New-Tech Europe Magazine | March 2018

aluminum fins [3]. This cannot be accomplished in a waveguide attenuator where the resistive card is suspended in the waveguide cavity as there is generally no effective way to place a heat sink for suitable heat dissipation. Since the dielectric naturally has far less thermal conductivity than metal, these attenuators cannot be used in high powered applications. Rotary Vane Rotary vane attenuators typically consist of three sections of circular waveguide that contain a particularly shaped resistive card. As shown in Figure 4, the middle section is rotated axially while the ends -- typically a rectangular-to-circular waveguide -- remain static. When all three resistive cards are flat, or normal to the electric field, no attenuation takes place. Full absorption occurs when the central resistive card is rotated 90o, or parallel to the electric fields [9]. Variable attenuation values occur at particular angles with reference to the electric field, allowing for a highly precise insertion loss. The benefit of this device is the precision and bi-directionality while some design considerations may be the phase shift and length that vary as a function of frequency. Furthermore, cooling for these types of attenuators may be difficult to achieve as the dielectric substrate is once against hard to reach the resistive vane within the waveguide. High Power Waveguide Attenuators High Power Rotary Vane Another version of the rotary vane

Figure 1: A change in length of the broadwall attenuates the signal rapidly over the length of the waveguide. Naturally, there is also a high VSWR due to the reflections from the discontinuities. Source: Chegg

with wear and tear thereby risking oscillations [6]. Some constructions involve a disc-shaped insulator material suspended into the waveguide via a mounted holding structure. The continuous rotation of one side of the disc yields a sinusoidal attenuation while the other side of the disc has been shaped to compensate for the phase shift in precision applications [5]. More precision flap-type attenuators are adjusted via a plane orthogonal to the moving arm so that the element is inserted into the waveguide as a function of angular position [4]. These attenuators are not designed specifically for high powers due to the poor thermal management of the resistive component. For instance, high powered coaxial attenuators contain resistive elements on a ceramic insulating substrate that are in direct contact with a heat sink where heat can be carried away rapidly by the anodized

Continuously Variable A continuously variable attenuation is accomplished in a waveguide by means of vertically adjusting the lossy dielectric fins to incrementally reduce the energy level of the signal at the output. This can be done with either a micrometer screw (Figure 3a) or with a flap-type adjustment where the attenuating material is mounted to a movable arm (Figure 3b). The greater the penetration of the lossy vane, the greater the attenuation. The dielectric slab can be specifically shaped (and is often disc-shaped) so that the attenuation can vary linearly with insertion. The flap-type waveguide attenuator has gone through several iterations to improve the linearity of the insertion loss over mechanical positions. Older constructions involved suspending the element manually to preset fixed positions that could vary over time

Figure 2: The resistive element causes attenuations while the taper minimizes reflections. The taper shape can vary with a single taper or a double taper. Source: [2]

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