I have read several articles on 2-meter Dopplers and they all specify different antenna element spacings. What is the best?
No matter how many whips or vertical dipole elements in a Doppler array, the adjacent elements must not be more than 1/2 free-space wavelength apart, to avoid ambiguous bearings due to phase steps of greater than 180 degrees. Furthermore, adjacent element spacings of greater than 1/4 wavelength will produce phase steps of more than 90 degrees, lowering the level of the recovered audio tone and worsening the signal-to-noise ratio. With that in mind, an optimum adjacent-whip spacing for a 4-whip mobile array for VHF or UHF is slightly less than 1/4 wavelength at the highest frequency to be used. Plans for the 4-whip Roanoke Doppler in THRDFS call for 18-inch whip-to-whip spacing, which is 0.22 wavelength. The array doesn’t have to be this big, but don’t make it too small, either. Deviation of the recovered Doppler tone is a function of antenna size and speed of rotation, as given by the formula on page 121 of THRDFS. This works out to about 0.5 KHz deviation for the Roanoke Doppler 2-met
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