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Smith Charts
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I never intended to add a Smith chart to my S-parameter plotter. I can appreciate how useful they must have been before computers, but I don't see their value today. A computer program, especially one with automatic optimization, can make quick work of a complex matching problem. However, since I see references to Smith charts here all the time, before I dismiss them I thought I should ask why people use them. What do they offer that rectangular plots versus frequency don't?
Brian |
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Yes, and if you let the PC do all the optimization work, what have you
learned? Not much other than how to use the PC for the applications. Rectangular plots vs. frequency, Bode plots, give you absolutely no information of the complex portions of impedances. Yea, poles and zeros. They're like an SWR meter. The VECTOR reflection coefficient gives you far more information than just frequency response!! Bode plots address frequency response, and, yes, some additional information. But they are certainly not as powerful as the Smith Charts when it comes to dealing with complex sources and loads. By complex, I mean the reactive components of a circuit, not a "complex" circuit. Sorry, I live on the Smith Chart and have no intention of letting the PC do all the work. What have I missed? Everything..... EXAMPLE: The EE students at CSU up in Fort Collins, Colorado, "learn" electronics sitting in front of PCs doing simulations. I've worked with a few of them before retiring. They are useless in both designing and troubleshooting. If it were up to me, they would not have been hired! They can't even put Ohm's Law on the white board, let alone use it!!!! Sure, they learn how to use the modeling applications. But those change. After a decade or so, they're useless at even running the newer applications. Then they need to find a new career. I'll stick with my Smith Charts, thank you....... Dave - W?LEV On Sat, Apr 26, 2025 at 11:18?PM Brian Beezley via groups.io <k6sti= [email protected]> wrote: I never intended to add a Smith chart to my S-parameter plotter. I can-- *Dave - W?LEV* -- Dave - W?LEV |
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After working with Smith charts for awhile, and understanding them - they provide in one 'glance' a visualization of the complex impedance across frequency, at the same time as a visualization of the SWR across frequency. And at the same time, for a length of transmission line, you can see the wavelengths/half-wavelengths directly. I don't know of another single plot that can do that (I guess you can overlay multiple traces on the same XY plot to acheive it, but it is IMO not as convenient).
Stan KC7XE |
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On Sat, Apr 26, 2025 at 04:32 PM, W0LEV wrote:
If you can plot the real and imaginary parts of an S-parameter as well as magnitude, phase, and SWR, what additional information does a Smith chart provide? Is the advantage that you get SWR and impedance in one curve as Stan suggests? Brian |
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Convenience for graphical design of a matching network. There’s also sort of a qualitative thing that you recognize particular “shapes” on the chart as having particular significance.
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And, if you’re tuning a filter by turning a screw, sometimes seeing the Smith chart in real time is more “intuitive” in some sense. I tune filters looking at magnitude S11 and S22 and S21. For a three section filter, you can see the three resonances, and looking from one end or the other tends to emphasize the section closest to the port you’re looking at. These days, with computers, seeing the Smith chart isn’t as useful. I use plots of magnitude and phase for design. And I work with a lot of more than one port systems, for which the Smith chart is less useful. For instance, if you’re looking at the coupling among antennas in a phased array. On Apr 26, 2025, at 16:46, Brian Beezley <k6sti@...> wrote: |
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Jim,
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You are exactly right about the graphical design of a matching network. Some years ago I worked up a network which involved solving a quadratic equation, easily done. So I had an analytic solution for that network. I wanted a similar, but not identical network that also involved a quadratic. Well, sort of: the equation for that network involved a multiplicative term that made its solution difficult to visualize, at least for me. But in plotting what I wanted each network to do on a Smith chart, I could see a reverse symmetry between the networks. Modifying the first solution using that information yielded an exact analytic solution for the second network. I don't known whether or not I would have seen that solution without resort to the visual presentation of a Smith chart. 73, Maynard W6PAP On 4/26/25 17:09, Jim Lux via groups.io wrote:
Convenience for graphical design of a matching network. There’s also sort of a qualitative thing that you recognize particular “shapes” on the chart as having particular significance. |
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On Sun, Apr 27, 2025 at 06:28 AM, Maynard Wright, P. E., W6PAP wrote:
Very interesting, Maynard. I had pretty much decided to skip implementing a Smith chart until I read your response. I'm all for facilitating insight. I never worked in RF professionally. The Smith chart is foreign to me. Perhaps many NanoVNAs users favor it because it is familiar from their professional work. Offering something familiar may be another good reason to implement it. Incidentally, I wrote my program mainly to plot all four s-parameters, provide reference impedance renormalization to allow measurement and optimization of filters without building a matching network, and to implement the Y21 series-through method that cancels stray reactance. I want to avoid unnecessary embellishments, but it sounds like a Smith chart might be useful. Brian |
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Hi Brian,
As an RF engineer, I depend on Smith charts for the intuitive understanding they enable. It is easy to look at at an impedance and instantly understand what would best match an impedance. It's easy to visualize what effects rotation through a transmission line would have, and pretty much how complicated a matching network will be, before ever designing it. It's invaluable, in my opinion. Just my 2 cents worth. Glad you found it easy to implement. 73, Dean W8ZF |
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On Sun, Apr 27, 2025 at 11:58 AM, alan victor wrote:
I appreciate the advice, Alan. Usually I only program features that I would use myself. I don't anticipate using the Smith chart and I have no experience with them so it's hard to know what detail is reasonable. I'll post a message when the program is available. Thanks also to Dean for your input. Brian |
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Yes, Dean points out an interesting aspect - if you look at it and it’s just spirals, you know a simple network is likely to work. If it’s back and forth zig zagging, probably not.
