50Ohm water cooled coaxial cable and connectors? - Page 1
Forum section: electronics / projects

author: Plasmateur
author_rank: Regular Contributor
timestamp: October 11, 2019, 05:20:43 pm
content: Hello,I've found several different high powered water cooled coaxial cables through searching online, however I'm having a bit of difficulty locating any kind of connector for them - (think SMA,BNC,N-Type bulkhead connections but for the watercooled coaxial cable).Does such a bulkhead connection even exist?

author: jbb
author_rank: Super Contributor
timestamp: October 11, 2019, 06:57:42 pm
content: I guess so, but have no idea where to find it. Maybe you could ask one of the cable manufacturers who they use?

author: jmelson
author_rank: Super Contributor
timestamp: October 11, 2019, 07:16:29 pm
content: Quote from: Plasmateur on October 11, 2019, 05:20:43 pmIf the cable needs WATER COOLING (!!) then you are dealing with power levels WAY in excess of any standard coax connector. There are some really big connectors used in broadcast transmitters that are several inches in diameter, and have springy contact fingers. These are roughly the same diameter as the "cable" which is actually rigid coax made out of silver-plated pipe, but have a flange for bolting, and usually an O-ring so they can be pressurized to keep out moisture.Jon

author: vk6zgo
author_rank: Super Contributor
timestamp: October 12, 2019, 02:55:16 am
content: Quote from: jmelson on October 11, 2019, 07:16:29 pmI've never seen rigid coax made of "silver plated pipe"-------- the "three & one eighth inch" cable common inTV Broadcasting has, for the really "rigid" stuff used inside the building, an aluminium (or sometimes bare copper) outside pipe, & a bare copper inner.The outside stuff may be constructed the same way, but more commonly these days, has a corrugated copper outer conductor.The connectors are very large, indeed.I have never heard of "water cooled " coaxial cable--------- if your cable is getting that hot you are doing something wrong!

author: calmtron
author_rank: Regular Contributor
timestamp: October 12, 2019, 03:21:05 am
content: Never seen a bulkhead connector for thoose, seems like they usually are bolted directly to a bus bar (for RF heating applications).

author: German_EE
author_rank: Super Contributor
timestamp: October 12, 2019, 08:05:14 am
content: The only reason your coax would get hot is a poor SWR. I've actually seen and felt coax in such a situation and you could run your hand along the feeder and sense the warm spots due to standing waves.Note that I saidwarmhere. This was coax about 35mm diameter and the transmit signal was about 10KW with no cooling of the transmission line. So, tell us why you need water cooled feeder when even a megawatt TV transmitter gets by with gas cooled feeders (although maybe that is to keep the moisture out).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2019, 08:21:28 am
content: It should be hard to even get coax that hot, since it arcs over (if not in the cable itself, then at the connectors) at about the same point or earlier. Unless SWR is the real problem, as mentioned.High power of course is easily handled by going to a larger cable -- better HV clearances, more copper.Tim

author: awallin
author_rank: Frequent Contributor
timestamp: October 12, 2019, 08:34:54 am
content: fwiw the tv transmitter (500kW:ish and more) rigid line mentioned above is probably along the lines of these:https://www.eriinc.com/wp-content/uploads/2019/09/Rigid_Transmission_Line_Catalog.pdf

author: Plasmateur
author_rank: Regular Contributor
timestamp: October 13, 2019, 05:19:46 am
content: Quote from: German_EE on October 12, 2019, 08:05:14 amThecoaxI've been using is the HP-900 in that link. It gets hot because it is used for plasma production. A similar setup can be visualizedhere.A matching network insures I have almost no reflected power. I'm working with up to 3kW of power in the 10's of MHz range, so all that current is riding on the surface of the conductor. I believe ohmic heating is just something that's going to happen at those powers without larger diameter wire, which isn't feasible for the plasma source - or litz wire, which I've never seen anyone use for these types of plasma sources.I basically had to punch two holes in the matching network and fabricate connectors for the high power cable I'm currently using.I have to make another one of these soon, but it would be so much nicer if there was a bulk-head for this type of coax.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2019, 05:44:24 am
content: It's rated for 80kW and 200°C, what's your problem?!Tim

author: Plasmateur
author_rank: Regular Contributor
timestamp: October 13, 2019, 06:25:09 pm
content: Quote from: T3sl4co1l on October 13, 2019, 05:44:24 amThere's no problem on that front, although it can easily get hotter than 200°C on the surface of the glass containing the plasma and the coil producing the plasma if water isn't run through it.The problem is finding a nice aesthetically appealing and easy to connect solution.

author: coppercone2
author_rank: Super Contributor
timestamp: October 13, 2019, 07:53:30 pm
content: the idea of hard line that is flow cooled and moving dielectric around the conductor is interesting

author: DaJMasta
author_rank: Super Contributor
timestamp: October 13, 2019, 08:10:42 pm
content: But probably extremely hard to execute, any residual oil, magnet/shaft wear from a pump, or other impurities would change the dielectric and resistive properties of the insulator of the cable.... it would be a nightmare to keep consistent.I presume it would have to exists if the cables exist - I assume the water wouldn't cool that effectively unless the terminated end was part of the loop, so unless the water channel actually is split for out and back and it's terminated before the bulkhead, they would have to be out there somewhere.Maybe a place to start is contacting the manufacturer of such a coax cable? I'm sure they'd be happy to point you to their own parts if they have a commercially available stock, and maybe that can give you a physical feature to look for in other connectors or a baseline to have one machined yourself. Maybe there's even a water cooled coax that just has the water in a jacket, so that you can use a standard connector and then the water line goes to a separate connection straight out of the jacket.

author: coppercone2
author_rank: Super Contributor
timestamp: October 13, 2019, 08:48:18 pm
content: particulate filters and teflon mag drives would help but it is a nasty problem

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 13, 2019, 10:23:56 pm
content: Quote from: Plasmateur on October 13, 2019, 06:25:09 pmInteresting problem you have there. This sounds an awful lot like a vacuum sputtering system and if so, I wonder if you couldn't inductively couple RF to the chamber, neatly sidestepping issues with cables heating as a byproduct of plasma generation.Sometimes the best solution is an orthogonal one... but sometimes brute force is, indeed, the only option. If the latter is all that is left then look for coax/hardline with teflon dielectric. It is also possible to pressurize some types of hardline with a custom gas fill like helium or SF6 for both better thermal conductivity and higher dielectric strength.

author: TomS_
author_rank: Frequent Contributor
timestamp: October 14, 2019, 06:11:31 pm
content: Quote from: German_EE on October 12, 2019, 08:05:14 amThey pressurise the feeders to keep moisture out, never heard of cooled feeders before.

author: coppercone2
author_rank: Super Contributor
timestamp: October 14, 2019, 09:51:16 pm
content: what would be a reasonable length of custom hard line to test this on? i want to build a flow cooled coax line