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Assume we have an antenna in receiving mode, connected to a matched load. The equivalent circuit is a voltage source in series with the antenna feedpoint impedance and the load impedance. As we know, the feedpoint impedance is made up of a reactive component, an ohmic resistance and a radiation resistance. Now, since the load is matched, half of the intercepted power will be delivered to the load, while the other half will be delivered to the ohmic and radiation resistances respectively. The power delivered to the ohmic resistance will be converted into heat, but how to interpret the power delivered to the radiation resistance? Is this part of the total power reflected by the antenna?

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  • \$\begingroup\$ Where does the reactive component come from? \$\endgroup\$ – Andy aka Dec 6 '15 at 22:29
  • \$\begingroup\$ @Andyaka I'm not an antenna expert, but it is my understanding that all antennas produce a field comprised of a radiative part and a reactive part, where only the former is present in the far zone. The direction of the Poynting vector (power density) for the radiative part is constant in time, while it for the reactive part is oscillating back and forth. Across the feedpoint, this translates into a resistance and reactance respectively. \$\endgroup\$ – Troels Folke Dec 7 '15 at 21:11
  • \$\begingroup\$ No, an antenna, if matched to its optimal frequency will look resistive. Sure, if it's not tuned exactly then it will present a complex impedance. This is why I asked because it seemed you were taking an obscure line with the question. For instance a half wave dipole is 70 ohms + 42 ohms but if the length is reduced to 0.48 it becomes 70 ohms resistive. \$\endgroup\$ – Andy aka Dec 7 '15 at 21:45
  • \$\begingroup\$ The fields up close are complex and non-coherent regards a proper EM wave but, if the antenna is tuned it will produce a radiating power that makes its input impedance (and o/p impedance when receiving) purely resistive. \$\endgroup\$ – Andy aka Dec 7 '15 at 21:46
  • \$\begingroup\$ You are right, wrong on my part to say anything general about shape of near field. I wonder, however, whether a perfectly tuned antenna will store and retrieve energy from it's near field or only a nontuned antenna will do this? \$\endgroup\$ – Troels Folke Dec 8 '15 at 13:15
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Yes, it is effectively radiated back by the antenna. If the antenna is receiving from a point source, but its radiation pattern is not pointing only at the source (e.g. is isotropic), this radiation will go in 'all' directions. Thus the antenna will redirect (reflect) the incoming signal back in a broader beam.

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No, in general, there is no such effect as reradiation. This heavily depends on the antenna type. The radiation resistance is a model for the radiation. Due to reciprocity, one should still match to this resistance in the receive case, but the simple model as you describe it (a voltage source in series with the antenna feedpoint impedance) is not valid in the receive case.

Generally, all power accepted by the antenna (described, e.g., by the effective aperture size) is received. Any other amount can be scattered by the antenna, but this is related to geometrical features of the antenna and, in the matched case, by no means always 50% of the power, but a rather arbitrary value.

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