Reflection coefficients

The Reflection coefficients are based on concepts introduced in our childhood. Imagine the case when you throw a ball at the vertical stone wall. The ball having its incident power will travel towards the stone wall, hit the wall which will soak up some of its power and then the remaining power, reflected power, will cause the ball to bounce back towards you.

The ratio of (reflected power)/(incident power) is known as the reflection coefficient. The reflection coefficient is often represented by the Greek letter gamma (G).

This simple equation is very practical for the following reasons.

• the value of G is independent of incident power because if you double incident power, if incident power is doubled the reflected power will also be doubled.

• It gives the measure of the hardness, impedance, of the wall to incident power. For example if the wall is stone wall, it is will absorb little incident power will be absorbed and most of the incident power will be returned to you as reflected power. You will get a high reflection coefficient (Ã=1). In case of wodden wall bounce will be weaker giving reflected power. You will get a lower reflection coefficient (Ã < 1). In another case if the wall is made of straw, it is more than likely that almost all of the incident energy will be absorbed and there will be very little rebounce(reflected energy) (Ã=0). Finally, if wall is made of air, the ball will simply go through the air wall and carry all its incident power with it (G = 0). There will be no reflected energy due to fact that the incident energy would simply be exhausted in carrying the ball further. Note there that the transmission medium resistance is air, and it is the same as the air wall resistance which is the load and we simply say that the load is matched to the transmission medium.

• By measuring the angle of the rebounce relative to the incident direction, it is possible to say whether the wall is at some angel to thrower or the wall was vertical. Hence, we can determine the direction of the wall.

• The path via which the ball travels is called transmission path.

• Lastly, you need not even physically touch the wall to find out some of its characteristics. In other words, measurement is indirect. This is very useful in the measurement of transistors where the elements cannot be physically or directly touched. It is also very useful when we want to measure the impedance of an aerial on top of a transmitting tower when our measuring equipment is at ground level.