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BNC connector as rotary joint
Posted by
CopperCone
on 22 Apr, 2018 17:53
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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.
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#1 Reply
Posted by
W9GFO
on 23 Apr, 2018 06:34
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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.
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#2 Reply
Posted by
T3sl4co1l
on 23 Apr, 2018 16:43
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I wouldn't think it would be good for more than the low hundreds of rotations.
Tim
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#3 Reply
Posted by
edpalmer42
on 23 Apr, 2018 18:27
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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
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#4 Reply
Posted by
ericj
on 23 Apr, 2018 19:42
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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
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#5 Reply
Posted by
CopperCone
on 24 Apr, 2018 16:00
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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.
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If it's not a transmitting station... What would the most likely failure look like?
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#7 Reply
Posted by
CopperCone
on 24 Apr, 2018 18:44
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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?
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#8 Reply
Posted by
ericj
on 24 Apr, 2018 18:46
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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
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Up next, BNC robustness testing...
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#10 Reply
Posted by
richard.cs
on 30 Apr, 2018 20:45
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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.
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#11 Reply
Posted by
GerryBags
on 30 Apr, 2018 20:59
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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
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#12 Reply
Posted by
dazz1
on 10 May, 2018 03:07
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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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#13 Reply
Posted by
CopperCone
on 11 May, 2018 18:53
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i kinda thought it might look cool if its spinning in a circle constantly
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what do those spinning radar antennas, like those on a ship or airport, use?
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#15 Reply
Posted by
CopperCone
on 11 May, 2018 19:01
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expensive rotary joint SMA connector that's at least 100$
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#16 Reply
Posted by
slurry
on 21 May, 2018 20:03
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Why not use two loops and spin them electronically instead, using phase.
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#17 Reply
Posted by
CopperCone
on 22 May, 2018 16:11
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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?
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#19 Reply
Posted by
ericj
on 22 May, 2018 18:47
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#20 Reply
Posted by
CopperCone
on 22 May, 2018 19:06
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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.
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#21 Reply
Posted by
Intermod
on 02 Jun, 2018 14:57
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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?