What is the purpose of matching?

[sundarmeenakshi]

matching questions?

Hi Guys
I had a question on matching. What is the purpose of matching? Well, the following is my calculation.
Suppose we have a perfect matching for 1mW for 50 ohms.
Then the voltage at the input points is
V^2/(2*50)=10^-3,
V=0.316 V.

Well, let's suppose there is a resistor of 10^5,
then the reflection coefficent is gamma=(10^5-50)/(10^5+50)=1-1*10^-3.
So the power at the input point is
(1-gamma^2)*10^-3=2*10^-6.
So the voltage at the input is
V^2/(2*10^5)=2*10^-6
V=0.63V
So in the dismatch case, the voltage is actually twice the input voltage for matching case. I am very confuse about this. Can anyone explain what happened here. In some applications, maximum voltage is better than maximum power, so why do you have to have power match in all the place from antenna to LNA.
Thank you

 

[ieropsaltic]

Re: matching questions?

Hello,

For the LNA matching to the antenna case, you're really interested in matching to prevent reflections. If there's a mismatch between your antenna impedance and your LNA, waves can get reflected back to the antenna and radiated. This can cause interference.

 

[heylal]

Re: matching questions?

Hello,
Regardless of your calculations and even interference, you usually want the most sensitive channel that you can get. Meaning transfer as much energy as possible to your detector.
A mismatch is a situation when some of the power is reflected back and not transmitted forward.
If the antenna is not mismatched some of the power will be reflected back to space.
The other reason is of course interference of these reflections that can limit the operation of the channel: Bandwidth and distortions.

Heylal
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