EM Waves - Conducting Materials

[pavannanduri]

As we know Electromagnetic waves get reflected when then hit the conducting surfaces

when we use two simple copper wires as Tx and Rx antennas, the EM wave thats generated at the TX end should get reflected when it reaches the Rx..but rather it induces a voltage according to faraday's law of induction

for the EM wave to be generated we need a conductor,..but when this travels and hits another conductor what exactly is the phenomenon thats taking place?

sounds like a basic physics question..but can anyone please answer this?

 

[WimRFP]

You may know that you need an oscillating charge to generate AC fields and EM radiation. So an AC current will produce an EM field (unless fully counteracted by other currents). This is no matter the cause.

The TX EM field that arrives at the receiver introduces an AC voltage into the RX antenna. This will result in an AC current in the receiver antenna's conductor. That AC current will create a new field that will add (constructive or destructive) to the original field. The shape of the reradiated radiation pattern depends on the current distribution.

A well-terminated halfwave dipole reradiates same power as it receives and this can really be measured. The situation becomes interesting when you short-circuit this half wave dipole. The antenna current doubles, hence the reradiated power becomes 4 times higher (both E- and H-field doubles).

Changing the load seen by a receive antenna is used in passive RFID systems. When used at UHF, they call it backscatter systems.

Only structures (surfaces, mesh, arrays, etc) that are significantly large w.r.t. wavelength can reradiate in a beam-like (re)radiation pattern.

So reflection, scattering, refraction, etc is all caused by the fields generated by the currents that were induced by the driving EM wave.

A bit OT: Also induced polarization currents in dielectrics do reradiate.

 

[pavannanduri]

The TX EM field that arrives at the receiver introduces an AC voltage into the RX antenna. This will result in an AC current in the receiver antenna's conductor. That AC current will create a new field that will add (constructive or destructive) to the original field. The shape of the reradiated radiation pattern depends on the current distribution.

A well-terminated halfwave dipole reradiates same power as it receives and this can really be measured. The situation becomes interesting when you short-circuit this half wave dipole. The antenna current doubles, hence the reradiated power becomes 4 times higher (both E- and H-field doubles).

So reflection, scattering, refraction, etc is all caused by the fields generated by the currents that were induced by the driving EM wave.

thanks for the info..but if we consider the reradiation scenario,
then this will effect the source from where the waves are generated ?
can we estimated the reradiated field stength? or is there any such expression to find it?
is there any such mechanism, such as coating th Rx so that we can minimise the reradiation effect?

 

[WimRFP]

If some of the reradiated fields reach the TX antenna, a voltage is induced that interferes with the transmitter voltage. To a certain extend, the fields can be calculated. The half wave dipole and some other structures can be done manually based on basic antenna theory. Large structures can be treated with diffraction theory, but soon the calculations become too complex and you have to divert do simulation and/or measurement.

To get a more specific answer, please give us some more details about your actual application.