Voltage Reflection Coeeficient

[devonsc]

voltage reflection coefficient

Hi there,

Do you guys mind to briefly explain what does a negative voltage reflection coefficient means?

I understand that it is calculated as => (Zl - Zo)/ (Zl + Zo). Where Zl = line impedance and Zo = characteristic impedance. But don't get what does a negative voltage reflection coefficient means. Please?

 

[glenjoy]

voltage reflection

As far as I know, that formula is used in computing the SWR, and that shows the mismatch between the line impedance and antenna impedance which is being matched to the impedance of the atmosphere to have an efficient transmission. It is also know as the maximum power transfer, applicable also to electrical sources and loads.

As SWR is commonly in maximum is infinity and the lowest is 1, the practical measured SWR is between 1 and 1.2 to be ok.

SWR values corrected, got confused with reflection coefficient and its reciprocal.

Thanks for the correction and reminding me.

 

[devonsc]

negative reflection coefficient

Hi there,

Confused. Newbie here. I thought SWR is always greater than 1?

Help needed. Thanks in advance

 

[fekete]

voltage reflection coefficient derivation

Negative voltage reflection coefficient means that the reflected wave has it’s voltage in opposite phase to the incident wave’s voltage. It also means that the total voltage at the reflection point will be lower that the incident. The extreme example is the short circuit when the voltage reflection coefficient will be –1 and the reflected voltage in exactly – the incident voltage at the short. The total voltage is thus 0 there, as expected for a short.
This reflection coefficient can be complex valued also in which case it expresses a certain phase and magnitude relation between the incident and reflected voltage at the reflection point.
You are right SWR is always >= 1. And it is not used only antenna related issues.

 

[biff44]

reflection voltage

Way you help!

That IS the formula for voltage reflection coefficient
ρ= (Zl-Z0)/(Zl+Z0)


The relection coeficient is a complex number. In other words, if you send a signal down a transmission line towards a load, the reflected signal has a magnitude and a phase angle.

If it is a passive load, the magnitude of the reflection coefficient is always less than unity. That is because a passive load can not "amplifiy' the reflected signal.

But the phase angle can be anything, from 0 to 360 degrees.

If the load is a short circuit, and transmission line is 50 ohms, Zl=0, so ρ = (0 - 50)/(0 + 50) = -1, or a magnitude of 1 and a phase angle of -180 degrees.

If the load is an ideal capacitor, the load impedance is Zl = 1/( j 2ΠfC), or in other words the load is purely imaginary. You use the same reflection coefficient equation as above, and find that the magnitude is still unity, but the phase angle is some negative number that is between 0 and -180 degrees.

So, in general, if the reflection coefficient ρ is real and positive, the load impedance is purely real and greater than or equal to 50 ohms.

If ρ is real and negative, the load impedance is purely real and less than of equal to 50 ohms.

If ρ is complex, the load is some combination of real resistance and reactance.

It helps to play around with a SMITH CHART to understand some of these concepts.