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Electrical Length / S Parameter Phase Offset

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Full Member level 1
Feb 1, 2009
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Perth, WA, Australia
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My VNA has an option for entering the electrical length and the S parameter phase offset.

I understand that the option for electical length is so that I can move the plane of measurment from the end of my cables right upto the component on my DUT.

Why on earth do I need to enter an S paramter phase offset? I would have thought the S paramater phase offset would be calculated from the electrical length.

What then is difference between these two options? Why do I need both?


Electrical line length may include tranmission line loss, out and back, where the phase adjustment does not.

I use them both all the time. Lets say I have a small antenna that I want to measure the impedance of. I cal the vna normally (on my machine I just do an open/short "response" cal). But after this cal the reference plane is at some connector plane, not necessarily at the exact antenna feed point. I then take the antenna and solder on a short wire between the feed point and the ground plane. Hook the antenna up, and sweep the frequency (say from 902 to 928 MHz). As I look at a polar display I SHOULD see a small reflection coefficient dot at mag=1, ang=-180 deg.

What I usually see instead is a squigly worm somewhere else. I go to the electrical delay, and change the value until the squigly worm display looks the most like a single dot (it will never look exactly like a dot, just make it as small a radius of squigles as you can). THEN you go to the phase offset and turn it until the center of the "dot" is right on the -180 deg angle.

THen you unsolder the jumper wire to ground, and can measure the antenna's true impedance.

It is a little crude, but for under 6 ghz it works fairly well.
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