New-Tech Europe Magazine | March 2018

Waveguide Attenuators at a Glance

Tim Galla, Pasternack Enterprise

Attenuators are 2-port passive devices that can incrementally dissipate microwave energy. They have a myriad of uses and are often applied in test scenarios as padding to prevent damaging sensitive and expensive test equipment. Variable attenuators can be used to control the gain of a system via the output of the microwave source as opposed to varying the input in order to maintain a favorable signal-to-noise ratio. Attenuators are designed to have minimal reflections and a low Voltage Standing Wave Ratio (VSWR) to avoid potentially catastrophic reflections back to the source. Theory: The ‘cut off’ principle Waveguide attenuation is often based on the cut off principle where a section of the waveguide is operating frequency far below the cutoff frequency -- a frequency that directly correlated with the dimensions of the waveguide,

below which the waveguide is unable to carry signals. For a rectangular waveguide, the length of the broadwall is generally equal ½ wavelength of the lower cutoff frequency. Microwave energy will therefore dissipate very rapidly in sections where the H-plane is stunted as shown in Figure 1. This rate of decay is exponential along the transmission path of the signal. As the cutoff frequency increases, so does the rate of attenuation of the modes below cut off frequency. And since waveguides can operate with many modes existing simultaneously, the dominant mode is used; the mode with the lowest decay rate and lowest cutoff frequency [1]. The attenuation rate for a rectangular waveguide with the dominant TE10 mode in frequencies under the cutoff can be defined by Equation 1 [2]. The attenuation for a circular waveguide with TE11 mode in frequencies under the cutoff is defined by Equation 2 [1]. Where α is the attenuation, δis the skin

depth of the guide, a is the radius of the circular waveguide, fc is the cutoff frequency in GHz, and 〖 λc 〖_is the cutoff wavelength in centimeters. Fixed Waveguide Attenuator Fixed waveguide attenuators typically include a resistive film such as glass placed inside of the waveguide. As shown in Figure 2, the resistive card is placed at the center, parallel to the longitudinal axis and to the maximum electric field with each end tapered sharply to point directly at the incoming wave in order to minimize reflections due to discontinuities. The length of the material is typically two wavelengths (2ƛ) while the length of the tapered portion is about a half a wavelength (ƛ/2). In essence the bulk resistive material accounts for a uniform insertion loss across select frequencies while the taper is fine tuned for a lower VSWR.

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