fullsquare
@fullsquare@awful.systems
- Comment on What is a good Matrix (public) home server besides the big matrix.org one? 3 days ago:
join some matrix room that you like and look around for people’s homeservers
- Comment on 3D printed motorized variable capacitor, butterfly-trombone hybrid (also question on dielectric losses in capacitors) 1 week ago:
i was thinking more like, thin external plastic shell and empty cells inside, perhaps with another thin plastic shell on inside, and internal metal shell (on plastic support?) fitting in snugly, for mechanical stability, idk 3dprinting
keeping leads short and nonmagnetic (dramatic reduction in skin layer depth) would be a good thing because of losses, but the longest object in capacitor would be just capacitor plates, and either way in wavelength terms it’s rather small. more precisely you can model it as open transmission line stub with some weird and low impedance, but it’s so small that you don’t have to
- Comment on Testing a shortened OCFD 1 week ago:
Found a piece about RFI and baluns, audiosystemsgroup.com/RFI-Ham.pdf multiple turns on ferrite aren’t really an option for VHF and up, ended up ordering extra beads (non-split)
- Comment on 3D printed motorized variable capacitor, butterfly-trombone hybrid (also question on dielectric losses in capacitors) 1 week ago:
The dielectric between the plates in this case is 0.4mm of ABS plastics (+ a bit of air in the 3d print layer lines).
in terms of losses, PP or PE is a bit better than ABS, teflon or FEP is a bit better than PP, but air is superior to either (this is part of the reason why foam coax is a thing). not sure which ones are printable, or whether it’s practical at this size, but try to introduce as many voids as possible (perhaps requires larger thickness of dielectric). it doesn’t matter much in your case, because of low power (warping of plastic because of excessive heat is probably not a problem). if your coax has solid dielectric, then by introducing enough air in 3d-print your variable might become less lossy than that
The Capacitors allows my 80cm diameter loop to tune from 20Mhz to 37Mhz. Sweeping the whole range is a bit slow due to the low RPM of the motor and takes about 6min. But that is kinda nice when fine adjusting to a frequency.
you have probably noticed that position vs resonant frequency relationship is rather nonlinear. you can get higher sweep speeds at lower end without losing much accuracy at higher end by tapering end of side plates into a triangle shape (it will get longer overall). it doesn’t matter much in your case, because it’s all approx monoband, but if you want to go multiband with this, then it’ll be a nice enhancement. similar effect happens when air variable capacitors have moving plates shaped in such a way that one end is longer than the other, and external edge has shape roughly like a section of logarithmic spiral. precise movement of variables like this is done by use of worm drive with large wheel
I am not sure what is causing this, but i assume it could be due to increase of dielectric losses in the capacitor getting bigger when more of the plates overlap because then the electric field has to flow thru a bigger area of dielectric, increasing the potential for losses.
loss tangent of dielectric is material property, that is ratio of equivalent loss resistance to capacitance should remain constant at given frequency. so i guess that losses should remain roughly the same, if dielectric is to blame, but at any rate lossy capacitor should make bandwidth broader and SWR lower. my guess would be that it’s a matter of coupling loop becoming wrong-sized or wrong-positioned at some point with change in frequency (try moving it up or down? there’s gotta be some optimum position for your entire range of interest)
- Submitted 1 week ago to amateur_radio@lemmy.radio | 1 comment
- Comment on Series coax stub capacitors for more precise loop antenna tuning 2 weeks ago:
Skin depth is larger in aluminum but not enough to balance out its lower conductivity, copper is better material taking all into account, in practice both are good. If opposite was true we’d use lead or zinc for conductors. There are satellite microwave parts made out of aluminium (low weight) coated sequentially with zinc (bonding layer), copper (better conductivity), thin layer of silver (even better conductivity) and then gold (actually not thick enough to contribute, this one is for corrosion protection)
- Comment on Series coax stub capacitors for more precise loop antenna tuning 2 weeks ago:
