There are three main types of stacking :
i) vertical stacking, with both antennas mounted on the same mast.
ii) horizontal stacking, with side-by-side mounted antennas.
iii) The quad stacking, with four antennas is a combination of the two previous approaches.
Purpose of stacking is to improve weak signal DX results, especially with regard to tropo and meteor scatter work.Usually, this technique of combining the outputs of two antennas is used to sharpen their forward receiving patterns, making the antenna nulls extremely “deep,” while producing a theoretical 3dB increase in forward gain.
Next, it is also to concentrate in a specific direction instead of be radiated in the secondary and additional smaller lobes.