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scubasteve AntennaTheory.com Newbie
Joined: 19 Feb 2017 Posts: 6 Location: USA

Posted: Sun Feb 19, 2017 10:44 am Post subject: Transmission Line Tutorial from Main SitePossible Mistake? 


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 Site Admin
Joined: 03 Jan 2007 Posts: 199

Posted: Mon Feb 20, 2017 7:16 pm Post subject: 


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 AntennaTheory.com Newbie
Joined: 19 Feb 2017 Posts: 6 Location: USA

Posted: Wed Mar 01, 2017 6:06 pm Post subject: 


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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