwave energy to be terminated some sort of high power absorbing load. One construction involves a 3-port circulator within the transmission line that controls the signal transmission such that the input energy is sent into the second port where the tuning occurs. In this port, the waveguide can be tuned so that the reflected portion that is sent back into the circulator and to the output (third port) to yield a particular attenuation value. The rest of the signal at the second port is absorbed by the load which can much more easily be convectively cooled with water or circulating air [11]. Summary There are essentially three types of waveguide attenuators: fixed, continuously variable, and rotary vane. While all these attenuators exploit the cutoff frequency principle, each subset has many electromechanical design variations that can be optimized for more precision, lifetime, VSWR, or power handling. The best construction for a particular application depends on some of these variables. Waveguide attenuators offer a simpler design with far less VSWR than their coaxial attenuator counterparts since an engineer will have to take into account the losses due to the waveguide-to- coaxial adapters.

Figure 3: Manual variable attenuators employ some type of mechanical adjustment such as a screw, micrometer, or knob. Motorized attenuators require continuous spinning of the resistive plate where variable control is often accomplished with voltage-tuned motors. Source: [5] [6] [7]

References: Adair, Robert T. “A precision 30 MHz waveguide- Below-Cutoff attenuator with an absolute electronic readout :” 1976, doi:10.6028/nbs. ir.76-833. h t t p s : / / w w w . e l e c t r o n i c s - c o o l i n g . c om/ 2 0 1 7 / 1 1 / t h e rma l -ma n a g eme n t- c o n s i d e r a t i o n s - h i g h - p owe r - c o a x i a l - attenuators-terminations/# h t t p s : / / w w w . e l e c t r o n i c s - c o o l i n g . c om/ 2 0 1 7 / 1 1 / t h e rma l -ma n a g eme n t- c o n s i d e r a t i o n s - h i g h - p owe r - c o a x i a l - attenuators-terminations/#

h t t p s : / / www . g o o g l e . c o m / p a t e n t s / WO1999001906A1?cl=en https://www.google.com/patents/US3209288 https://www.google.com/patents/US2619538 Das, Annapurna, and Sisir K. Das. Microwave engineering. Tata McGraw-Hill, 2009. https://www.google.com/patents/US2603710 Fundamentals of Microwave and Radar Engineering By K K Sharma. https://www.google.com/patents/US3562679 https://www.google.com/patents/US6066992

Figure 4: Rotary vane attenuators have three sections with the center rotating with respect the electric-field. These types of waveguide attenuators are generally the most precise. Source: [8]

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