The letter is talking about Several methods to decrease the interfering effect among antennas with close mounting position.
1. Isolate by distance

Keep distance between antennas as far as possible. In theory, the distance between antennas should be more than half of the operating wavelength. For GSM band, there commended distance value is more than 20 practical application, the experiment value is no less than 15cm for better isolation.

2  Isolate by metal shielding baffles among antenna

Put a metal shielding baffle between antennas, which can help to prevent radiation signal coupling from each other. If the metal baffle is big enough, the isolation will be unlimited big in theory.

3   Isolate by antenna orthogonal polarization

In theory, the isolation is unlimited big if the polarization is really orthogonal, like vertical polarization VS horizontal antenna; RHCP (Right Hand Circular Polarization) VS LHCP (Left Hand Circular Polarization).
In real practical application, the antenna are all elliptic polarization, they have a certain ratio. So putting the same type antennas in orthogonal position will be helpful for isolation.

4    Isolate by antenna radiation pattern 

  • Try to modify the antenna mounting position, avoid the major lobe of both antennaoverlap, which is also an effective way for high isolation among different antennas,especially for high directivity antenna.
  • It might be difficult to find a perfect position to stagger the major lobe for lowdirectivity antennas, two factors cause this, one is the major lobe is very broad,the other is the radiation pattern is affected by surroundings sensitively.
  • For common antenna we often use, most of them belong to low directivity type, we can put the antennas in a nonmetal shelf, and place antennas in vertical form, not in horizontal form, which can help to avoid antennas major lobe overlap and make the isolation bigger in some degree. See figure 4-3,

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