English: Drawing comparing the RF current distribution in a T antenna (b) with the current in a vertical radiator (a), of the same height, demonstrating how the horizontal wire improves its efficiency. The T antenna is used at frequencies where a full quarter-wave (λ/4) vertical antenna can't be built. So the vertical portion of the antennas are much shorter than λ/4. These "electrically short" antennas have low radiation resistance and can't radiate much power. The horizontal wire in the T antenna doesn't radiate radio waves but instead functions as a "capacitive top-load" to add capacitance to the antenna, increasing the current in the top of the vertical wire.

The red areas are graphs of the current distribution on the wires, with the width perpendicular to the wire proportional to the current. The current in the antennas (assuming they are brought to resonance with an impedance matching network) is the tail end of a sinusoidal standing wave. In the vertical antenna (a), the current must go to zero at the top, reducing the current in the upper part of the wire. As can be seen, in the "T", current flows from the top of the vertical wire into the arms of the "T", increasing the current in the vertical wire.

The power output from each antenna is proportional to the square of it's "effective height", which is equal to the area of the red current distribution graph next to the vertical wire. In (a) the current distribution is roughly triangular, so the "effective height" is about L/2. In the "T" the current distribution is more trapezoidal, approaching constant current for long horizontal wires, so the "effective height" approaches L. Therefore the radiation resistance and power output of the T antenna can be up to 4 times that of a simple vertical antenna of the same height.