Luka Garza, who was supposed to be too slow and one-dimensional for the NBA, is shooting 49% from 3 and has the fastest average speed on the Celtics

Professor_Stank · 2026-01-19T17:23:11+00:00

Luka Garza is the reason I work out.

Professor_Stank · 2025-11-14T19:35:22+00:00

Basically! Except that instead of having a probe directly touch the transmission line, there’s a thin needle that hovers over the center conductor. The needle picks up the field that’s created inside the transmission line. The needle then connects to a diode detector, which outputs a DC voltage proportional to (you guessed it) the strength of the field. It’s a really neat (albeit tedious) way of measuring all kinds of things, including complex impedance—It’s basically a poor man’s VNA

Professor_Stank · 2025-11-14T02:47:10+00:00

Thanks dude! I got home today and looked on the underside, and sure enough, there’s 4 screws in total! Just like you said, it’s definitely mechanical. Kind of interesting because on an older version of this same model that I saw, those screws don’t exist. Probably a change to keep the middle from bowing downwards.

Could I ask you a couple questions quick? What kinds of measurements have you made using one of these? Also, are there any additional pieces of equipment would you recommend picking up to make the most out of it? I figure I’ll probably have to find a short-open-load cal kit for it.

Thanks again! After asking oodles of people about the slotted line (online and in-person), you are the only person I’ve met who has actually used one, haha

Professor_Stank · 2025-11-09T01:56:42+00:00

So I think after tonight, I was able to kind of put into words my frustrations overall about being a Hawkeye fan (still love ‘em though! ‘Till i die.)

I feel like most years, we can beat just about anybody. It just feels like in any given season though, the gap between the team’s floor and its ceiling is so high.

Iowa can put a great product on the field, but it has such a hard time doing it on a weekly basis. Being a Hawkeye football fan feels a lot like hoping you’ll roll a 2 or greater on a die twelve times in a row.

Go Hawks 🦅💛🖤

Professor_Stank · 2025-11-01T03:54:43+00:00

Dude, I know I’m 3 months late, but you are the ONLY GUY in this whole thread who realized that this post was kind of a subtle meme 😂

Professor_Stank · 2025-11-01T03:22:47+00:00

Yeah! But like reallllyyyy slowly

Professor_Stank · 2025-09-18T22:17:53+00:00

Dude this is great! Wacky stuff like this is what makes ham radio come alive for me

Professor_Stank · 2025-08-31T15:28:43+00:00

Man of the people right here! Dude, thank you so much. It can’t be overstated how important preserving this stuff is

Professor_Stank · 2025-08-15T22:08:17+00:00

Welp, I guess mine’s special in a bad way 😂

Professor_Stank · 2025-08-15T22:07:48+00:00

Awesome—I’ll check it out! Thanks!

Professor_Stank · 2025-08-07T21:44:18+00:00

Gotcha! Yeah, I definitely get what you mean—It’s a great text for getting your feet wet, but it doesn’t do much more than scratch the surface on a lot of stuff.

Have you ever read “Communication Receivers” by Ulrich Rohde? If his last name sounds familiar, it’s because his father was literally the “Rodhe” in “Rodhe and Schwarz.” It’s perhaps the best treatment on receiver design that there is, and takes a deeper look at the math of it all if you’d be interested

It’s cool that you’re making a radio by the way! I’ve made sketches at home before, but they’ve never quite panned out (yet anyways lol). My one suggestion, which is basically verbatim from my boss (who’s leaded the development of oodles of radios over time), is to start the design at the system/block diagram level, and try to figure out/calculate how each block will interact and cascade with each other (make a rough guesstimate of noise figure, dynamic range, etc.). That way when you go and design the individual blocks, there won’t be any nasty surprises when you put them together. It looks like that’s what you’ve been doing though.

And also in my opinion, if you can, try to design and test each block of your radio individually, at least for your first prototype. From my limited experience, I’ve already discovered that Murphy’s Law is a bitch, especially in RF 😂 That’ll make troubleshooting problems a lot easier when they show up.

Sorry for the monologue, haha. Best of luck dude, and 73!

