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Tube❄️Time

the app note wants me to have a bunch of vias connecting the two copper planes, so i used a very fine PCB drill and a Dremel Stylus (RIP) to make these holes.

copper board with tiny holes drilled at the edge of the large copper plane.
42 comments
Tube❄️Time

here's a technique i'm trying for making my own vias. i snip off part of a component lead (like a capacitor or something) that fits through the hole snugly, then cut it off, leaving ~0.5mm protruding from both sides of the board. then i clamp it tightly in a vise to smoosh it down.

a row of tiny drilled holes. some of them are filled with wires that have mushroomed out, locking them in place.
Tube❄️Time

for the antenna connection, i've soldered some very thin coax to the launch. the other end was pre-terminated to an SMA connector. you can often buy these online as U.FL to SMA adapters; just snip off the U.FL connector. (the ferrite beads limit common-mode currents.)

antenna board with a thin coax cable soldered to it. a row of small ferrite beads are strung on the coax. at the opposite end of the coax is an SMA connector.
Tube❄️Time

let's see how it performs on my cheap NanoVNA. for the frequency of interest, i want the SWR to be as low as possible. here we are hitting SWR=1 at 2.08GHz. this frequency is below my target of about 2.4GHz.

a NanoVNA-F V2 Vector Network Analyzer with the antenna SMA connected to port 1. the VNA is showing two superimposed graphs, one SWR which has a trough at 2.08GHz, and the other a very swirly Smith chart showing 50 ohms at 2.08GHz.
Tube❄️Time

to increase the frequency, i removed some of the tail of the F until the frequency shifted up to around 2.4GHz. it's always easier to remove material than to add it back, so it's a good idea to start longer than what you think you need.

same setup as before, but the F is shorted, and there are some scoring marks where i used a sharp knife to cut small pieces of copper off the end
Tube❄️Time replied to Tube❄️Time

now for the sorcery part: to get a rough idea of the polarization of the antenna and to make sure it is radiating, i've switched the VNA to log-mag S11. the metal thing in the foreground is a pair of scissors (really!) which, when i hold it nearby, absorbs energy and causes the S11 to fall.

same setup as before but the VNA screen is showing log-mag. the scissor's in the foreground but a bit blurry.
Tube❄️Time replied to Tube❄️Time

let's try another angle. holding the scissors perpendicular to the "F" shape, the S11 dips strongly (compare to the previous image). i like these because the handle is insulated so my body capacitance doesn't get too involved.

the pair of scissors is more visible now.
Tube❄️Time replied to Tube❄️Time

from the NXP app note, the radiation pattern looks like this. i'm sure there's a correlation with the scissors pattern but i'm not smart enough to understand it.

3d isometric view of the antenna
the radiation pattern looks like a donut
katch wreck replied to Tube❄️Time

@tubetime maybe the spiral out reflects the way it bulges as you go up from the bottom along the vertical plot axis

Henning Paul DC4HP replied to Tube❄️Time

@tubetime this is a perfect case for OpenEMS. I think there's even a tutorial you only need to slightly modify.

F4IHX replied to Tube❄️Time

@tubetime in fact the emitting part is the vertical middle part. The long open wire act as a capacitor.

doragasu replied to Tube❄️Time

@tubetime Nice experiment! From my experience, I would have expected the null to be along the X axis, but the radiation pattern shows it on the Y. Yeah, I'm really bad at RF, it's basically wizardry for me.

EDIT: I have checked and yeah, in fact the radiation pattern is correct, so I can confirm I am really bad at RF.

Tube❄️Time replied to Tube❄️Time

continuing on, @hennichodernich suggested i try out OpenEMS. it's a bit hard to use; i can't just sketch out the shape of the antenna and use it. i have to create it with code, but that code means it can be parametric. here's what the resulting antenna looks like.

AppCSXCAD window showing the geometry generated by my code. it looks just like the inverted F antenna board.
Tube❄️Time replied to Tube❄️Time

side note, to get openEMS to work, i had to downgrade my numpy version to 1.26.2. (yes, i know i'm supposed to be running Python with venv in a chroot jail in a Docker container under vmware, but i just want software to work...)

Tube❄️Time replied to Tube❄️Time

the sim appears to work, S11 looks reasonable. it's dead on at 2.4GHz.

