VNA cable characterization by car tire

[jjoonathan]

It turns out those thick armored VNA cables are actually quite tough! In conclusion, you should immediately gather all of your most expensive VNA cables and run over them repeatedly with a car because they might survive 

Not sure about the tangential direction though. I'm not convinced that the strong outer armor is any benefit against tugging. Or tweaking a mated connector sideways -- I'm pretty sure that the compressive contact area of the seemingly-25mm-diameter mated connections is actually just the inner 5mm, which is a pity. But in compression the armor can apparently withstand being run over by a car, so that's nice.

[joeqsmith]

Maybe you could connect it to the VNA, then drive over it.  See if you can detect a change.   Try some different coax to compare results.   

[DaJMasta]

But what can you learn about the car from the changes in the response of the cable?

Is your VNA fast enough to work out the time between axles while it's driving?

Can you tell the difference between a truck on the cable and a car?

Can you even see a bike going over it?


We have to go deeper

[jjoonathan]

Hey, now that I have the FieldFox I could actually measure a cable squish in the S domain 

Tomorrow I'll look at the bottom of the cable drawer and see if anything inspires me 

[pdenisowski]

I would definitely want to run a distance to fault measurement before, during (!), and after.  Phase stability would also be interesting to look at.

Incidentally, I just (last week) did a video on DTF using one of our handheld VNAs and the "pure technology" DTF video is already in the can (should be uploaded in a week or two).  Phase stability and cable loss videos are coming as well.



When I was still working as a field applications engineer, I was traumatized by some of the things I saw done to cables  

Oh, and here's how I insert controlled faults into coax

[jjoonathan]

Cool! I really enjoy the R&S Measurement Fundamentals series, you do a good job with each of the videos and the back catalog that fits together is even better.

I like that the thumb tack looks like a peak on the DTF! If I tried that on the armored cable though I think the cable might win.

But yeah, put this in the field, bang it up with some construction equipment, give it water and time... nothing wins against that.

[joeqsmith]

Or better, try to repair the coax after the damage has been done.  Is it possible?  I tried and failed. 

https://www.eevblog.com/forum/rf-microwave/is-is-possible-to-repair-a-hairline-crack-in-semi-rigid-coax

[jjoonathan]

Those are some solid experiments, you really gave it a go! Too bad it didn't work out. With the "signal travels on the inside of the shield" intuition that's sort of what I would expect, but it's still a bummer to see that none of the patching techniques panned out. Even so I can't help but wonder about a few more "one little change and it just might work" theories that are almost certainly a waste of time but hope springs eternal.

[pdenisowski]

Or better, try to repair the coax after the damage has been done.  Is it possible?  I tried and failed. 

I used to do a LOT of fieldwork in interference hunting and direction finding as part of my job.  When LTE was first coming out, there was massive interference between cable systems (due to egress) and the 700 MHz LTE bands.  When we localized (using a portable receiver and a directional antenna) a section of hardline with a crack in it, the cable guys simply cut out that section and replaced it - no attempt at all to repair the crack.

And there was no repairing the animal chews .... 

[joeqsmith]

That was one of those deals where a lot of what I consider bad advice was being given and I thought I would try and add some data to the picture.   If there is a way to fix such damage, I'm sure a lot of people would be appreciative of learning how.   I was experimenting with printed plastic waveguides and had purchased some conductive coating from MGC.  I had thought about trying to add some of that into the damaged area but was never motivated enough.  It did work well for my experiments where I compared a production metal horn with a coated plastic horn and saw no difference. 

[coppercone2]

for reliable shield cable repairs in aviation they show that you need another segment of braid that you put over the junction and then put two lap joints around it or for temperature reasons I think they also use constant force spring tape to join the new segment of braid with the two other segments if you can't solder it, but usually its teflon wire there so its OK to solder over that.

but that is just shielding telemetry from noise. I would imagine you could do a coaxial splice like that, but then you would need to put some heavy duty heat shrink on it that can clamp the braid down to the original diameter and tightness if it was heated very uniformly. The solder is going to show up as a discontinuity.


If you wanted it like really good I think you would have to splice the braids together using a laser welder wire by wire with one hell of a cable fixturing aparatus to get it all together. But it also would involve super precision cutting and fitting of new dielectric (unruly plastic) with minimum gap, and I don't know how you could keep the middle conductor, which is often load bearing (steel) spliced to its original diameter. Again, I think you would need laser welding for high frequencies, followed by having to sand it flush, and replate it with silver to get it anywhere near original strength with the same diameter.

Since its only a few thousand strands in the shield, maybe you can weave them in place over the wire in the same pattern as the cable with some kind of laser welding robot... that way you would have no interface really other then a bunch of really small welds. Or you could fusion splice them mechanically (cold weld machine).


Yeah actually to get the middle conductor the same and equally strong you would need to cold weld it with a 50 pound small diameter wire cold welding pressure machine.


Maybe they would do this if they had to fix an antenna wire on a modern armored air defense frigate or something, if it saves having to cut and patch 20 steel armor walls to run a new wire


And even if you do all the correct wire fusion techniques, you are still left with the dielectric which has a slit in it so it can be fit over a joined cable. Its going to be a problem if the wire moves at all ever. To get rid of that you would need some kind of dielectric splicing machine extruder thing that can weld the dielectric together



I think this is the only machine that can repair your center conductor to the correct tensile strength and dimensions.


The braid and dielectric is left up to your imagination. Braid seems doable and less critical. Dielectric... you need a plastics guy, or magic glue. I think pretty much all glue would be shit but who knows. Maybe you could 3d print a extension in there that is fused to the original if you find the right filament and put the print head on a complex robotic arm?


then it might be close to the original cable...