A broadband antenna has a substrate, a coupling conductor, a conductor string, a ground conductor and a ground plane. The coupling conductor has a first coupling member and a second coupling member being separated from each other. The conductor string and the ground conductor are connected to the second coupling member. The conductor string extends along a direction opposite to the second coupling member. The ground conductor is connected to the ground plane. The broadband antenna uses the coupling conductor and the ground conductor to adjust input impedance for impedance match. The conductor string functions as a multi level resonance circuit to increase impedance bandwidth.