Antenna Signals: Reflection, Distance and Phase

[dervast]

Hi let's suppore that an antenna creates two signals... The two signal are received from an other antenna but with a main difference. The one of the two signal is reflected in a surface which means that it will travel bigger distance... The two signals are received with a difference in the phase... But what do we mean phase in a signal?

 

[Astronuc]

Phase has to do with the shift between corresponding amplitudes between signals.

For example, given signals $A cos (\omega\,t)$ and $A cos (\omega\,t + \theta)$, the phase shift is given by the angle $\theta$.

The phase shift can be spatial, as for sound and water waves, or temporal, as in two EM signals.

Constructive and destructive interference are caused by phase differences.

 

[dervast]

Thx a lot ... but what do we mean that two signals are received with a difference in the phase?

 

[Ouabache]

Thx a lot ... but what do we mean that two signals are received with a difference in the phase?

The differerence is in time
The phase shift is temporal in EM (electromagnetic) signals (as Astronuc explained). Meaning the two signals will arrive at the receiving antenna, at two different times. This is useful information, if you would like to know the direction the signal is coming. But creates a problem when you are trying to utilize information from more than one propagated signal.

This happens (for example) when you use a cell phone and are moving (walking, driving, flying). For the antenna at the cell tower to most efficiently utilize all of your signal, it should add all of your phase-delayed signals and then shift each one by the appropriate time, so that they become reconstructed in the same phase.

 

[SGT]

Thx a lot ... but what do we mean that two signals are received with a difference in the phase?

If this happens with a TV signal, you will see a ghost image, slightly apart from the original one.

 

[Ouabache]

If you listen to an AM broadcast signal at night time, even from a stationary reciever, you may hear a distant broadcast slowly become softer and then louder. You are getting destructive & constructive interference (as Astronuc mentioned) between the ground waves and the ionospheric reflected waves.