Non-Magnetic Coax Connectors? - Page 1
Forum section: electronics / general

author: Floopy
author_rank: Regular Contributor
timestamp: April 18, 2023, 08:53:57 pm
content: Hello Everyone,I'm working on a project that involves putting sensors in the bore of an MRI machine. I've been using BNC connectors outside of the area but now that it needs to be tested I've realized that this is problematic. It can't be ferrous or else it's going to be flying all over the place. It also needs to be able to work up to 40W of power. I've been looking, but can't seem to find anything suitable.Suggestions?

author: daqq
author_rank: Super Contributor
timestamp: April 18, 2023, 09:01:15 pm
content: I think that these are custom parts or special options only - I remember reading in a datasheet "Contact us for non-magnetic versions of this".Molex seems to confirm this:https://www.content.molex.com/dxdam/literature/987651-2392.pdf; ordering information: Contact Molex

author: Floopy
author_rank: Regular Contributor
timestamp: April 18, 2023, 09:03:20 pm
content: Thank you, that is a great lead, I'm not surprised that it's a custom request.

author: coppercone2
author_rank: Super Contributor
timestamp: April 18, 2023, 09:07:16 pm
content: Arent they just brass? I figure that the only thing you would need to do is not plate it with nickelMaybe just silver plated ones?

author: mendip_discovery
author_rank: Super Contributor
timestamp: April 18, 2023, 09:07:25 pm
content: Does,https://www.radiall.com/products/rf-coaxial-connectors/non-magnetic-connectors.htmlNot sure if it's the same line to the other poster buthttps://www.molex.com/molex/products/family/nonmagnetic_rf_solutionsThat was from just from a search for "non magnetic bnc"

author: tchiwam
author_rank: Regular Contributor
timestamp: April 18, 2023, 09:09:09 pm
content: Some machinist can handle this type of request. Be ready for mid to high 3 figures...

author: coppercone2
author_rank: Super Contributor
timestamp: April 18, 2023, 09:13:38 pm
content: You would figure its cheaper to make a BNC connector out of brass then stainless because its easier to cut. The center pin you can reuse from a normal one, I think you can get non magnetic there pretty easily, and you can reuse the dielectric, so you just need to make the body.

author: wraper
author_rank: Unknown
timestamp: April 18, 2023, 09:22:17 pm
content: https://eu.mouser.com/new/amphenol/amphenolNMConn/

author: Floopy
author_rank: Regular Contributor
timestamp: April 18, 2023, 10:30:30 pm
content: Thank you all, very good resourcesQuote from: mendip_discovery on April 18, 2023, 09:07:25 pmFunny enough, I've been typing "non-ferrous BNC", no wonder I wasn't getting much. A bit embarrassing on my part.Thank you!

author: Someone
author_rank: Super Contributor
timestamp: April 18, 2023, 10:40:35 pm
content: FAKRA connectors will also have some non magnetic versions, but probably no cheaper than suitable BNCs and you'll still have to go through distributors/sales agent.

author: srb1954
author_rank: Super Contributor
timestamp: April 19, 2023, 02:06:40 am
content: Quote from: coppercone2 on April 18, 2023, 09:07:16 pmEven if the main body is brass the spring inside the bayonet coupling nut of a standard BNC is probably a ferrous alloy.IDK if it would be possible to disassemble the connector and replace the spring with a phosphor bronze or beryllium copper spring.

author: coppercone2
author_rank: Super Contributor
timestamp: April 19, 2023, 02:43:46 am
content: lol I think it might pull out with a fixture, I had them pull out before (with pomona banana to bnc)this is something someone can probobly start cheap production of I think, fuck expensive medical equipmenthaha I wonder if you can strip the nickel off of it and put a new spring, if there are any connectors made with brass

author: coppercone2
author_rank: Super Contributor
timestamp: April 19, 2023, 02:51:08 am
content: I ripped one apart, there is a wavey washer in there. You can definatly get wavy washers that are not magnetic, it just got damaged.I think they roll the edge over after its pressed in. You would need to unroll the edge.

author: coppercone2
author_rank: Super Contributor
timestamp: April 19, 2023, 02:53:03 am
content: If you look at it carefully it almost looks like they use a C-clip (jesus clip) to keep it together, but one without the hole in it for the tool. I wonder if you can mill two slots in it so it loses integrity.It is washer, spring washer and a C-clip in a groove. You can probobly get rid of the C-clip with minimal intrusion into the connector. If you disable that, I think it should come apart.So drill 2 holes to disable C clip, take it apart, unroll the edge, (I think you can heat soften this part too, its just the outside to make it ductile again, then put a new c clip, and roll the edge over again.Two thin slots would be a fair price to pay for a nonmagnetic cheap connectorI tested it with a magnet, it looks like only the spring is the strong magnetic part, everything else is weak, it might just be nickel plating being picked up.Kinda ridiclous anyway because you can put like a wire there to tension it from the outside like a lockwire. I wonder if a MRI machine cost needs to be shaved like a abandoned sheepYou can probobly also just weld or spot braze or something a leaf spring on the outside of the connector to push up on that thing in a circle, it just wont be as nice, hide it with heat shrinkSo maybe theoretically if you can get rid of the nickel some how by asking or removing then drilling out the c-clip or bottom washer you can make a non magnetic one by replacing a non magnetic wavy washer in there and rolling it over with some kind of press (maybe collet press) after softening the curled bit and rolling it flat

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 06, 2023, 09:33:59 pm
content: Maybe late, but I thought I'd contribute since I have experience with MRI engineering.It depends on what you mean by "non-magnetic". If you're just concerned with the components becoming missiles in the MRI's field, then you need to avoid large solid bodies of ferrous material. But small amounts (relative to the mass of the whole device) can be fine (so long as they're securely fixed in place). Many connectors are commonly available with solid brass bodies, and are safe to use in MRI applications. I've used SMA, SMB, SMC, and MCX at fields up to 3T. I've also used BNC, but only specific parts which I knew were safe (I think we used 11_BNC-50-2-13/133_NE from Huber $uhner).However, most of those connectors I used did have some amount of ferrous material, most often nickel underplating (extremely common underneath gold plated contacts). The overall mass of the nickel is very small relative to the rest of the part, so it won't turn into a missile in a strong field. However that will still disqualify it from being sold as a "non-magnetic" part.Some other things to be aware of:1. Always check your components and assemblies for excessive magnetic attraction before introducing them to the MRI.2. Many connectors have stainless steel parts. A simple google search will often suggest that SS is non-magnetic. Never, ever take this at face value, even if they claim it's an Austenitic alloy or something. See #1 above.3. Many coaxial cables use copper clad steel (CSC) as the inner conductor. This is definitely not good for MRI. Usually this is determined by the cable's standard, for example all RG-58 should have solid copper, while all RG-316 should have CSC. Best to check the datasheet for the specific cable you buy.4. Even if the nickel plating isn't enough to turn the parts into missiles, they may distort the B0 field enough to degrade image quality, depending on how close they are to the imaging FOV. Usually this is pretty obvious, but it depends on what sort of sequence/application you're working with...