Re: Design of Directional microstrip patch antenna at 433mhz

Regarding directivity of electrically small antennas. Without referring to "superdirectivity", a small dipole with l << λ (also called Hertz dipole in literature) can still achieve regular cosine law directivity. The principle is e.g. used in field probes which can have either one, two or three (x/y/z) axis.

An electrically small loop antenna has basically the same directivity. Loop antennas are traditionally used for direction finding in the SW and lower VHF band.

Small dipoles and loops have both low radiation resistance, resulting in low bandwidth and considerable losses in a matched impedance (transmitter application). As a receiver antenna, it can be operated non impedance matched with higher bandwidth, but also low sensitivity.

FvM, Can i do some changes in ground plane so as to put it onto metal onboard for direction finding? so as to increase the directivity?

Don't see how. I believe the best way to achieve directivity is either a symmetrical dipole or a loop.