Appraising the Effect of Null-fill to the Broadcast Coverage of Four-bay Collinear Array

Abstract

Null-fill is a method of improving the signal level in areas where pattern nulls occur. To take advantage of itin improving FM broadcast coverage, factors such as antenna height in relation to the average terrain, the number of bays in the array and the location of the target area are considered. It is therefore important to understand the radiation characteristics of the antenna system after the application of null-fill. This paper presents an analysis of the radiation characteristics of a four-bay FM collinear antenna array composed of circularly polarized elements with varying upper and lower bank power ratios. The analysis leads to the discussion of the capabilities and limitations of null-fill with the different bank ratios. Results show that, in the elimination of pattern nulls, the bank power ratio of 95:5 produces the best results. Also, irrespective on the value of the bank ratio, there exists a pattern null near the tower base. If left uncorrected, this pattern null poses reception difficulty in that area. An explanation on the existence of this kind of null and a method of eliminating it is worth considering in future studies. Keywords—Broadcast antenna, Collinear array, FM antenna, Null-fill, Pattern null, Polarization