1 Answers
Radiation resistance is that part of an antenna's feedpoint electrical resistance that is caused by the emission of radio 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.
Radiation resistance is an effective resistance due to the power carried away from the antenna as radio waves. Unlike conventional resistance, radiation resistance is not due to the opposition to current of the imperfect conducting materials the antenna is made of. The radiation resistance is typically defined as the value of resistance that would dissipate the same amount of power as is dissipated by the radio waves emitted from the antenna, with the same input current passing through it. From Joule's law, it is equal to the total power P R {\displaystyle P_{\mathsf {R}}} radiated as radio waves by the antenna divided by the square of the RMS current I R M S {\displaystyle I_{\mathsf {RMS}}} into the antenna terminals: R R = P R / I R M S 2 . {\displaystyle \,R_{\mathsf {R}}=P_{\mathsf {R}}/I_{\mathsf {RMS}}^{2}~.}
The radiation resistance is determined by the geometry of the antenna, the operating frequency, and the antenna location , and the point on the antenna at which the feedline is attached.The total feedpoint resistance R i n {\displaystyle \,R_{\mathsf {in}}} at the antenna's terminals is equal to the radiation resistance plus the loss resistance R L {\displaystyle \,R_{\mathsf {L}}} due to "ohmic" losses in the antenna and the nearby soil: R i n = R R + R L . {\displaystyle \,R_{\mathsf {in}}=R_{\mathsf {R}}+R_{\mathsf {L}}\,.} 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 by the antenna.