This antenna is a variation of a basic planar monopole antenna. An additional section is attached to excite a second resonance at a higher frequency. As with several other planar antennas, this monopole can be integrated on the same substrate as other electronics. The T-shape monopole makes the antenna more compact than a straight monopole.
The 2D graph below shows the input impedance (S11) with dual resonance. When adjusting either of the resonant frequencies, the impedance at the other resonant frequency will be influenced.Typical S11 impedance of the Printed dual band double T monopole
The following figure shows typical radiation patterns at both resonant frequencies. At the lower resonant frequency, the radiation pattern of the antenna is similar to that of a conventional monopole in free space, with a so-called 'doughnut' shape. The radiation pattern at the higher operating frequency becomes more irregular. For both cases, the shape of the ground plane affects the radiation patterns....Typical gain pattern at (a) lower frequency and (b) upper frequency