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Series 244 / 245:

Introduction:
The use of the Sectoral Horn Antenna is a very effective method of achieving the  'broadcast' type coverage required of the base station transmission system.  Generally speaking, the area to be serviced is divided up into equal squares and a base station situated so as to illuminate the four squares around it from the corner of each square. This arrangement gives a 4:1 ratio of revenue generating cells to cost generating transmission sites, an important factor in the consideration of commercial viability.

The problem is that there appears to be some confusion on the matter of the Sector Antenna radiation pattern that  gives the most efficient illumination. In Britain and Europe it is agreed that an antenna having a 3dB beamwidth of 64 degrees is best suited to the  application, a relatively simple device which provides high levels of illumination efficiency for a variety of conditions. In the USA and Canada, it appears that the 90 degree sector antenna is favoured. This fact remains somewhat of a mystery because the coverage pattern of the 90 degree antenna produces more signal wastage and therefore significantly lower illumination efficiency. The theoretical arguments supporting the previous statements are presented here.

Illumination Efficiency

It is possible to take each antenna pattern and calculate and compare the  amount of signal transmitted to and beyond the cell, taking into account the free space loss, rain attenuation and the atmospheric loss. Using typical cell sizes between 2 Km and 10 Km, one is able to calculate the illumination efficiency for each antenna pattern by dividing the amount of signal transmitted  to points within the cell boundary by the total.  The results for variations in cell size and rain rate are shown in Figure 2 and it is clearly seen that the 64 degree antenna achieves a considerably higher efficiency factor for all  conditions. The extra signal allowance for rain conditions means that the system is less efficient in no rain conditions, i.e. too much signal is transmitted.

Cell Edge Signal Variation

One possible argument in favour of the 90 degree antenna is that the difference in free space loss between the opposite and adjacent corners is 3 dB and that the difference in angle is 45 degrees, therefore 3 dB at +/-45-degrees  equals a 90 degree 3 dB beamwidth.  However this does not take into account any loss due to rain, a serious omission since the link budget should be calculated under worst-case conditions! Since rain attenuation is proportional to distance, the differential between signal levels at opposite and adjacent corners increases, thus favouring a narrower beam antenna. Plots of the signal variation along the cell edge are shown (Figure 3) for a 5 Km cell and a rain attenuation of 3.67dB/Km. Higher rain rates and larger cells produce larger differentials and hence require narrower beamwidths.

Conclusion

Overall, it looks like the 90 degree sector antenna is not as suitable as the 64 degree sector antenna for this application. Figure 1 illustrates this point quite nicely, showing a coverage contour plot for each antenna type overlaid on the cell.  In addition, the 64 degree antenna is a relatively simple device to manufacture, whilst the 90 degree antenna requires the use of beam shaping techniques, making it more expensive both to develop and produce. To sum up, one could replace a 90 degree sector antenna with a 64 degree version and at worst,  see no difference in system performance. More probably, one would see an improvement in the quality of the signal coverage, as indicated by these theoretical results.

Specification:
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