antina and earthing

(Frequency, power)-antina relation? I saw some circuits are connected with earth. What is the uses of earthing?

You need a Ground or return in order to help boost the input signal.

To prevent electrocution by lightning.

We know any electrical or electronic circuit needs two wires to tap electricity or connect electricity. Similarly to tap electromagnetic energy radiated in space for radio communication we need two wires, these wires are taken from tuned circuit or resonant circuit. One wire is connected to antenna and other wire to earth. Todays radios are more sensitive hence they need not be connected to earth. Other reason for earthing is, if we are using AC mains radio, chasses may be live (phase might have been connected to chasses due to reversal of mains wires) in this case operator gets shock, to avoid shock chesses is grounded.

If the antenna was a quarter wave dipole (or similar) then ground might have been functioning as the other pole of the antenna (making it a half wave antenna).

Some antennas have a "ground plane" that simulates this same grounding effect.

BTW, the antenna on a walkie-talkie is usually quarter wave, and the hand holding the radio becomes the (less than sufficient) "ground plane". And, a quarter wave "whip" antenna, mounted on a car, uses the metal of the car as a rudimentary ground plane. Also, for an antenna mounted on the top of the car (i.e. roof of the passenger area), the radiation pattern is more omnidirectional than if the antenna is mounted on the bumper. This is because on the top of the car, the antenna is surrounded by a metal "plane", whereas on the bumper, the antenna's ground plane exists on only one side of the antenna. This is all predicated on the wavelength -- i.e. if the wavelength is such that one fourth of the wavelength is longer than the radius of the ground plane surface, the effectiveness of the ground plane is reduced. Thus, a CB radio antenna mounted on the top of a VW Beetle will be less effective than a CB radio antenna mounted on the top of an 18 wheeler cabin, but will be much more effective than a hand holding a walkie talkie.

On the other hand, a hand holding a 2 Meter radio will be more effective, and the top of a VW Beetle will work quite well.

And, correction: above should read: "If the antenna was a quarter wave _monopole_ ..."