Unless you hold a First Degree RF Wizard rating, chances are good that coax stubs seem a bit baffling to you. They look for all the world like short circuits or open circuits, and yet work their magic and act to match feedline impedances or even as bandpass filters. Pretty interesting behavior from a little piece of coaxial cable.
If you’ve ever wondered how stub filters do their thing, [Fesz] has you covered. His latest video concentrates on practical filters made from quarter-wavelength and half-wavelength stubs. Starting with LTspice simulations, he walks through the different behaviors of open-circuit and short-circuit stubs, as well as what happens when multiple stubs are added to the same feedline. He also covers a nifty online calculator that makes it easy to come up with stub lengths based on things like the velocity factor and characteristic impedance of the coax.
It’s never just about simulations with [Fesz], though, so he presents a real-world stub filter for FM broadcast signals on the 2-meter amateur radio band. The final design required multiple stubs to get 30 dB of attenuation from 88 MHz to 108 MHz, and the filter seemed fairly sensitive to the physical position of the stubs relative to each other. Also, the filter needed a little LC matching circuit to move the passband frequency to the center of the 2-meter band. All the details are in the video below.
It’s pretty cool to see what can be accomplished with just a couple of offcuts of coax. Plus, getting some of the theory behind those funny little features on PCBs that handle microwave frequencies is a nice bonus. This microwave frequency doubler is a nice example of what stubs can do.
No offense but this is literally RF 101 covered in the most basic of EM fields and waves classes and well known by high school diploma ham radio enthusiasts everywhere. Maybe better to tone down the comments pumping this up as some sort of “wizardy” and replace it with the basic math used to calculate RF stubs… You are trying to make something easy sound difficult which is going to scare some away from learning these basics.
Paul Falstad has a nice simulator for filters that also supports working with transmission line stubs[1].
It’s not exactly rocket science, but since RF is always finnicky, measuring equipment such as VNA is very handy.
[1]: https://www.falstad.com/afilter/
Yeah… Maybe your experience isn’t the only experience.
This stuff might not be Graduate level work, but I guarantee it is outside the knowledge base of nearly everyone. Radio stuff IS wizardry, just like every other specialization.
And saying something is “highschool” level anything is a meaningless statement, because what is taught in highschool varies wildly from place to place.
I went to a “normal” public highschool, and every student rebuilt an automotive engine in regular shop class, played an instrument even if not in the music specialty(which required 3 different ones from various types. Woodwind, string, percussion, or brass), either managed plants or animals depending on the specialization. I’ve since learned that none of that is a “normal” highschool experience. This was a school in the city too. This isn’t some agricultural/farming area.
Did you learn to sail a ship in highschool? One of my good friends did. He wasn’t even in the marine biology specialization. And yes I mean a ship, not a boat. It was a 65 foot twin mast.
How many languages did you learn in highschool?
Languages were an elective in my school.
A coworker had to take a single language for all 4 years to and prove full fluency, and had to take 2 other languages for at least 3 semesters each to pass a ‘basic’ comprehension test. And none of those languages could be from the same region/family. They learned Mandarin, Farsi, and…Welsh I think?
An ex of mine says they learned calculus and linear equations in highschool. We only got precalc in the AP program.
My cousin’s kid learned ARM assembly last year in middle school (grade 8). I helped him with his work and it wasn’t “hello world” stuff. His project was doing stuff I hadn’t learned in a full semester undergrad course.
Heck, my 7 year old niece is learning Boolean logic in 2nd grade this year (just saw the lesson plan).
Don’t jut assume people know things just because you do.
A lot of smart people lack what others consider “basic” knowledge, simply because their particular education/life happened to go a different route.
Most radio stuff requires quite a bit of math and physics experience and intuition to understand. It is not surprising tht many people would never have the basis to pick it up on their own.
You might need the math, a Smith Chart, or an online calculator to design an actual application of transmission line stubs, but you don’t need any of that to understand the basic principles of how they are used. Whether or not anyone bothers to learn that doesn’t make any of it “wizardry” any more than you need to learn organic chemistry to understand how to bake a cake.
I can’t design an airplane wing but I know how they work.
Ian,
Just because you haven’t taken the time to understand something doesn’t make it “wizardry”. There is no education required for these RF concepts beyond algebra. In that sense, maybe you could call it “7th grade wizardry”? You wrote a whole lot of text here for nothing. Anyone has the basis to pick this up on their own unless they are completely illiterate without even a basic middle school education. Even in that illiterate state, they can learn it, they will just have to learn a little basic algebra first.
Plot twist: Ian didn’t graduate highschool.
Coax cable stubs are also useful to operators of λ½ dipoles.
To adapt the symmetric side to the unsymmetric one (dipole coaxial feed line).
Alternatively, an 1:1 balun can be used.
A hand made stub is cheaper though, and may handle more power due being usually more sturdy.
This article makes the topic sound way more exotic than it is. Any science looks like wizardry when you don’t bother to understand the basics, and transmission lines are a good example.
Completely agree hence my original comment. Describing such simple concepts in such a manner does not encourage people to learn these simple concepts. I continue to be amazed almost every day when people tell me that they are not “smart enough” do something that doesn’t even take a G.E.D. to accomplish.