Smith Charts: Designing a Series Stub Match with a Shorted Stub (00h6)
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- เผยแพร่เมื่อ 16 ก.ย. 2024
- This is the first of the application videos in the Smith Chart series.
In this video I will use a pencil and paper exercise to create a Series Stub Match with a Shorted stub.
I will explain how they work, how they are constructed and how they are designed with a few practical hints along the way.
00:05 Introduction
01:15 What is a Stub Match?
04:22 What is a Series Stub Match?
05:41 STEP 1 = Measure the actual Impedance
06:11 STEP 2 = Know Your Coax (Measuring the Velocity Factor)
08:20 All the Facts in One Place - What are we working with?
08:58 STEP 3 = Normalize your measured Impedance
09:32 STEP 4 = Plot the Normalized Impedance on the Smith Chart & draw the SWR circle
13:13 STEP 5 = Move to the R = 1.0 Circle & read Impedance(a) here and lambda(a)
15:51 STEP 6 = Calculate coax length from Load to Stub (Impedance Transforming)
16:13 Calculate Length in Wavelengths
19:39 Calculate Length in Inches from Wavelengths
22:18 STEP 7 = Determine the lambda(Stub)
26:47 STEP 8 = Calculate the length of the coax for the Stub (Reactance Eliminating)
27:42 Final comments and Toodle-oots :-)
Excellent Ralph ! The graphics and explanation are the best I have ever seen.
Thank you, Todd! My hope is that I didn't leave anything out so someone can watch this video and have everything they need to do what's got to be done. :-)
Beautiful videos of a great teacher!!!
Just one question: in an unbalanced transmission line like coax, doesn't a series stub radiate? If so, it will also carry common mode currents outside the braid along the rest of the line as well.
Isn't it better to use a balanced line with the same characteristic impedance (and known speed factor) at least for the series stub?
Thank you!
An interesting question.
With a series stub, the stub never is connected to the braid of the impedance transforming coax (main feedline) of the assembly. The braid of the main feedline is connected across the point of where the stub lives. The stub lives in series with the center conductor of the main feedline. So, I suppose, if there is some common mode current on the braid, this would continue on.
Thinking about the business of the stub radiating ... it sure seems that it would being that the signal runs down the center and back by the shield. As far as using balanced vs coax (unbalanced), it actually would make no difference. The stub doesn't know the difference between the two.
Me? I prefer parallel stub matches as the physical implementation is a LOT easier. Open stubs tend to radiate some. The issue (open or shorted) is more a matter of how long do you want your stub to be. One will be shorter that the other. Open radiates more, but how much? And, is this an issue? 🙂
@@eie_for_you I agree: parallel is better and the rest is not a problem. 73s.
@@picwiz2 Cool! 🙂
At 4:25 you show a diagram of the stub which I think you arrive at around 9" in length. Effectively the outer surface of the stub outer conductor is connected to the inner conductor on the source side of the stub, so this almost eight wave conductor is a radiator and applies some impedance in shunt with the match. You seem not to have considered any of this?
It took me a while to get what you were getting at in your comment. I hope I didn't put you off with my initial reply.
If we remember that a transmission line (like coax) acts like a transmission line as defined by transmission line equations, then this demystifies the phenomenon (or further confuses us, cuz this stuff is often not even close to being intuitively obvious).
The Smith Chart shows us that a piece of coax shorted at one end will, first, look like a small value inductor and, as it gets longer and longer, this inductance value will increase in value until it electrically reaches 1/4 wavelength. Then, all of a sudden, this inductance in all of its magnitude, will instantly change to a high value of capacitance (and we say, "WHAT??? But its shorted!"). It's transmission line magic!
As it proceeds to get longer yet, the value of capacitance that it exhibits will begin to diminish until it reaches near zero when the coax is electrically 1/2 wavelength in length. Then it starts its journey again increasing in its inductive value and so on every 1/2 wavelength.
Of course, this all assumes a lossless transmission line which real world coax certainly is not. This also plays into the need to make adjustments to the lengths in a stub match after initial construction. Yeah, there is a LOT of MAGIC here ... but, it works! :-)