BNC connector as rotary joint  - Page 1
Forum section: electronics / rf-microwave

author: CopperCone
author_rank: Super Contributor
timestamp: April 22, 2018, 05:53:48 pm
content: I want to mechanize my loop antenna. This means putting it on a platform that can be rotated remotely.I was going to put it on a bearing, and rotate it using a belt.I noticed that you can rotate BNC connectors. I am aware of designed for the job rotary joints, but i am wondering how a simple BNC connector will hold up to the task. It seems OK so long this is not spinning all day for days on end, but I would like opinions.

author: W9GFO
author_rank: Regular Contributor
timestamp: April 23, 2018, 06:34:43 am
content: I am sure it would work in the short term, but would probably cause trouble later on. Is it important that it be able to rotate continuously? If not then I wouldn't bother, just make sure that you have a way to keep track of rotations so that you don't wind up the cable. You also have the added difficulty of needing to mount the BNC "slip ring" coaxially with the bearing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2018, 04:43:26 pm
content: I wouldn't think it would be good for more than the low hundreds of rotations.Tim

author: edpalmer42
author_rank: Super Contributor
timestamp: April 23, 2018, 06:27:24 pm
content: All it would take is for one of the outer conductor 'fingers' to get caught on something and the entire connector would be shredded. I wouldn't even consider it for a test.Ed

author: ericj
author_rank: Newbie
timestamp: April 23, 2018, 07:42:36 pm
content: If it is acting as a thrust bearing for the weight of the antenna, and the only support, the connector is not very substantial and will eventually fail (as everything does). It probably isn't going to take side loads from winds very well.But all this is conjecture. Try it. You'll learn a couple of things. The connector will or will not be satisfactory. You'll know better what the problem is which will lead to a more satisfactory solution. If you destroy a $1 BNC connector in the process, who cares?Pack it full of grease. Figure a way to put a dust cap over it. Protect it from direct weather (rain, etc). See what happens. Nobody experiments with things we already know.Eric KE6US

author: CopperCone
author_rank: Super Contributor
timestamp: April 24, 2018, 04:00:21 pm
content: Nah i wanna put it in the middle of a ball bearing that will handle things mechanically. I just noticed unlike most rf connectors it spins freelyGood point on the grease you dont have much rf reasons to worry about 50MHz tops and it will reduce friction some.

author: metrologist
author_rank: Super Contributor
timestamp: April 24, 2018, 05:14:26 pm
content: If it's not a transmitting station... What would the most likely failure look like?

author: CopperCone
author_rank: Super Contributor
timestamp: April 24, 2018, 06:44:15 pm
content: I don't know what mechanically worn RF connectors do to be honest, especially a connector rated for 4GHz being used at 30MHz.I'm pretty sure there won't be a gross mechanical failure (unless its the center pin breaking the center conductor?), but some kinda slow degredation.I suppose the correct thing to do would be to connect the joint to a VNA once in a while to see what happens to the impedance.I guess you get insertion loss?

author: ericj
author_rank: Newbie
timestamp: April 24, 2018, 06:46:59 pm
content: White grease or silicone grease.I suspect some additives in wheel bearing greases are conductive, but I don't know that for sure.Eric KE6US

author: metrologist
author_rank: Super Contributor
timestamp: April 24, 2018, 07:38:26 pm
content: Up next, BNC robustness testing...

author: richard.cs
author_rank: Super Contributor
timestamp: April 30, 2018, 08:45:54 pm
content: I've seen a commercial product where a rotating connection to a cable drum was done with a standard 1/4" audio jack. It seemed to work quite well and they said it gave them a long and trouble-free life. They were using it at around 50 kHz but I suspect it'd probably not be too ugly at HF.

author: GerryBags
author_rank: Frequent Contributor
timestamp: April 30, 2018, 08:59:24 pm
content: The way it's done in tank turrets is to use slip rings. I wondered if these were also used to route the comms and apparently there are coaxial slip rings available which go into the GHz range, and then wave-guide slip rings. You might be able to use the same principles, or maybe even find a used example of an RF coax one from a military disposal company?https://www.moflon.com/mhf.html

author: dazz1
author_rank: Frequent Contributor
timestamp: May 10, 2018, 03:07:49 am
content: HiIf you limit the rotation of you antenna to say 720 degrees, you can just use a cable rather than a joint.Dazz

author: CopperCone
author_rank: Super Contributor
timestamp: May 11, 2018, 06:53:19 pm
content: i kinda thought it might look cool if its spinning in a circle constantly

author: metrologist
author_rank: Super Contributor
timestamp: May 11, 2018, 07:00:06 pm
content: what do those spinning radar antennas, like those on a ship or airport, use?

author: CopperCone
author_rank: Super Contributor
timestamp: May 11, 2018, 07:01:04 pm
content: expensive rotary joint SMA connector that's at least 100$

author: slurry
author_rank: Regular Contributor
timestamp: May 21, 2018, 08:03:44 pm
content: Why not use two loops and spin them electronically instead, using phase.

author: CopperCone
author_rank: Super Contributor
timestamp: May 22, 2018, 04:11:13 pm
content: Pretty sure the measurement would be partially degraded from what is supposed to be a lab instrument but i am very interested in the project. Do you have a decent tutorial?

author: metrologist
author_rank: Super Contributor
timestamp: May 22, 2018, 04:24:29 pm
content: links at bottom of page:http://www.jproc.ca/marconi/mdf5.html

author: ericj
author_rank: Newbie
timestamp: May 22, 2018, 06:47:48 pm
content: Quote from: metrologist on May 22, 2018, 04:24:29 pmLooks simple enough...

author: CopperCone
author_rank: Super Contributor
timestamp: May 22, 2018, 07:06:39 pm
content: I don't know if I want to deal with tubes even if they are more robust. That thing looks huge and I think that a directional finding antenna should be portable.

author: Intermod
author_rank: Newbie
timestamp: June 02, 2018, 02:57:17 pm
content: I note that 'C Series' (An N series sized bayonet variant) has the ability to spin infinitely. Perhaps that would be a more robust alternative?