How to change Antenna Radiation Pattern?
I need help from all you guys out there. I am designing some very closely spaced antennas. These antennas needs to appear as operating as diversity antennas. The requirement is to create nulls in different angles in the azimuth plane for each antenna without any external device but only changing the feed position, antenna placement position and the antenna geomentry. For example I have 2 simple monopoles both operating at 444Mhz. They are placed within 5 cm apart from each other, how do I change the antenna radiation pattern so they might appear to operate as diversity antennas?
Any help will be very much appreciated.
Many thanks in advance!
Element7k
I think that the antennas located so close to each other cannot become "diversity" antennas.
Diversity means that two antennas are located far away from each other; if an obstacle appears in the path from a remote transmitter to one of the antennas, the other one may be not obstructed at the same time.
Two antennas close to each other form an interferometer; the radiation pattern changes into two or more "main" beams, with deep nulls. By phasing the antennas or varying their distance and position, you can suppress interference source signal by moving pattern null in that direction. This mode is often used to suppress interference but it is not a diversity system.
Ideally the distance should be greater than λ/2 but studies shows that even λ/4 can be used sometimes. Anyway in this case 5cm is much shorter than λ/4.
There is an other case, when the antennas itself have physical dimensions << λ/4 (small antennas). In this case a 20dB isolation (or greater) between antennas applies.
Placing objects in the reactive near field of the antenna change the antenna pattern.
Yeah, you could have the ground plane segmented, and turn on/off pin diodes to connect the segments to the antenna ground. Will somewhat modify the radiation pattern.