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.
fullsquare@awful.systems 6 days 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
einfach_orangensaft@sh.itjust.works 6 days ago
Yeah i am planing to build a remote controlled coax capacitor eventually (probably something like this: …dxfreun.de/…/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 6 days 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