An essential performance metric for a high-power microwave (HPM) array antenna’s power handling capability is mainly determined by the magnitude of the electric field at the slots of the array. The power handling capability of an L-Band waveguide longitudinal shunt slot array with different slot widths is determined through simulation and measurement. Analysis shows that there is an optimum slot width resulting in maximum power handling capability. Different slot shapes are examined, including rectangular, end-rounded rectangular and elliptical. The results indicate that a longitudinal shunt elliptical slot array provides the highest power handling capability.
With the development of HPM, more attention is given to the array antenna for its high gain and high efficiency.1-3 Recently, some traveling wave arrays have been used as HPM antennas.4-6 A traveling wave array has the advantage that its power handling capability is more than that of a standing wave array; however, its radiation efficiency is lower. Also, the longitudinal shunt slots of the rectangular waveguide array are standing wave antennas.7 For radiating gigawatt HPM, the longitudinal shunt slot array is faced with the problem of significant electrical breakdown at the slots.8-10
A slotted waveguide HPM antenna array consists of a slotted waveguide with dielectric windows. A 100 megawatt (MW) high power handling capability can be realized by maintaining a vacuum in the waveguide. The thickness of the slot wall is only approximately 1 to 3 mm and the maximum electric field of the waveguide is near the slot, so the power handling capability of the array is determined mainly by the electrical field near the slots.8,9
Some researchers have verified that an elliptical slot can increase the power handling capability of the waveguide array.9-11 As is reported by Bemal et al.,8 when an elliptical slot is used, the maximum electric field encountered at the slot decreases by approximately 10 percent with respect to the fields associated with the rectangular slot. Baum9 guessed that an elliptical slot might be a good starting point. These references, however, do not provide the detailed structure of the elliptical slot as well as a comprehensive analysis of the power handling capability.
The goal of this work is to determine the best longitudinal shunt slot configuration for the highest power handling capability. The power handling capability of an L-Band waveguide longitudinal shunt slot array with different slot widths is determined. There is an optimum width and slot length for maximum power handling capability. Longitudinal shunt slot arrays with rectangular, end-rounded rectangular and elliptical slots are considered. The longitudinal shunt elliptical slot array provides the highest power handling capability. The optimum slot length is 0.51 free-space wavelength (λ0), which is the longest of the three configurations. This is supported by simulation and measurement. In addition, the array antenna can work in a vacuum (∼10-2 Pa) for higher power handling.
SLOT CONFIGURATION
Figure 1 Waveguide slotted arrays: rectangular slots (a), rounded rectangular slots (b) and elliptical slots (c).
Schematics of the considered slot waveguide array antennas are shown in Figure 1. The slot lengths are almost half a wavelength in free space. Slot conductance can be adjusted by its length and offset from the central axis of the waveguide wide edge. Although the slot is not fed at the waveguide center, the transverse electric mode (TE10) passes underneath it.
The field distribution of the slot is approximately equiphase half-cosinusoidal.12-16 The length of the waveguide’s wide inner wall is 120 mm, the length of the narrow inner wall is 40 mm and the thickness of the waveguide wall is 3 mm. The working frequency is 1.575 GHz. Each waveguide has nine wide edge longitudinal shunt slot elements and the slots are staggered on both sides of the waveguide’s center line.
SIMULATION
When the waveguide structure is fixed, the slotted array power handling capability is mainly determined by the width of the slot.1,11 According to Elliott,14 the electric field distribution in a longitudinal slot array is:
Figure 2 Maximum electric field for the three slot geometries as a function of slot width.
where 2l is the slot length, w is the slot width, Vs is the slot voltage measured across the slot at its center, x’ a point in the width of the slot aperture and z’ a point in the length of the slot aperture. The width of the slot is optimized to yield the maximum handling power capability.
Arrays with different slots are analyzed using Ansys’ High Frequency Structure Simulator (HFSS) software. Power handling capability is determined by the maximum electric field in the array slots. Because the thickness of the slot wall is only 3 mm, to maintain simulation accuracy the mesh is no greater than 1 mm. The simulation results show that when the mesh is smaller than 1 mm the simulation results tend toward stability.
Figure 1 shows three waveguide slot arrays with nine elements each. When the slot width changes, the slot offset from the central axis of the waveguide wide edge and the slot length must also be adapted to meet the resonant condition of the array. Power handling can be increased by increasing the slot width.11
Figure 2 shows the maximum electric field in the slot for the three arrays when the input power is 1 W. When the width of the slot is larger than 10 mm (0.05 λ0), the maximum electric fields drop rapidly in agreement with Equation (1). The minimum value for the maximum electric field of the end-rounded rectangle slot is 3260 V/m, which occurs at a slot width of 20 mm (0.1 λ0). The minimum value for the maximum electric field of the rectangle slot is 3064 V/m, which occurs at a slot width of 15 mm (0.08 λ0). The minimum value for the maximum electric field of the elliptical slot is 2667 V/m, which occurs at a slot width of 27.5 mm (0.14 λ0). The power handling capability of the rounded-end rectangular slot array is almost 1.1x that of the rectangular slot array and the power handling capability of the elliptical slot array is almost 1.5x that of the rectangular slot array.
From Figure 2, when the end-rounded rectangular slot width increases from 20 mm, the maximum electric field in the array varies around 3500 V/m. When the rounded-end rectangle slot width increases from 10 mm, the maximum electric field in the array varies around 3000 V/m. In the elliptical slot array, when the slot width increases from 17.5 mm, the maximum electric field in the elliptical slot array varies around 3250 V/m.
Figure 3 Electric field as a function of slot length.
Figure 4 Optimized resonant slot length as a function of slot width.
Figure 3 plots the electric field near the edges of the slot near the center of the waveguide. The field distribution across the slot is approximately equiphase half-cosinusoidal.14 Figure 4 shows the resonant lengths of the three types of array antenna slots. The lengths of the optimized rectangular, end-rounded and elliptical slots that provide maximum power handling are 87.5, 92.5 and 96.8 mm, respectively (0.46, 0.49 and 0.51 λ0). The elliptical slot’s resonant length is the largest.