Tube❄️Time replied to Tube❄️Time

now i'm curious as to why i had to shorten the antenna on the PCB so much. perhaps the dielectric thickness didn't match up, or maybe the trace was too wide (i didn't cut it precisely to 1mm)

Charles J Gervasi ⚡🛡️🥥 replied to Tube❄️Time

@tubetime Did you calibrate out the effects of the thin coax cable?

Martinius replied to Tube❄️Time

@tubetime It could also be caused by the anisotropic material (eps_z neq eps_xy) which FR4 is famous for

Tube❄️Time replied to Tube❄️Time

most likely it is a difference in dielectric constant, slight dimensional differences, and the coaxial cable interacting with the VNA calibration. i calibrated to the end of the blue cable but this doesn't account for the thin coax. the antenna doesn't have a matching network either which will have an impact.

Chip replied to Tube❄️Time

@tubetime Ah, the wiggly F. For experts only.

Tube❄️Time replied to Tube❄️Time

figured out the problem! I neglected to include the thin coax in my calibration. it's much better now, I've even restored it nearly to the original length, and the impedance match looks better as well.

Tube❄️Time replied to Tube❄️Time

open and short at the end of the coax was easy but for the 50 ohm load I had to solder this tiny smd resistor.

Tube❄️Time replied to Tube❄️Time

another way is to calibrate to the end of the SMA cable and use the "electrical delay" setting. this didnt work well for me. I'm used to larger, more professional VNAs that let you adjust it with a knob and watch the Smith chart in real time. the NanoVNA makes you play a game of Guess-The-Number...

Daniel O'Connor replied to Tube❄️Time

@tubetime you should be able to measure the electrical length with the far end open (or shorted) by measuring the phase and halving it. Would be nice if the tool could do it for you though! I have some QoL ideas like this I keep meaning to try implementing in my nanoVNA after using it at work a bit

Tube❄️Time replied to Daniel

@Darius yes it might actually be good enough to do it that way. i'll have to experiment some more.

Dick Telder replied to Tube❄️Time

@tubetime
This means the thin cable is not 50 Ohms?

Tube❄️Time replied to Dick

@dtelder it's 50 ohms but it is longer, so the calibration plane needs to move.

Dick Telder replied to Tube❄️Time

@tubetime
There is magic involved. I understand that not including the thin cable will affect levels and propagation delays.
But I don't understand why it changes the frequency with lowest swr.
Perhaps the cable is to be considered a part of the antenna?

Tube❄️Time replied to Dick

@dtelder i'm not 100% sure myself about why it caused a frequency shift, but you may be right about it being part of the antenna. the ferrite beads along the thin coax are supposed to prevent RF currents from flowing along it, but it's not perfect.

Felix replied to Tube❄️Time

@tubetime a couple of weeks ago I tried to make a 1.7GHz patch antenna, I got the resonance right after the second try but until today I haven’t figured out why the impedance is wayy off…

✧✦✶✷Catherine✷✶✦✧ replied to Tube❄️Time

@tubetime isn't a venv easier to get to work than system python...

Tube❄️Time replied to ✧✦✶✷Catherine✷✶✦✧

@whitequark how many versions of python exist on an average desktop ubuntu install. hmm

✧✦✶✷Catherine✷✶✦✧ replied to Tube❄️Time

@tubetime probably one? system python is there for system packages and not for you, counterintuitively (which is the cause of much confusion and strife. I wish it wasn't on PATH given that...)

Tube❄️Time replied to ✧✦✶✷Catherine✷✶✦✧

@whitequark maybe analogous to using a cross compiler vs the system compiler.

✧✦✶✷Catherine✷✶✦✧ replied to Tube❄️Time

@tubetime I'm used to rustc and clang, which make no distinction between the cross and native compilers >.>

(to be fair, there is a little bit of magic in native clang that tells it where to find the libs, but also I wish it wasn't needed...)

F4GRX Sébastien replied to Tube❄️Time

@tubetime You make all of that look deceptively simple, good job! The vna on curve mode instead of smith would have given prettier graphs, but thats a minor detail.

Chuck

@tubetime FWIW when I was doing my own double sided PCBs and etching them at home I would do this but use solid copper wire, put it through the hold, solder it, cut it off on the other side, solder it there too. Basically a one lead component (a vertically mounted zero ohm resistor 😆)

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