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Microstrip Short Circuit

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New for September 2020: Short circuits are often used in S-parameter measurement calibration standards, such as SOLT (short ,open, line and through). In microwave engineering, rarely is something as simple as it seems.  From our good friend Lou (a.k.a MadEngr on the Microwaves101 forum) at Greybeard Microwave, we offer two videos about short circuits you can used as calibration standards. Check out these and other videos over on his Greybeard Microwave Youtube Channel; we recommend you subscribe.

Coaxial short circuit: bonus material!

Before you learn how to make a microstrip short circuit, it is worthwhile learning how a coaxial short circuit behaves. In this video, Lou shows that a short placed across a coax line is for all intents and purposes an ideal element: it does not come with any parasitics such as ground inductance that could cause a time delay or other issues.  Below the cutoff frequencies of undesirable coax modes, the pure TEM mode of coax, that short circuit acts like a mirror.  When you use a coaxial short circuit as a calibration standard, it requires no corrections, something you cannot say about a coaxial open circuit.  During the video you will learn a few things about meshing and accuracy, pay attention.

Microwave Office with Analyst 3D EM solver is used to analyze a conducting disc (short circuit) at the end of a coaxial cable.

GBM001 video

Microstrip short circuit

You are given the job of fabricating a microstrip short circuit, calibration standard and the performance of the lead engineer's design depends on the accuracy of it. You might think a plated via hole will be good enough, or maybe an "edge-wrap". And you would fail. What is needed is an end wall to reflect the wave above and below the strip, without acting like a radiator. If a coax short circuit acts like a mirror, a microstrip short acts like a fun-house mirror!  What size wall is required to short-circuit the quasi-TEM field on a microstrip line with the best results? And how do you actually fabricate this? In the video below from Greybeard Microwave, Microwave Office with Analyst 3D EM solver is used to estimate the wall size using the electric field around the line. Check out how a simple short circuit (without a wall) really stinks, but with the optimum wall you get close to (even appraching) ideal performance: magnitude 1 at 180 degrees on the Ol' Smith Chart.

GBR002 video

Editors note: there is more than one way to fabricate thie microstrip short circuit. This video shows how to use a plated slot with a metal tab.  You  could also cut the microstrip line where you want the short, and ribbon bond it to a  gold-plated shim that is the same height. Then place a second shim on top to achieve the ideal dimension of the shorting wall.

 

Author : Unknown Editor

Source : Greybeard Microwave

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