the thing with using aluminum tape is that you can get away with very small thickness, because current flows only in top tens of micrometers depending on band. you can just roll up, say, 5cm wide, 0.5mm thick aluminum tape and have riveted or maybe brazed short length of 2mm thick bar to the ends for connecting capacitor. the problem is with mechanical stability of this setup, which is why you see pipes and thicker bars, bicycle rims etc
with braid you get a lot of contacts between wires, and i’m not sure that resistance of them would be low unless tire is fully inflated. maybe you can put short U-turn within loop at end opposite of capacitor and have adjustable shorting bar there. adjustable capacitor is more common by far, because if you can adjust it widely enough, you can get to different bands
- Comment on Series coax stub capacitors for more precise loop antenna tuning 2 weeks ago:
i’d expect shield to fray and core to bend with arrangement like this. if you just slide piece of pipe (can be rectangular, or U-shaped) it should be more durable. you’d be surprised at voltages developing there, even with 4W online calculators suggest something in 1kV range. 100W is over 5kV
- Comment on Series coax stub capacitors for more precise loop antenna tuning 2 weeks ago:
you can add in parallel small adjustable capacitor, made from two or maybe four coax cores with some kind of sliding conductive sleeve around them all (piece of copper pipe moved by screw) this way you should be able to tune to any channel within cb band
- Comment on Can turkey motion machines be used for producing electricity? 2 weeks ago:
it’s a type of heat engine. heat engines require temperature difference to work, and the lower it becomes, the less energy is there in the first place and very fundamental limitation, that is carnot cycle efficiency, goes down very quickly. in practice, all heat exchangers have some thermal resistance, and the temperature gradient you can afford to use up on this, the bigger heat exchanger becomes, making low grade heat powerplants extremely big and expensive on top of barely generating any electricity
i don’t think there’s a lot of energy to be squeezed from daily variations in air temperature vs lake temperature, you’d be better off just by using solar panels on the same area
- Comment on What do you recommend I carry in my first wallet? 2 weeks ago:
take any aluminum can, cut it open, cut a plate fitting in your wallet, insert it there so that it sits on external surface if your walket, done, that’s your yeehaw rfid blocking sleeve, extra mass 1g
- Comment on More CUDA please 3 weeks ago:
real. every prediction i got from computational chemists was wrong
- Comment on NanoVNA vs. Loop antenna SWR testing [Question] 3 weeks ago:
I wouldn’t bother with air core. Ferrite beads allow you to use old calibration. If you make 1:1 balun just by threading coax through toroid, you can use old calibration as well provided it’s the same coax. Keep in mind minimum bending radius of coax. There are other designs, like using twisted pair on toroid
I’ve seen people using PE-Al-PE pipe for variables, this gives you layer of good dielectric (polyethylene) in dimensionally stable form. One connection is aluminum layer inside the pipe, and for the other you’ll have to figure it out on your own. Retuning might be required anyway within the band (magloops are narrowband) Common way to make variables is to bolt two of them in series, so that no sliding contact is used, moving part is the same for both. This is good for high voltages also but i’m not sure if you’ll need it
- Comment on NanoVNA vs. Loop antenna SWR testing [Question] 3 weeks ago:
Yeah this lower one looks better but still probably your capacitor value in loop is way off, try to find frequency where impedance is real (purely resistive; green line on smith chart crosses horizontal line in the middle) and work from there, then you’ll know whether to increase or decrease it. resonance is narrow so you might miss it. there’s a reason why magloops are made with variable capacitors (sometimes retuning is required due to changes in ex. humidity)
- Comment on NanoVNA vs. Loop antenna SWR testing [Question] 3 weeks ago:
i’ll add that in a way SWR chart is more resistant to misuse, because if nanovna is calibrated with wrong length of 50 ohm feedline, or without feedline at all, then smith chart will be rotated by angle depending on difference in length of that feedline, while SWR chart should look the same. for example, if real part of impedance at resonance is too low (ex. 20 ohm), and feedline is quarter wavelength different from what nanovna was calibrated with, then impedance will be still real but too high (ex. 125 ohm), while SWR chart should look the same (1:2.5 SWR minimum) (barring losses in feedline). (this works the same way as quarterwave long feedline impedance matching scheme). for different feedline length differences (non-multiple quarterwave) impedance will be complex at antenna resonance. this problem is avoided by calibrating nanovna with feedline
