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Tube❄️Time

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

18 comments
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…

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