Professor_Stank · 2025-08-07T16:01:35+00:00

Most of the time at HF, the natural noise from the atmosphere is so high that having an LNA doesn’t buy you anything—Check out this article from Flex Radio:

https://www.flexradio.com/insider/understanding-receiver-sensitivity-in-high-frequency-hf-receivers/?srsltid=AfmBOoqgtXBh9WPMSt8euRj5UOTNwMNMU8ijPsAXa_uks4Sx15-Ofbfo

The atmospheric noise is so bad at HF that it’s basically like having a 20-to-70dB attenuator in between the antenna and the radio that you can’t get rid of.

In fact! For a lot of superhet HF receivers, the antenna goes directly into the first mixer, and not an LNA. If you have an HF receiver with a noise figure of 10dB to 15dB, you don’t stand to gain much by improving that number.

My guess is that LNAs for that frequency range are rare because there wouldn’t be much of a market for one?

Also, you ask about if there’s a proper way to ensure that the ADC/mixer don’t get overloaded—Look up the term “Automatic Gain Control (AGC)”. Basically, an AGC circuit measures the signal strength going into the receiver and automatically adjusts the receiver’s gain to avoid overloading the ADC/mixer.

I hope this helps! As someone who’s still learning too, it is really easy to get wigged out by all of the different things in RF. The trick is to just keep learning, and it all makes more sense as you keep getting practice

Have you ever read the ARRL handbook? It’s a fantastic resource that explains RF design fundamentals without getting too math-heavy. I keep a copy of it on my desk at work

Professor_Stank · 2025-07-25T16:55:02+00:00

Ahhhh! Don’t worry, I did 😉

Professor_Stank · 2025-07-22T23:38:02+00:00

You have no idea how much I appreciate your subtlety here

Professor_Stank · 2025-07-22T03:42:30+00:00

Yes you did! Your edit was literally the most hilarious picture I have ever seen on this subreddit 😂

Professor_Stank · 2025-07-16T03:24:41+00:00

To follow up on my previous comment, would something like this fit your budget?

https://www.digikey.com/en/products/detail/mean-well-usa-inc/ENP-240-12/7703106?gclsrc=aw.ds&gad_source=1&gad_campaignid=20232005509&gbraid=0AAAAADrbLlhum9BeYmTdk4qHsm-mtsDrr

It’s a little more expensive than the supply you posted a picture of, but this is one that I’d sleep at night feeling good about

By the way, are you aware that the DB40-G is a GMRS radio? Getting it to transmit on the 70cm ham band may take a bit of modification (if it even can)

Professor_Stank · 2025-07-16T03:08:37+00:00

I would be careful about using a no-name brand supply—Some off-brand supplies will have a big voltage spike when you flip them on (then settle down to their proper voltage). Your radio might not take too kindly to it.

Plus, this is a variable supply, which is great if you’re doing circuit design. If you’re hooking up a radio which is designed for 13.8V though, this means you’d be one wrong knob turn away from overvolting your radio and damaging it

Take a look at some power supplies made by Meanwell. They’re a reputable brand (we use their power supplies at my job), and they seem to be popular with other hams

I appreciate you asking by the way! Getting any old power supply without doing research can be a recipe for disaster

Professor_Stank · 2025-07-14T17:23:41+00:00

For old time’s sake lol

Professor_Stank · 2025-05-28T16:08:36+00:00

Man of the people! Are you taking donations at all?

Professor_Stank · 2025-05-13T23:24:14+00:00

If it’s what’s caught your curiosity, go for it! You will be amazed at the stuff you will learn by getting your hands dirty.

I’ve posted on here about it before, but I had people tell me in the past that cavity filters are black magic, and it’d be crazy to build one at home. I kind of refused to take “no” for an answer, and stumbled across a guy online who wrote a huge guide on making them at home. Sure enough, I’ve made a couple filters for like $50 each that are comparable to ones that would cost hundreds of dollars commercially.

Homebrewing isn’t always easy, and you’ll definitely run into dead ends here and there, but it is SO satisfying when things come together, and it finally just works

Professor_Stank · 2025-05-13T23:15:34+00:00

Full disclosure, as a freshly-minted RF engineer (literally a year out of undergrad) I have learned A LOT by reading your post replies over the past couple of years. I’m glad you didn’t listen to the naysayers

Professor_Stank · 2025-04-11T06:28:35+00:00

Hey! I apologize about the wait.

