PDF of this tutorialCharacterizing the performance of slant-45 polarization diversity antennas is considerably more involved than vertically polarized space diversity antennas or polarization diversity antennas with vertically and horizontally polarized receive and receive diversity ports. Thus, antenna pattern measurement techniques used to determine the performance of slant-45 dual polarized antennas are likewise more involved. The technique for measuring the performance of vertically and horizontally polarized antennas is well known and quite robust. When these types of antennas are measured properly, the polarization of the source matches the polarization of the antenna being tested for the entire test. This type of measurement is said to be a copolarized measurement. Hence, as the polarization of the source antenna matches that of the antenna-under-test, no coupling loss occurred.
Coupling loss (i.e., the polarization loss factor) is a result of a mismatch in the polarizations of the source antenna and the antenna-under-test. If the polarizations of the two antennas do not match over the entire pattern measurement, then antenna parameters such as gain, horizontal beamwidth, and front-to-back ratio may not be accurately reflected in the antenna pattern measurement. The polarizations of the source and of the antenna-under-test do not match over the entire test with a slant-45 source and a slant-45 antenna. The polarizations of the source and the slant-45 antenna match only at boresite. This type of measurement is not truly copolarized, but is often referred to as copolarized, and is depicted in Figure 1.
Figure 1. Copolarized Measurement with a Slant-45 Source on a Slant-45 Antenna
For this single pattern cut with a slant-45 source, the source antenna and the antenna-under-test are copolarized at boresite (see position A in Figure 1), cross-polarized in the back (see position B in Figure 1), and varyingly noncopolarized elsewhere. The same can be said in converse fashion for a cross-polarized measurement on a slant-45 antenna with a slant-45 source. Such a pattern is shown in Figure 2, where the source antenna and the antenna-under-test are cross-polarized at boresite (see position C in Figure 2), copolarized in the back (see position D in Figure 2), and varyingly noncopolarized elsewhere.
Figure 2. Cross-Polarized Measurement with a Slant-45 Source on a Slant-45 Antenna
The difference in signal strength between position A in Figure 1 and position D in Figure 2 is known as the copolarized front-to-back ratio. If the power level at B and D are added together in a linear fashion, this gives the total radiated power in the backward direction. Comparing this total radiated power to that of position A in Figure 1 gives the total power front-to-back ratio. For this case, the copolarized front-to-back measurement is approximately 22 decibels (dB), whereas the total power front-to-back ratio is approximately 20 dB.