- Comment on NanoVNA vs. Loop antenna SWR testing [Question] 3 weeks ago:
idk how you have done that, maybe i have older version but for me this marker just reads CH0 SWR 1.00/(value)
you can pull up a smith chart, this will tell you whether impedance is too low or too high, since it’s still not matched at resonance
- Comment on NanoVNA vs. Loop antenna SWR testing [Question] 3 weeks ago:
you can get away with very inefficient antennas on HF reception, so i wouldn’t take SDR reading very seriously. (atmospheric noise dominates all noise, so amplification will get you useful signal with amplifier not introducing significant noise on its own. reverse is true on vhf, and especially on uhf and up)
resonance should happen no matter what power level. do you mean SWR 50 or 50 ohm? i’m not even sure if nanovna can measure SWR that high. it sounds like you have a short or open somewhere it shouldn’t be? you need to calibrate it after changing tested frequency range, have you done that? (calibration can be saved). at the vhf-ish frequencies, it would make sense that your loop becomes full wave or even larger. circular loop has impedance of some 100 ohms, but you have capacitor at the ends of it so it’s gonna be different
with magloops, with set size of loop, tune is via changing capacitance, match is via changing position (closer or further from loop, tilt away from plane of loop), shape or size (cross sectional area) of feed loop, you can match it exactly this way. coax stub can be lossy, if it’s just 4W then probably not a problem but with higher powers check if it’s not overheating
- Comment on Should I replace my laptop battery, and are third-party batteries safe? 4 weeks ago:
there’s nothing wrong with 3rd party batteries, if it’s worn down so badly that it no longer does the thing that it is supposed to then replace it, idk why it’s a question. I replaced mine when they’re at 60% of original capacity
- Comment on If the color of the Sun was orange, wouldn't the clouds and everything white also be orange? My friend is adamant that 30 years ago the "real" Sun was orange but got replaced with a white LED. 4 weeks ago:
Your best bet is to just not engage with them when topics like that come up
real soon it will include any topic
- Comment on If the color of the Sun was orange, wouldn't the clouds and everything white also be orange? My friend is adamant that 30 years ago the "real" Sun was orange but got replaced with a white LED. 4 weeks ago:
if sun was orange, it wouldn’t be orange, it would be just precieved as regular white light, and new-orange would be even more oranger than what we know today
- Comment on If a Space Elevator became a reality, wouldn't the cable act as a kind of wick for all of the unfiltered radiation from outside our atmosphere? 5 weeks ago:
you need really spicy photons for activation to happen
- Comment on Do people actually believe those "gurus" on the internet that supposedly "give advice"? These seems very sussy and feel scam-adjacent, isn't it? 1 month ago:
dude, people join irl face to face cults, of course they do
- Comment on [deleted] 2 months ago:
- Comment on Firefox dev clarifies that AI features will be 'opt-in' and there will be a 'killswitch' to disable them 2 months ago:
no no no no it’s on by default so it’s opt-out, and switch for that isn’t even implemented yet
- Comment on Industrial Strength Shitpost 2 months ago:
at least he didn’t say he “fell on it” and it was totally an accident
- Comment on YOLO 2 months ago:
it can be bought in italy as a cleaning agent without going through entire process as for reagents purchase iirc
- Comment on Could gunpowder be chemically addictive for humans ? 2 months ago:
apparently ww1 era british soldiers figured out that cordite works like amyl but shittier (more specifically, nitroglycerin part) pdfs.semanticscholar.org/…/009c8713aadd8accbb03b2…
- Comment on Could gunpowder be chemically addictive for humans ? 2 months ago:
smokeless powder is, sort of en.wikipedia.org/wiki/Nitroglycerin#Industrial_ex…
- Comment on How does the private equity bubble compare to the AI bubble if at all? 2 months ago:
Or you could use different hardware, maybe from competition, because result isn’t worth electricity it used
- Comment on How does the private equity bubble compare to the AI bubble if at all? 2 months ago:
gpus as used for genai aren’t really suitable for normal loads like aerodynamic simulations, genai uses low precision data like fp8, fp4, blackwells and such are optimized for it so hard that you can’t really do anything else on this thing