Rrad meaning in Electronics ?

Radiation resistance is that part of an antenna's feedpoint electrical resistance that is caused by the radiation of electromagnetic waves from the antenna. In radio transmission, a radio transmitter is connected to an antenna. The transmitter generates a radio frequency alternating current which is applied to the antenna, and the antenna radiates the energy in the alternating current as radio waves. Because the antenna is absorbing the energy it is radiating from the transmitter, the antenna's input terminals present a resistance to the current from the transmitter. Unlike other resistances found in electrical circuits, radiation resistance is not due to the opposition (resistivity) of the material of the antenna conductors to electric current; it is a virtual resistance due to the antenna's loss of energy as radio waves. The radiation resistance R R {\displaystyle R_{\text{R}}} can be defined as the value of resistance that would dissipate the same amount of power as radiated as radio waves by the antenna with the antenna input current passing through it. From Joule's law, it is equal to the total power P R {\displaystyle P_{\text{R}}} radiated as radio waves by the antenna divided by the square of the rms current I rms {\displaystyle I_{\text{rms}}} into the antenna terminals: R R = P R / I rms 2 {\displaystyle R_{\text{R}}=P_{\text{R}}/I_{\text{rms}}^{2}} .

The radiation resistance is determined by the geometry of the antenna and the operating frequency. The total feedpoint resistance at the antenna's terminals is equal to the radiation resistance plus the loss resistance due to ohmic losses in the antenna. In a receiving antenna the radiation resistance represents the source resistance of the antenna, and the portion of the received radio power consumed by the radiation resistance represents radio waves reradiated (scattered) by the antenna.