i bet they have some preferences about contractor

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. you can also make capacitor shorter and wider, or even add more layers like how vacuum variables are made. in nesting design you can get taper effect just by making inner layers longer

Found a piece about RFI and baluns, http://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)

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)

and i told you before that it’s officers (pilots are generally officers) that will be doing aerial bombing, and because large force on ground is prerequisite for situations you’re describing, it’s not going to happen

i mean that saudis were somewhat restrained about airstrikes, at least publicly, but this action would cause them to not be so. even if they tried, there are extra air defences dragged to saudi for exactly this purpose; every cargo flight and every extra warship makes odds worse for iran, as more missiles would be intercepted, but even if nobody dies, shooting missiles would have diplomatic consequences. another action that would result in rising oil prices would be iran shooting ships in strait of hormuz, but this would also close access to their own single large oil terminal, and there are american warships nearby anyway, so it’s perhaps unwise decision to make today

at this point, i think that decision to strike already has been made, and they’re just stalling so that more metal can come from across the atlantic. dragging an aircraft carrier out there is not done for no reason, and the second one they want to put out there would need to have some of pre-deployment training shortened and done on the way, which is unusual and avoided because there were accidents that this training was supposed to mitigate

we’ll see in a month tops

i don’t think it would go the way you think it would go

right, let’s see… they propped up unpopular (some 70% iranians oppose islamic republic rule, source), authoritarian, religious minority rule (only 32% iranians reported to be shia in 2020 survey, so before 2022 protests, it might be even less today), that was in a constant state of crisis and when president nho dinh diem got couped and killed, people danced on the streets (iranians burned down mosques and statue of soleimani during january protests). yeah, maybe iran is like vietnam after all, to be more specific like south vietnam during buddhist crisis

i don’t think that regular iranian population, that protested every year in almost decade and these protests were all met with crackdowns, mostly deadly, would like to see islamic republic rule to continue. neither i have heard anything about ground invasion, so all you’ve got out there is couple of pilots (officers) in the air and some more on ships

except irgc and friends, nobody’s sticking out their neck for theocracy and ayatollah, definitely not majority of iranian population (survey from 2024, and this is before irgc did 4 srebrenicas), and i’m not even sure about allegiances of regular military (artesh, not irgc. i heard that irgc disarmed artesh just before january crackdown, not very sure about it). let alone handful of pilots and marine officers that would conduct bombardment

have you? vietnam was backed by soviet union and china when these states were at peak performance, and north vietnamese government was genuinely popular. iran today is not it, who’s gonna stand up for them, fucking north korea? mullah’s regime doesn’t seem to be popular either https://gamaan.org/wp-content/uploads/2020/09/GAMAAN-Iran-Religion-Survey-2020-English.pdf also iranian air defences were already shown to be unable of touching american aircraft last year, and nobody’s talking about ground invasion

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)

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/brazed/spot welded 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, and here you would need some kind of horizontal bars for loop to more or less keep shape

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. keep in mind that copper in contact with some grades of rubber develops copper sulfide film. 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

if you’re going for portable operation, wire dipole is probably the better way to go. cheaper, lighter, more efficient, you can roll it up and fit in your pocket. if you’re operating out of a car, you don’t need to fold magloop just lay it flat in the trunk

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 (voltage scales as square root of power)

btw if you don’t need it collapsible, consider using bicycle rim as loop, or some kind of wide aluminum tape, as it has much higher equivalent diameter than coax (less losses)

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

additionally, you made your loop suitable for higher power than was previously (magloops tend to be limited by voltage across capacitor). if you use coax with foam core, capacitance per mm will be lower still. for adjusting, you can get away with only clipping away shield with nail clippers

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 a very fundamental limitation, that is carnot cycle efficiency, goes down very quickly. in practice, all heat exchangers have some thermal resistance, and the lower 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

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

if you want to scan bus pass you can put that card on external side of it

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, then you have to include balun in calibration as well (it adds some electrical lenght). If you noticed changes after making air core, this suggests that you do have some common mode current, this will make your measurements sensitive to random changes as rf current flows on the outside of cable where it shouldn’t

I’ve seen people using PE-Al-PE pipe for variables, this gives you layer of good dielectric (polyethylene) (but not as good as air) 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

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. what LH0ezVT said makes more way sense than that, i forgot how magloops work. but you still might want variable capacitor

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)

how have you made your capacitor anyway?

you can put some ferrite beads on your coax close to feedpoint in order to eliminate common mode currents. better yet, use a balun. this might help you in getting more reproductible results

e: note how swr gets much higher when off resonance with properly calibrated nanovna. when measuring antenna with cable, you’re seeing loss in cable as a degree of lowered swr but only with high swr, because energy is lost in cable when it bounces around and never goes back to nanovna

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