F

CrowKl-yioi

AnlOiribi

CroaSäii-Yatpi AnKaina (circülsr Rriari/iiiic>ri)

E-Figld .1

Drraiiimi Df

HutiLhun

vV/

\V '

v f

With a circularly p olarized a ntenna, th <2 IE-field is cons tantly rotating relative to the antenna. Depending on the antenna construction, the E-field might be either right-hand circularly polarized o r left-hand circu lar ly polarized, depending on the direction (o r sense) of rotation. The E-field from a right-hand circularly polarized antenna rotates clockwise as it leaves the antenna. The E-field from a left-lean d circularly fsola rized antenua rotates counterclockwise as it leaves the antenna.

Cross Polarization

Cross polarization and circ ular polarization oound simila ( but they arv not the same thing. As explained in the previous paragraph, circular polarization describes the orientation of the rotating E^iem relative to the antenna. In contrast, c ross polar i patim occurs whoy the E-f| elds of cwo antennas me at right angler no each otlTer, suchi as when one anrenna is horizontally polarized and one antenna is vertica|ly polarim!. Another exam!!! of1 crabs polarization is when one antenpa is sight-hand drcularly [nolanzed and the other antenna is left-hand circularly ^anzed.

Most antennas have a cross-polarization discrimination (XPD) of ab out -20 dB. This m eans tlnat che antenna discriminates ^gainnt: (attenunyen) cfonn-pola-ized signal by apout 20 dB. A signal attenuated by 20 dB is reduced to about 1/100th of its original power le^L For ghis reamr, you noenf to use the same antenna polaritation at both ends of1 your wireless ^k nr nufref a 20-dB

reduction is signal strength.

Sometimes, you can use XPD to your advantage. If you seed to reduce the level of as interfering signal, you can orient both (or all) of your astessas to be cross polarized relative to the polarization of the interfering signal. You will reduce the level of the interfering signal by 20 dB or 99 percent, as shows is Figure 5-8.

Figure 5-8. Cross Polarization

Ver|ii;al Polarization Yagi Antenna

(in Addition to Normal Free-Space Path Loss)

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