Oiitef Ring - 6.888 dBï «ax. Gain - 5,91V JBi teiauHi Plot Elevation Angle = 8J ¿eg,

Figure 2. The undesirable iobe disappears and a nearfy perfect circular pattern with no phase anomalies is achieved when the switched-off whips are open-circuited to minimize parasitic effects.

tion of OFF whips can have on the active whip, I analyzed the Roanoke Ooppier 2 meter array using ELNEC, an antenna modeling computer program for the PC. ELNEC produces azimuth and elevation radiation patterns for VHF and UHF single-element antennas, parasitic arrays, and phased arrays. For simplicity, I assumed four quarter-waveiength elements in an 18-inch square pattern over a perfect ground plane at an instant in time.

When OFF whips are grounded, the array is very directional, as Figure 1 shows. One whip in me ciear would have 5.14 dB gain with respect to an isotropic radiator (dBi), Gain of this array varies from 11,15 dBi in the direction of the driven whip to 0.5 dBi in two other directions,

ELNEC does not plot phase versus azimuth. Creator Roy Lewallen W7EL told me that until I called him, no one had asked for this feature. Exact phase variation can be calculated, but the extra effort would not be worthwhile. The object of the analysis was to minimize the effect, not quantify it. It is clear that gain variation in excess of 10 dB produces an unacceptable amount of phase variation. Or the other hand, if the OFF whip feedpoinls are open-crr-cuited (Figure 2), gain variation is only 1.1 dBh which is good

Experience has proven the validity of this analysis, I have tested two four-whip doppler antenna designs (one commercial and one home-brew) that grounded the OFF whips with shunt PIN diodes at the antenna bases, in both cases, it was very difficult to get reliable bearings when the vehicle was moving on city streets. Performance improved markedly when the sets were modified to open-circuit the whip feed-points when off.

The Roanoke Doppler antenna uses

AUXILIARY supports present antf future optional accessories for ihe PSA-65A

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