Reusing rigid/semi-rigid coax - Page 1
Forum section: electronics / rf-microwave

author: artag
author_rank: Super Contributor
timestamp: September 11, 2022, 01:55:54 pm
content: I'm a bit of a lightweight in RF practice, so although I have a few bits of test equipment for higher frequencies I tend to use ready-made preterminated cables, usually older secondhand from good sources rather than unknown quality new ones from ebay.I have an application where I want to make small links between N-types from a spectrum analyser to a directional coupler. They need a bit more routing than is possible with right-angle adapters, and one possibilty is some semi-rigid cable (if thath's what it's called - the stuff that has braid filled with solder).But for a bit more robustness, I could use some copper rigid assembly, probably recovered from some ancient military kit with N or SMA on the ends. Is it possible to reuse this by bending it to shape, or will that destroy it or it's properties ? Any special techniques for bending it (filling with sand is not an option) such as using a former to avoid too-tight corners ? Or are the used parts like this I can find at a radio rally useless unless they happen to fit already ?

author: coppercone2
author_rank: Super Contributor
timestamp: September 13, 2022, 01:55:51 am
content: I had a whole bunch of stuff here about tubing cutters in this post initially ,but rereading it makes me feel like I am a crazy man.. maybe try cutting it with a thin jewelers saw on a fixture to make a nice sharp cut without the bend from a tubing cutter, or try silicone carbide abrasive disk. Dealing with the detent formed in the semi ridgid cable with a conventional tubing cutter is annoying as fuck and you should not have to put up with it, if you care about the tiny 0.3mm discontinuity formed by the tube cutting process, since you can't deburr the ID of a filled cable. You can try precision filing and micro widdling if thats what you go with, but I recommend finding another process, I don't know why I always reached for the tubing cutter when making semi ridid tubing, I think its because the frequency and precision was always too low for me to care (the tubing cutter makes a interior bend of the tubing to make a restricted 'apature' at the end of the coaxial cable so its not a flush cut cylinder in a tube anymore, but one encroached by copper.When you make something out of that it will likely still be superior to a cable, because think about it, a cable does the same thing as your tubing when you bend it (attenuation and phase shift).A bunch of adapters have variation too because their mechanical joints, most likely this solution will be equal, and you will have problems measuring the problems it causes.For bending it, you can use a tube bender if you want to be anal about it, and some people bake their tubing after bending also (there is a cycle you can find online).The materials I think will out last us as human beings by a long margin (teflon and copper).. the only thing I might recommend to do is polish it lightly with a jewelers cloth to make it look nice. You can also cut a segment, pop out the middle and inspect it for corrosion/oxidation on the inside of the tube.The only property that cable has when you take it out of something is that the problems it causes are calibrated by that piece of equipment, so if you take it out, bend it into another shape (it was probably only bent once when its in a old equipment) and then bend it back into shape your calibration will be effected, but its also effected by tightening and untightening the connectors).Just calibrate what you make out of it, and it will be fine for everything but the most demanding requirements like avionics service... Just don't make a tight bend (the instrument is full of bends, study the most tight bend there to see what your limit should be for that cable. you don't want to bend it near the connector after soldering.I also don't know if you can bend it after installing the connectors, I try to get it into the correct shape BEFORE I add connectors because you can imagine that it might put some pressure on something or another.Of course I bent soldered finished rigid tubing before after making and well... nothing bad happened, it worked fine... but I assume you will be cutting it smaller etcbut I always say... give it a inch infront of where its soldered that its not bent if you are desperate to avoid buying or reusing connectorsIf you are sure that you did not effect center pin alignment or cause a crack, kink/cusp or tear in the cable, dielectric or solder joints, calibration will take care of whatever you did. You can't avoid calibration anyway. So after you bend do a good inspection to make sure there are no cracks or weird things going on.. thats where the jewelers cloth might help you identify defects on a clean surface, it might be hard to see on a nasty blackened tube. And of course your bending tools will thank you from working on clean polished material rather then roughened oxidized surface (you should use oil when you bend these over a mandrill or use a bending tool to further lower friction)... it should cause less wear on the tube bender and bend smoother... you want it to elongate over a surface IMO, you don't want friction anywhere but where you are clamping it.*Fun story, when I got my VNA this summer, when I was unpacking it, I bend the hardline loops on the back of the instrument when I tilted it because they don't have a rear guard. I bent it back into shape and it calibrates OK with the cal kit, I guess its OK but I have been meaning to replace them anyway because it was a pretty tight bend *but not to the point of 'cusp' deformation, just more then I would think is a good idea) .... I don't think its 100% guaranteed that there wont be a crack, that will at least defiantly be a point where I check for problems if there is a service interruption. If you add some tubing like that, I recommend a roll-cage type thing for the back of the instrument if you can add one, because eventually you are gonna forget about it and tilt the instrument during service or transport.