Does a receiving antenna radiate the power received?

Question

Let's consider the case of linear dipole antenna or Hertz dipole.

I know that the antenna acts as a transducer and it radiates the E-M waves due to the acceleration and decelaration of the electrons or charges in the wire. While receiving the signal, it is said that the E-M waves induce a voltage at the input end of the antenna. (Let's call this open circuit voltage Voc).

My doubt:

1. If the incoming E-M wave can induce a voltage won't it accelerate or decelerate the electrons in the wire and doesn't this acceleration cause further radiation (lets call it secondary radiation) of E-M waves?

2. Won't this secondary radiation lead to loss in the effectively received power?

It would be really helpful if I get this clarified. Thank you in advance. (PS: I am reposting this in the physics site. Previously I posted it in Electrical site.)

Answer 1

An antenna has an equivalent impedance both in transmit and in receive. If the load in receive mode is matched to that antenna impedance then there is no reflection from that load back out the antenna, the incident energy on the load will be completely absorbed. Note the emphasis on energy incident on the load not on the antenna. The actual reflectivity of an antenna will depend on the incoming direction of the wave, for example.

Whether the load is matched, therefore, just concerns the load and the way it is connected to the antenna ports via a transmission line, that is a matched load for one incident direction maybe mismatched for a different one. This problem does not show up in transmission mode because there is only one way and one transmission line mode used that the source energy may excite the antenna ports. (This is the main reason why antenna designers calculate the resulting antenna pattern in transmit mode instead of analyzing it in receive mode, by reciprocity the transmit and receive antenna patterns are the same.)

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Following @JanLalisky perfectly correct answer I would like to add that the physical reason for the secondary emissions from a receive antenna is because the incident wave is usually emitted from a far away source so it is either spherical or planar. In either case it is not the type of EM field that a "perfectly matched" antenna would emit in its transmit mode. For example, around the antenna there is a reactive field (non-radiating) that would have be recreated by the incident wave so the antenna metal not reradiate parts of it in some unknown directions, a simple plane wave cannot do just that.

Answer 2

1.If the incoming E-M wave can induce a voltage won't it accelerate or decelerate the electrons in the wire and doesn't this acceleration cause further radiation (lets call it secondary radiation) of E-M waves?

Yes, exactly. It is indeed sometimes (e.g. in physics of EM wave scattering rather than in engineering) called secondary radiation, while the incident wave from the source is called primary radiation.

2.Won't this secondary radiation lead to loss in the effectively received power?

That's not a good formulation (it is misleading). Because without secondary radiation, there would not be any change in the EM energy flow compared to empty space without the receiving system. One cannot assign energy to secondary radiation directly; EM energy is property of total EM field, and secondary radiation changes its flow in space in such a way that some EM energy is captured by the receiving system with help of the antenna, and some other (often the major part) goes past the antenna, around it, further away.

Amount of energy that is captured depends on amplitude and phase of current oscillations in the antenna (relative to phase of oscillation of the primary EM wave), which in turn depend on details of the receiving system connected to it, such as whether all parts of transmission path are impedance matched. Impedance matching is designing/tuning the circuits in such a way that power transfer from source to load is maximal and power reflection back to source is minimal. If impedances are not matched, then the antenna will redirect less EM energy into the system, and more energy will remain in the EM field.

Of course, different antennas will change the flow in different ways, and some antennas will "capture" more EM energy than others, based on their different geometry. But all antennas need to produce some secondary radiation in order to work at all.

Answer 3

Speaking not as a physicist, but as an elderly radio amateur (and retired building surveyor), isn't it the case that our antenna doesn't match the characteristic impedance of free space, hence there is a mismatch at the interface, and thus a reflection. So some of the incident energy doesn't go down the feeder and into the receiver, and that's a loss of incoming signal.

Answer 4

Antenna's by their nature fall into distinct categories.... Either a dipole (omni-directional) or a Yagi (directional) all Antenna's need to be attached to a receiver or a transceiver. In order for the antenna to be effective and efficient it needs to be matched to the resonant frequency that it is designed to operate in. Any piece of wire, as in the style of a Marconi or 'long wire' can radiate "spurious harmonics" however these are usually of a very low value and due to the nature of "coaxial" feeders then any such out of phase harmonics are usually neutralised by the return to earth via the natural pathway. In the case of dipole Antenna's there is an unseen invisible antenna equal to the driven element.... The adage "as above so below" applies. In my experience of over forty years designing and matching/using my own Amateur Radio home brew Antenna's I have never encountered problems with receiving antenna radiating significant amounts of RFI....