MIMO and Smart Antennas for Mobile Broadband ... - 4G Americas

--Co-Polarization--Cross-Polarization(Source @ 90°)F/B Ratio @ 1800 dB – 25 dB = 25Figure 67A — Front-to-back ratio measured by comparing gainat bore site to gain at point 180° behind bore site.Figure 67B — Cross-pol ratio is a measure of the de-correlationof the two polarizations used in an X-pol antenna—one at +45°and the other at -45°.Figure 67 – Front-to-back and cross-pol ratio.To demonstrate the huge amount of degradation that can happen to both F/B ratio and CPR whenmechanical down tilt is applied to an antenna already having a large amount of electrical down tilt, themeasured patterns of Figure 68 are presented. The patterns are for a typical 4 foot, 65° antennaemploying 15° electrical down tilt along with 5° mechanical down tilt. The co-pol pattern is shown in blueand the cross-pol pattern is shown in red. With a vertical beam width of ~16° at 850 MHz, the tiltcombination is well beyond even the 20% blooming curve of Figure 59. In fact, the horizontal beam widthis in the neighborhood of 160° or approximately 250% blooming!The F/B ratio has degraded to approximately 18 dB at 180° and the cross-pol is actually worse than theco-pol pattern. Taken over a ±30° angle around 180° this F/B method measures only 8 dB!Finally, the CPR over the desired sector degrades to only 5 dB at the sector edge – far short of the 10 dBexpectation.4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved87