Choosing base station antennas

In order to extend and improve signal coverage requires more radiated radio frequency power. So, look not only for a high gain antenna, but also for a high efficiency antenna write Bo Jonsson and Torbjorn Kampe.

The choice of antennas for base stations rarely receives any attention. Looking at the installed base, one could get the impression that the obvious, or even optimal, choice for almost all sites would be the traditional 18 dBi antenna. This antenna has 65° of horizontal beam width and around 6.5° of vertical beam width, as do about 80 per cent of all installations on 1,700-2,100 MHz. The 15 dBi antenna is still quite common, especially on the lower frequencies, with a vertical beam width of around 14°.

There are also high gain 21 dBi antennas and new so-called ultra high-efficiency antennas using air as dielectric and virtually eliminating power losses. These antennas improve base stations’ transmission capacity, resulting in higher signal strength, an increase in geogra¡phical area coverage, improved indoor penetration, increased traffic, improved data throughput and reduced production costs per call.

Choice of antenna: The decision to use this antenna comes down to historical reasons. The 18 dBi antenna was the highest gain available from antenna vendors able to deliver antennas in mass quantities, and was such an obvious choice that it became almost a de facto standard and installed almost without a second thought. This was of course not without reasons; it was a good compromise and has served us all very well.

But the 18 dBi antenna does not deliver the highest gain anymore-high gain ultra high-efficiency antennas do. The 18 dBi antenna was also always questioned both for its narrow vertical beam width and also its horizontal beam width of "only" 65°.

With data surpassing voice in new 3G and 4G mobile networks, interference is different and so must the antennas be to stay effective. Most sectors would benefit significantly from an antenna with higher gain and a sharper upper roll-off curve than the standard 18 dBi can offer. If there is a tall building nearby, a small antenna with lower gain like 15 dBi would do a good job. At a rural site with maximum coverage as the most important objective, a 21 dBi antenna is clearly the best option.

At suburban sites the focus shifts towards capacity, suppression of interference and less on coverage. In can be seen that a high gain antenna will provide a 3.5 dB stronger signal level in the center of the cell, and suppress interference from next cell better than the 18 dBi antenna. The 3.5 dB extra will improve in-building coverage and data transfer speed. Also, the lower interference will improve C/I and further improve performance. A high carrier-to-interference ratio (C/I) is the key parameter for efficiency, data rate and general success. So a high gain high efficiency antenna is again the best choice.

Simulations and real-life tests show that there is a very sharp "cut off" at 650 m from the high gain antenna at 6° tilt. This will provide a significant reduction of interference and a magnificent control of soft handover load. The high gain antenna still outperforms the 18 dBi antenna in dense urban sites and the sharper upper roll off is the main asset, actually even more important than the stronger signal level.

Going further in tilt, it can be seen that the high gain antenna provides a stunning 5.5 dB stronger signal and even more in interference rejection from outside 450 m. That does wonders for soft handover overhead and data transfer speed; so again a clear win for the 21 dBi high gain antenna with very sharp upper roll off.

So in general, we can see that the 21 dBi antenna is a better choice in surprisingly many situations, for much the same reason as the 18 dBi antenna once replaced earlier antennas with lower gain. They very often offer significant improvements to the traditional 18 dBi antennas. But they have a significantly more complex feeding network so the efficiency becomes a very important parameter. After all, to extend and improve coverage requires more radiated radio frequency power, not less. So look not only for a high gain antenna, but also for a high efficiency antenna.