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Transmission Line Tutorial from Main Site-Possible Mistake?

 
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scubasteve
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Joined: 19 Feb 2017
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PostPosted: Sun Feb 19, 2017 10:44 am    Post subject: Transmission Line Tutorial from Main Site-Possible Mistake? Reply with quote

Hi.

First post here so if I've gotten any of the etiquette wrong please let me know.

In this page from the main site I believe there may be a mistake in the calculation of the 800 ohm input impedance, or if not a clarifying sentence could be made.

Looking at the second to last image, copied below:



In the last line, I believe what is happening is that the real parts of the second expression drop out as they are dominated by the tan(pi/2) terms which tend to infinity. So then the (j) terms drop out leaving you with just 200*inf/(-50*inf) and the infinities drop out. Thus I think the result should be 200*-200/50=-800 rather than positive 800. How could it be positive?

If I am incorrect somewhere can someone please explain where? Or if not could a clarifying statement be added to the tutorial page to explain why the answer is not negative?

Many thanks!
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admin
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Joined: 03 Jan 2007
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PostPosted: Mon Feb 20, 2017 7:16 pm    Post subject: Reply with quote

You are right on the math that tan(beta*L) goes to infinity so it dominates. It looks like the minus sign in the denominator is correct, but there should also be a minus sign in the numerator. The net result though should be positive (you would know something is wrong if input impedance is negative 800 Ohms)
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scubasteve
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Joined: 19 Feb 2017
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PostPosted: Wed Mar 01, 2017 6:06 pm    Post subject: Reply with quote

Quote:
It looks like the minus sign in the denominator is correct, but there should also be a minus sign in the numerator.


what causes either term to change from a positive in the first line to a negative in the second line? Why wouldn't they both be positives?
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