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On Apr 27, 2025, at 11:58, Dean W8ZF <dwfred@...> wrote: |
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Hi
The Smith chart is simply a specific graphical method for illustrating impedances—originally as a function of line length, and now, with the NanoVNA, as a function of frequency. Its advantage lay in simplifying the plotting process on paper using just a compass to draw the impedance circle based on the electrical length of the line, resulting in a neat circle centered on the chart's origin. With the NanoVNA, this also produces a clean circle around the center, but by varying the stimulus frequency.? This method has allowed me to measure the characteristic impedance of coaxial cables precisely and reliably at a given frequency by centering the impedance circle on the chart's origin. This is achieved by adjusting the normalization impedance using the feature provided in DiSlord firmware version 1.2.40. No other graphical method offers such performance and ease for measuring characteristic impedance (Zc) with this level of elegance and precision. It truly is the 'magic circle' of the Smith chart.? 73s Nizar ChatGPT peut faire des erreurs. Envisagez de vérifier les informations importantes |
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From the original publication in the early 1950s, the Smith Chart addressed
impedance space, not just line lengths. I once had in my hands while working for a living (now retired) the original publication which introduced the world to the Smith Chart. I offered the engineer who owned it $200 on the spot. He refused. I upped my offer to $300 on the spot. He again refused. He also refused my $400 offer. I gave up. At least I got to actually handle and leaf through it! Now I have the time in retirement, but no $$ for "frivolous" things like that..... As an EMC/RFI/RF design engineer and spending a good amount of my professional life as well as on the hobbies designing, building, and matching antennas and any number of RF/?W circuits, the Smith Chart will go to my grave with my decaying body...... Dave - W?LEV On Sun, Apr 27, 2025 at 9:30?PM Team-SIM SIM-Mode via groups.io <sim31_team= [email protected]> wrote: Hi-- *Dave - W?LEV* -- Dave - W?LEV |
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On Sun, Apr 27, 2025 at 02:33 PM, Dean W8ZF wrote:
Another useful input, Dean. I had not planned to do admittance, but adding it should not be hard. I've implemented a marker. You click near the curve to activate it. You can click again or rotate the mouse wheel to move it. I'm thinking of displaying frequency, SWR, and Z for the marker. Can I get away without displaying the reflection coefficient? I like to keep think as simple as possible. Brian |
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Do you refer to Phillip Smith's original paper?
DaveD KC0WJN On Sun, Apr 27, 2025 at 17:45 W0LEV via groups.io <davearea51a= [email protected]> wrote: From the original publication in the early 1950s, the Smith Chart addressed |
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Hi Brian,
With regards to the marker, I can offer an opinion. I rarely used the raw reflection coefficient directly. I was looking to find a return loss, VSWR, or impedance, all of which would require calculation from the reflection coefficient and phase. A marker that did that calculation on the fly is very useful, just as you are doing. Thanks for being open to input! 73, Dean W8ZF |
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On Sun, Apr 27, 2025 at 04:31 PM, Dean W8ZF wrote:
Good. The less clutter the better. So far I display frequency and SWR. Impedance is just about ready to test. I will have to add the upside-down ohms symbol to my homebrew fonts when I do admittance. I love that symbol. We'll see about return loss. Thanks for being open to input! Since I don't know what I'm doing, I welcome any help! Testing the Smith chart on various .s2p files I've collected, mostly from SMD L and C vendors, I immediately noticed characteristic curves. I don't know what all the whorls and wiggles mean yet, but the Smith curves are much more distinctive than the rectangular versus-frequency plots I've been looking at for weeks. Brian |
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The Smith chart, of course, can be replaced by any of many software packages that yield just numerical results, but not the feel for what's going on. One may be really happy with incorrect numbers that fall out of a computer or calculator where the Smith chart might make one think "That ain't right!"
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I still use slide rules for pretty much the same reasons. Although I am thankful that I don't have to work my way manually through hyperbolic functions of complex argument as I did early in my career when no calculators or computers were readily available. 73, Maynard W6PAP On 4/27/25 14:45, W0LEV via groups.io wrote:
From the original publication in the early 1950s, the Smith Chart addressedimpedance space, not just line lengths. |
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