Antenna impedance (continued)
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The conducting vane was introduced between the disc and cavity wall to generate an internal E field as shown in Fig. 5. This E field excites currents around the slot edges, a necessary condition for radiation. The slot E field and current flow was then assumed to be shown in Fig. 6.
Because of boundary conditions imposed by the vane, this field distribution should assume a magnitude variation proportional to E = A sin ψ/2 (vane position at reference ψ = 0°). The E-field assumption was initially confirmed during high-power breakdown tests which resulted in a bright corona glow being visible 180 degrees (ψ/2 = 90) away from the vane (Fig. 7). Later, in probing the E fields in the region of the slot, this assumption was further validated.
Impedance tests were made of this geometry, and VSWRs less than 2:1 over a 20-Mc band centered at 620 Mc were obtained. A Teflon dielectric disc in the plane of the slot resulted in a center frequency of 505 Mc.
The power handling capacity was increased by translating the metallic disc toward the vane. This increased the gap where the corona occurred. This new geometry is depicted in Fig. 8. The electric field, E, now becomes proportional to AK (ψ) sin (n ψ/2).
The input impedance was remeasured, and the VSWR was reduced to less than 2:1 over a 6-Mc band centered at 505 Mc. A single-element shunt-capacitance stub network, placed at the junction of the input coaxial connector and the cavity, was used for matching with Smith-Chart techniques. A model of the unit is shown in Fig. 9.
To further lower the operating frequency, the electrical length of the slot was increased by altering the disc in the vane region. This lengthened the path of current flow around the slot. The aperture region was analyzed and the current flow assumed as shown in Fig. 10.
Impedance tests were made, and a shunt capacitance was added using Smith-Chart data. A matched input impedance occurred from 439 to 444 Mc. Power handling tests were repeated with the unit operating satisfactorily at 17-W input at simulated altitudes in a vacuum test chamber from sea level to 280 kilofeet.