Author Topic: BNC connector as rotary joint  (Read 4001 times)

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Offline CopperConeTopic starter

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BNC connector as rotary joint
« on: April 22, 2018, 05:53:48 pm »
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.
 

Offline W9GFO

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Re: BNC connector as rotary joint
« Reply #1 on: April 23, 2018, 06:34:43 am »
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.



 

Online T3sl4co1l

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Re: BNC connector as rotary joint
« Reply #2 on: April 23, 2018, 04:43:26 pm »
I wouldn't think it would be good for more than the low hundreds of rotations.

Tim
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Online edpalmer42

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Re: BNC connector as rotary joint
« Reply #3 on: April 23, 2018, 06:27:24 pm »
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
 

Offline ericj

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Re: BNC connector as rotary joint
« Reply #4 on: April 23, 2018, 07:42:36 pm »
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
 

Offline CopperConeTopic starter

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Re: BNC connector as rotary joint
« Reply #5 on: April 24, 2018, 04:00:21 pm »
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 freely

Good point on the grease you dont have much rf reasons to worry about 50MHz tops and it will reduce friction some.
 

Offline metrologist

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Re: BNC connector as rotary joint
« Reply #6 on: April 24, 2018, 05:14:26 pm »
If it's not a transmitting station... What would the most likely failure look like?
 

Offline CopperConeTopic starter

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Re: BNC connector as rotary joint
« Reply #7 on: April 24, 2018, 06:44:15 pm »
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?
 

Offline ericj

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Re: BNC connector as rotary joint
« Reply #8 on: April 24, 2018, 06:46:59 pm »
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
 

Offline metrologist

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Re: BNC connector as rotary joint
« Reply #9 on: April 24, 2018, 07:38:26 pm »
Up next, BNC robustness testing... :-DD
 

Offline richard.cs

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Re: BNC connector as rotary joint
« Reply #10 on: April 30, 2018, 08:45:54 pm »
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.
 

Offline GerryBags

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Re: BNC connector as rotary joint
« Reply #11 on: April 30, 2018, 08:59:24 pm »
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
 

Offline dazz1

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Re: BNC connector as rotary joint
« Reply #12 on: May 10, 2018, 03:07:49 am »
Hi
If you limit the rotation of you antenna to say 720 degrees, you can just use a cable rather than a joint.

Dazz
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Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

Offline CopperConeTopic starter

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Re: BNC connector as rotary joint
« Reply #13 on: May 11, 2018, 06:53:19 pm »
i kinda thought it might look cool if its spinning in a circle constantly
 

Offline metrologist

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Re: BNC connector as rotary joint
« Reply #14 on: May 11, 2018, 07:00:06 pm »
what do those spinning radar antennas, like those on a ship or airport, use?
 

Offline CopperConeTopic starter

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Re: BNC connector as rotary joint
« Reply #15 on: May 11, 2018, 07:01:04 pm »
expensive rotary joint SMA connector that's at least 100$
 

Offline slurry

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Re: BNC connector as rotary joint
« Reply #16 on: May 21, 2018, 08:03:44 pm »
Why not use two loops and spin them electronically instead, using phase.

 

Offline CopperConeTopic starter

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Re: BNC connector as rotary joint
« Reply #17 on: May 22, 2018, 04:11:13 pm »
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?
 

Offline metrologist

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Re: BNC connector as rotary joint
« Reply #18 on: May 22, 2018, 04:24:29 pm »
links at bottom of page: http://www.jproc.ca/marconi/mdf5.html
 

Offline ericj

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Re: BNC connector as rotary joint
« Reply #19 on: May 22, 2018, 06:47:48 pm »
 

Offline CopperConeTopic starter

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Re: BNC connector as rotary joint
« Reply #20 on: May 22, 2018, 07:06:39 pm »
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.
 

Offline Intermod

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Re: BNC connector as rotary joint
« Reply #21 on: June 02, 2018, 02:57:17 pm »
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?
 


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