Fixed and Level Setting Attenuators
These attenuators are constructed with an E Plane resistive insert in the center of the rectangular waveguide.
The operating waveguide mode is TE10. The insertion loss or attenuation of this type of attenuator is frequency dependent due to its inherent mechanical configurations. As one can see, the same amount of insertion in the waveguide shows less attenuation in the lower end of the operating frequency band of waveguide than the higher end, because the electrical length is shorter relative to the frequency. The difference between these two attenuators is that level setting attenuator has a micrometer for attenuation value reset or memorization, while the attenuation value of the fixed attenuator is set at certain value at center frequency. Although the attenuation value of the level setting attenuator can be calibrated against the micrometer reading individually, it is not recommended to use the calibrated level setting attenuator as the attenuation standard as the attenuation level is frequency dependent. See the following typical chart for an E Band level setting attenuator.
In addition, the provided micrometer head on the level setting attenuator is intended for resetting or memorization only, but is neither for absolute correlation purposes of attenuation value nor unit to unit. In other words, the attenuation value versus micrometer reading may be different unit to unit. As stated above, the direct reading or programmable attenuator is recommended if such correlation is required.
While the fixed attenuator is offered for link or path attenuation setting at a fixed value, the level setting offers a low cost means to have a quick attenuation value set.
Direct Reading and Programmable Waveguide Attenuators
These attenuators are constructed with a precision resistive rotary vane in a circular waveguide.
The operating mode of the attenuators are circular waveguide TE11 mode. Unlike fixed and level setting attenuators, the attenuation value and phase shift of these attenuators are frequency independent. The reason is that the attenuation value of this kind of attenuator is strictly related to the area of the resistive vane exposed to the electrical field. Therefore, the frequency dependent element, length, remains constant while the rotating angle changes. Because of this unique feature, the rotary vane based attenuator can be made for direct reading when the relationship of the attenuation value vs. rotation angle is characterized. The programmable version of this type of attenuator is made by replacing the drum with the digital control driver circuits and motor. See following typical chart for an E Band direct reading attenuator.
While providing more precision and convenient means for attenuation settings and measurements, the direct reading and programmable attenuators are bulky and expensive compared to their counterparts, the level setting and fixed attenuators.