Antennas available in the Real world market

Give some antennas that work upto 3GHz having a minimum bandwidth of 20GHz, that are available i the real world market.

In the real world market (of the planet Earth) you cannot find a 3GHz antenna with 20GHz bandwidth.

Small Correction : Minimum Bandwith of 20MHz.

This sounds a lot like you have just cut and pasted a question from an assignment.

A small clue. There is a high likelihood you used such an antenna to ask the question.