1
\$\begingroup\$

I'm aware there are frequency filters, but at the electron and EM field level, if there are multiple waves directed at an antenna, how does it differentiate between 2 different waves and frequencies?

If wave1 is captured by an antenna, how does the filter/antenna know that there is another wave2 and that the E field is related to a different wave?

How does it know that those excited electrons are from wave1 and not a part of wave2 for example?

\$\endgroup\$
  • \$\begingroup\$ It doesn't differentiate. Linearity means that until you get to absurd power levels (or unless you have corroded connections) it doesn't need to - it just passes the mixed up combination on for the receiver to sort out. \$\endgroup\$ – Chris Stratton Feb 8 '17 at 5:42
  • \$\begingroup\$ @Chris Stratton, doesn't a simple dipole have a degree of selectivity around its design frequency band? \$\endgroup\$ – Chu Feb 8 '17 at 8:13
  • \$\begingroup\$ @Chu only to a relatively minor degree - that is not the principle at work here. Note that the question did not specify any substantial frequency difference, yet radios can still discriminate - as long as the system remains in a regime of linear behavior you can user filters to select the desired signal after the antenna, and do so in a way that is much more selective. \$\endgroup\$ – Chris Stratton Feb 9 '17 at 0:57
4
\$\begingroup\$

The antenna can't tell the difference between signals at all.
Even if an antenna is broadly tuned to a band, there could be many different transmissions within that band. It is only with tuning, filtering, mixing and finally decoding that the signals you are interested in can be differentiated from the rest.

For example, to receive simple AM broadcasts you could have a non-tuned antenna and feed the signals (all mixed up) into an RF amplifier and then into a mixer fed also by a carrier frequency of your choice (a local oscillator). When you mix the carrier signal with the modulated signals from the antenna you get product and difference signals. The difference signal turns out to be the audio you desire. You could start reading here.

\$\endgroup\$
3
\$\begingroup\$

Most antennas work by being resonant at the frequency of interest. The resonant frequency of the antenna is determined by its physical dimensions.

So actually the antenna reacts to all the fields it is exposed to, but it reacts 'better' to those where it is resonant. In a similar way, several guitar strings emit different sound waves when being played, each one proportional to its length, even if they are strummed with the same force.

\$\endgroup\$
  • \$\begingroup\$ Thanks. So it is by resonance mainly. But what about Frequency Hopping? How would those be received efficiently \$\endgroup\$ – Rain Feb 8 '17 at 4:52
  • \$\begingroup\$ While not untrue, this differentiation is in most ordinary cases fairly minor; the main factor is that the question is based on misconception. \$\endgroup\$ – Chris Stratton Feb 8 '17 at 5:44
1
\$\begingroup\$

All antenna has gain characteristic. It can received any signals so long the signal is within the antenna frequency range.

With so many signals received by the antenna, the engineer select his wanted signals by tuning to the frequency, implementing a band-pass filter.... followed by analog digital conversion (can be optional)...etc. (This is a simplified description and the real system is far more complicated)

enter image description here

Above: The antenna gain of OmniLOG 70600. The antenna is supposed to be used from 680MHz - 6GHz, according to specs. There are performance penalties outside the range.

\$\endgroup\$
0
\$\begingroup\$

Antenna is an electrical device which converts electric power into radio waves, and vice versa, you can imaging the effect on water when you put a stone in it, similar to this the electric power applied to the antenna produce a disturbance (electro magnetic wave) to the applied electric power which propagate in air, when this disturbance hits an antenna (receiver) it produce electric power depending on the strength of wave. radio waves are prone to interference

Now to interpret signal the receiver stage will have a wide band (the range of frequency the receiver should cover) amplifier which amplify the received signal then we will have one or multiple tuned (band pass) filter which allow only the required frequency that is of interest, the next stage will be decoding of the message from the signal.

radio waves are prone to interference, frequency hopping, FDM, TDM are few methods, techniques to address various issues that are dealt with radio waves as mode of communications, today there are various technologies available to make effective wireless communication, like WIFI, BLE, etc

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.