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Design >> RF Design >> impedance matching going the wrong way
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Message started by pgib8 on Apr 26th, 2017, 10:06am

Title: impedance matching going the wrong way
Post by pgib8 on Apr 26th, 2017, 10:06am

Hello everyone. I'm still new to RF and I'm trying to match the impedance of a PCB trace antenna to about 50 Ohms. However as I try different components for the matching, the impedance moves totally different than what would be expected. For example when I put in a series inductor, the impedance is supposed to go upwards but in my case it's going sideways.

I have 4 pictures prepared that I want to attach, I guess I have to make a few posts go get all the information onto this forum.

Title: Re: impedance matching going the wrong way
Post by pgib8 on Apr 26th, 2017, 10:07am

Just adding more pictures

Title: Re: impedance matching going the wrong way
Post by pgib8 on Apr 26th, 2017, 10:08am

I soldered an SMA connector on for this purpose. The VNA has been properly calibrated.

Title: Re: impedance matching going the wrong way
Post by pgib8 on Apr 26th, 2017, 10:13am

Here is the smith chart, this is all at 900MHz.

Point 1 is without matching (just 0 ohm jumper)

Point 2 added 4.7pF in parallel (component furthest away).
Point 3 reduced from 4.7 to 1pF.
Point 4 increased from 1pF to 3.3pF

As you can see it moves sideways.

Point 5 added series 2.2nH. (component in the middle position)
Point 6 increased from 2.2 to 3.6nH
Point 7 increased from 3.6 to 6.2nH

Also moving sideways

Point 8 added parallel 12nH. (component closest to source)
Point 9 increased from 12 to 22nH.

Finally point 10, replaced series 6.2nH with 0 ohm jumper (component in the middle).

Title: Re: impedance matching going the wrong way
Post by tzg6sa on May 9th, 2017, 6:37am


pgib8 wrote on Apr 26th, 2017, 10:13am:
Point 1 is without matching (just 0 ohm jumper)


Point 2 looks like as if point 1 would be mirror to the middle point. It is curious. With a parallel cap you never go from 1 to 2. The bigger the cap the mpre looks your circuit like a short and the closer you are to point (-1:0).
Point 3 and 1 isn't on a constant reactance curve.

So yes, something is strange in your results.

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