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Series coax stub capacitors for more precise loop antenna tuning (infosec.pub)

submitted 2 days ago by einfach_orangensaft@sh.itjust.works to c/amateur_radio@lemmy.radio

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I recently build a loop antenna and used a RG58 coax stub as capacitor.

While cutting the stub to length while checking with a NanoVNA for SNR i noticed that even very small size reduction of the stub (+-1mm) could make the resonance frequency of the loop jump up and down 40-50kHz.

Since the channels on CB radio are only 10kHz wide that made tuning the loop to a channel difficult because it turns out clipping sub millimeter of the end of a coax aint easy.

After a bit of thinking of how i could lower the capacitance of the RG58 coax i realized i could just take 2 lengths of RG58 and put em in series, since putting capacitors in series reduces there capacitance.

This increased the size of the coax stub quite a bit (see picture compared to old coax stub).

But it worked very well, now clipping of about 3mm of one of the coax ends will result in about 10kHz change of loop tuning, hence the precision was increased a lot and i was finally able to tune into the precise channel i wanted.

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[–] fullsquare@awful.systems 5 points 2 days ago* (last edited 2 days ago) (1 children)

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

[–] einfach_orangensaft@sh.itjust.works 3 points 2 days ago (1 children)

Yeah i am planing to build a remote controlled coax capacitor eventually (probably something like this: https://antennenbau.dxfreun.de/krampfader/image/Balkon-Loop/Abstimmbereich_MagLoop.jpg ) but right now i am still waiting for some parts to make that happen.

The additional high voltage durability of the serial coax capacitor is nice, (should be now 5kv, since a single RG58 has 2.5kv) but i doubt i gona need that with my 4W Tx ^^

[–] fullsquare@awful.systems 3 points 2 days ago* (last edited 2 days ago) (1 children)

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)

[–] einfach_orangensaft@sh.itjust.works 2 points 2 days ago (1 children)

i thought about a bicycle rim :D but i kinda wana keep this a mobile setup...

i have been thinking about inflatable loops recently, sadly i was only able to find a singular example of those (some guy slapping some aluminum foil onto a inflatable swimming tire and it didnt work that well).

My idea would be to take some copper braid sleeve (5cm diameter) and put a inner bike tire thru that. As far as i can tell nobody has tryed that before...and i keep wondering, if i add a small electric pump to the setup i could maybe tune the loop by inflating and deflating it while using a non variable capacitor.

[–] fullsquare@awful.systems 2 points 2 days ago* (last edited 2 days ago) (1 children)

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

[–] einfach_orangensaft@sh.itjust.works 2 points 2 days ago (1 children)

Good point with the copper sulfide, that wasnt on my mind at all, if i try ill put a protective layer between the braid and the tire rubber

Yeah i heared about the skin effect in aluminum being much shallower than in copper negating the better conductivity of copper, but i didnt found aluminum braids.

On the point of braids, i was thinking that would probably work well since the shielding in coax is also copper braid and many loops are made out of coax, but big diameter coax cant be folded as good due to the core, so i was thinking...maybe just use the shielding and no core at all.

You are right with the dipole, that would probably be the most sane solution. But i cant get the idea of a inflatable loop out of my head now....

[–] fullsquare@awful.systems 2 points 1 day 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)