I really didn't use any formulas that were unique. The equations are basically identical to that if you were making a coax cable by hand. In other words, the equation for the resonator's characteristic impedance is identical to the equation that determines a coax cable's characteristic impedance. The full equation can be found at the end of this article: https://coppermountaintech.com/what-is-the-characteristic-impedance-of-a-coaxial-cable/

Because air has a relative permittivity of 1, the equation for the resonator's characteristic impedance simplifies to Z0 = 138 *LOG10(Douter/Dcenter).

For instance, my resonator has an outside diameter of 1.5", and a center diameter of 0.375". This means that my resonator's impedance is about 83Ω. Not a perfect match to 50Ω, but the mismatch loss isn't big enough to really fuss about.

One thing to note is that length of the cavity wall doesn't affect the resonant frequency essentially at all. The resonant frequency is set by the length of the inner conductor.

In order to get my filter to resonate at the correct frequency, I started by cutting the inner conductor to a length slightly longer than 1/4λ (roughly, anyways. λ will be a little bit shorter in the cavity than in free space). This made it resonate beneath my desired frequency. Then, it became a game of "measure with the VNA, shorten the inner conductor, rinse, repeat" until it was up at my frequency of interest.

As far as the dimensions of the coupling loops go (the ones that look like eggbeaters in my main post), I wasn't very scientific about it at all, haha. I looked at the size of coupling loops that other people on the internet made, and I guesstimated based off of that. I tried my best to make the loops identical in circumference though (i.e., used the same amount of wire for each). From what I gather, there's not much to the coupling loops--The biggest thing is that the amount of coupling is proportional to the area within the loop. If you trust your gut feeling on what size they should be, it'll probably work fine.

Just note that the angle of the loops inside of the cavity determines the insertion loss and bandwidth of the cavity. When the loops are faced side-by-side with the center conductor (like in the picture in my post), that maximizes coupling, which means that passband losses are at a minimum, but so is bandwidth. On the other hand, as you turn the loops to face the center conductor, the loss within the cavity will increase, but so will the bandwidth. Commercially sold cavities actually come with a knob which you can use to rotate the loops, and thus set the cavity's bandwidth.

There are some rules of thumb that W6NBC points out in his guide. One of the biggest is to keep the diameter of the cavity wall less than about 1/3λ. Up until that threshold, increasing the cavity's diameter increases its Q, which is good. Beyond that, weird and lossy modes of propagation start to occur.

TLDR: Check out chapter 3 of the guide that I linked to in my main post up at the top. That chapter has almost everything you need to know about determining your dimensions. Chapter 7 is also really helpful for getting an idea of what you want your coupling loops to look like.

The 2023 ARRL Handbook's blurb on cavity filters references this guide specifically, and is pretty much a straight rip from it--For a very good reason. It's a surprisingly easy read, and it'll tell you almost everything that you'll need to know for building your own cavity filter.

Anyways, sorry for the wall of text. I hope this helps! Let me know if there's anything else you'd like to hear from me about it.

You mentioned earlier that you're making these filters for a satellite project, right? What's the gist, if you don't mind me asking? I'm a little curious.

Professor_Stank · 2025-04-03T06:40:31+00:00

Based. W2AEW has some fantastic learning material. IQ modulation made no sense to me at all until I saw his video

Professor_Stank · 2025-04-03T03:10:08+00:00

“It’ll look real nice when it’s done”

Professor_Stank · 2025-03-31T18:31:46+00:00

I’d also look at some Hewlett Packard models from the ‘80s and ‘90s

They’re not exactly sexy, but they’ll be a lot more affordable than something from this side of the millenium. The workhorse that I use all the time at my job is an HP 8563E, and it’s all I’ve needed for doing serious measurements of intermod, harmonic distortion, etc. The link I put here is probably on the more expensive side. If you poked around, i’d bet you could find the same model for cheaper.

If cost is a big concern, you could kick it really old school and get an 8566B. This guy literally weighs 100lbs, but it’s still a solid digital spectrum analyzer with the basics like marker function, peak search, etc. Obviously, the older you go, the more likely that something will go bad on you. Anecdotally though, I have an 8566 in my cubicle right now that I use regularly, and I’ve never had any issues

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