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S-parameters of Matching Unit with Different Port Impedances - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: S-parameters of Matching Unit with Different Port Impedances
Unread postPosted: Sun Mar 20, 2005 9:56 pm

The ultimate objective of this exercise is to be able to determine the input impedance of an antenna when it is connected to the output port of an antenna matching unit (AMU). The reason for making the measurement in this way because it is not possible to make the measurement directly at the input terminals of the antenna.

I have characterized the AMU on the bench (which includes a 50:100 ohm transformer) in a 50 ohm environment with a network analyser. What I would like some help with is, how to convert the S-parameters the I have measured in the 50 ohm environment, to a new set with 50 ohms on the input port and 100 ohms on the output port. (i.e. where the output is loaded with 100 ohms and not 50 ohms).

Can anyone offer any advice?


Post subject: Antenna Matching & Network analyzer
Unread postPosted: Sun Mar 20, 2005 10:40 pm

It seems that what you need to do is to characterize the AMU with the 50 Ohm port of the AMU connected to the 50 Ohm port of the network analyzer, and the 100 Ohm port of the AMU connected to a minimum-loss pad to convert the impedance to 50 Ohms. You will, of course, have to compensate for the loss of the pad.

Or it could be that I'm misinterpreting what you're trying to do, and that you're expecting an antenna load impedance (rather far) away from 100 Ohms, and want to be able to know what that is from the appearance at the 50 Ohm impedance side. Any reason not to use theory there, based on the topology and values of the AMU?

Good Luck!


Post subject:
Unread postPosted: Mon Mar 21, 2005 12:56 am

Thanks for your advice.

My strategy was to characterise the AMU at its nominal output impedance of 100 ohms and to then indirectly measure the complex input impedance of the antenna by measuring the input reflection coefficient at the AMU. The antenna's input impedance will, for part of the frequency range, deviate significantly from the nominal value of 100 ohms.

I have made a measurement as you've suggested with a 50:100 ohm pad. I wanted to know if there's a way of transforming the S-parameter measurements made in a 50 ohm environment. I could then check the results I have obtained using the 50:100 ohm pad.

Thanks again.


Post subject:
Unread postPosted: Mon Mar 21, 2005 10:30 am

I have an excel spreadsheet that will perform the conversion you are looking for , but I have been able to figure out how to place an attachment. If you could tell me how to place an attachment on the forum I will be happy to do so..


Post subject:
Unread postPosted: Mon Mar 21, 2005 4:15 pm

I'm not sure how to attach documents to replies in the forum.

If you would email it to me I would be most grateful. My email address is:

Thanks for your assistance.

Posted  11/12/2012

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