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# Why Input Impedence is asumed 50&#937; or 75&#937;?

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#### @YY

##### Member level 2
Input Impedence

for impedance matching, why we assume the input impedance as 50Ω or 75Ω?

Re: Input Impedence

Tradiationaly transmission lines and feeds have this impedance and we need to match the antenna to them.
However, in general fit your antenna to your selected transmission line with its impedance.

Heylal

Re: Input Impedence

This is a common question, and you should search this site for more answers!
But in short:
low impedance is good for Power transmission and High Impedance is good
for low loss, and 10*Log( {V1^2/R1 }/{V2^2/R2}) is easier to compute
when R1 = R2. So in RF that "R" is set to 50Ω and in Cable that "R" is
set to 75Ω.

As a new comer it's better to first search this site, then use google, and then
if you still need clarification ask hear. Because chances are that many of
these questions have been THOROUGHLY discussed and you will end up learning
more on the topic.

Cheers

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