Ground plane of antenna

Dear brothers,

what is meant by ground plane of an antenna, with reference to planar antennas?
Why do we need them? I saw that in planar antennas, ground plane is used in connection with feeding the structure. As the signal flows sufficiently deep into the structure, the ground plane is terminated. So is it for the sake of feeding?

I have one more doubt. In planar antennas, in some structures, the antenna structure and the ground plane lie on the same plane, and in some other, they are in different planes. Why is it so and what is the difference between them?

Thanks in advance

Planar antenna is a way of construction rather than a specific antenna type. Some antenna types, very generaly speaking monopoles, need a ground planes, others e.g. dipoles don't need it. You have to refer to the operation principle of a specific antenna to answer the question in detail.

If you are interested in antenna design in general, read a good text book like Balanis, if you are asking about a specific antenna type, review known working examples of this antenna (and the textbook in addition).

When the ground plane is under the radiating structure (for example a half wave patch), and it is larger then the patch itself, it behaves as a reflector to direct the radiation. Wwhen the antenna is with the ground plane on the table, most energy is radiated upwards.

Many times the ground plane is also used the same way as the ground in an ordinary electronic circuit (as a return path).

For simplicity assume a coaxial cable with a ridgid shield (not braid), with thickness many times the skin depth (penetration depth). So current cannot go through the screen, but can only run over the inner or outer surface of the cable screen.

If you have a coaxial cable and 100 mA leaves the center conductor (and goes to the antenna), 100 mA must return via the innerside of the screen. In this way the coaxial cable does not generate any external field. Both E-and H-field are confined within the dielectric between center conductor and cable screen.

In most cases you don't want current at the outside of the screen as that current radiates and interferes with the intended radiation of the antenna. Current that runs on the outerside of the cable screen is mostly called common mode current. The ground plane can be used as a low impedance path so that the return current (runs on innerside of cable screen) comes from the ground plane and not from the outside of the cable screen.

You probably saw so called "ground plane antennas" (use google or other search engine). These are quarter wave whips with 3 or 4 (often sloping) quarter wave radials. In such antennas the 3 or 4 radials serve as a floating ground plane. So if momentary 1A goes upwards into the whip, a total of 1A is pulled out of the radials (and goes back into the cable via the innerside of the screen).

You may also search for "balun" as that is mostly required for antennas that don't have a solid ground (such as a center fed half wave dipole).