Measure the ratio of the signal strengths of the radiated power lobes from the front and side of the antenna 

Draw two imaginary lines through the ends of the elements and measure the angle between the lines

Measure the ratio of the signal strengths of the radiated power lobes from the front and rear of the antenna

Note the two points where the signal strength of the antenna is down 3 dB from the maximum signal point and compute the angular difference



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