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# Inexpensive 2.4GHz rotary joint?2

## Inexpensive 2.4GHz rotary joint?

(OP)
I'm building a device where you can place a 2.4 GHz antenna on a disc that will rotate and then view the relationship between the power spectrum and rotation on a PC.  I would strongly prefer to keep the disc 100% freely rotatable rather than to worry about the wire between the antenna and radio module becoming too twisted or tangled.

I thought that using a rotary joint would be a good plan, but all the sites I've found look like they are buying in mass quantities, whereas I only need one.  I just received a quote from one website that said I needed to buy a minimum of 3 at $467 apiece -- I was really hoping to spend no more than$150 tops for the rotating wire part.

Does anyone know where I could find any inexpensive rotary joints that would operate with minimal losses at 2.4-2.5 GHz?  Or, are there any RF connectors that could rotate freely?  I'm thinking a BNC connector might work, but I have limited access to test equipment at the moment, so I don't know if that would be too lossy.

Also, I don't care if the signal/connector is noisy during rotation, because I will take measurements when the disc is not rotating.

Mark

### RE: Inexpensive 2.4GHz rotary joint?

BNC is no good at that frequency.

The key thing here is the amount of rotation. Do you want to rotate once for an experiment, or are you intending on doing thousands of rotations? If the number or rotations is limited, then a less rigourous design might be tolerable!

My feeling is that I would prefer to receive and down convert on the moving table and only send base-band down the rotating joint.

### RE: Inexpensive 2.4GHz rotary joint?

"I don't care if the signal/connector is noisy during rotation, because I will take measurements when the disc is not rotating."

The issue is that the "noise" is irregularity in the contact resistance, while will affect static tests

### RE: Inexpensive 2.4GHz rotary joint?

(OP)
logbook, the design is more of a proof of concept, so if it was necessary to use a cheaper part that would need to be replaced after maybe a couple hundred rotations, that wouldn't be a large problem.

I can't downconvert the signal because I'm using a Cypress module to grab the RSSI directly from the 2.4xx GHz signal.  However, the module is going to be communicating with another micro over SPI -- so maybe I would be better off placing the Cypress module on top of the disk but below the antenna, and then sending VCC, gnd, and the SPI bus signals to it through rotatable connectors.

I'm not sure what would be worse - having several BNC cables to route through the shaft of the device, or trying to use one BNC cable to possibly send modulated SPI bus signals riding on VCC.  The datasheet says the SPI interface goes up to 2MHz, so maybe I could encode the signals, use an I/Q modulator and demodulator pair, then decode it -- but there's two way traffic between the master and slave.  I've got some research to do there in any case.

IRstuff, good point about the contact resistance.

Thanks for the responses so far.

### RE: Inexpensive 2.4GHz rotary joint?

Use an N type with the collar only slightly loose, it will probably suffice if the rotation is not to fast.  You could lock the collar to prevent it slipping with a short strip and a couple of tyraps.  Lightly grease the mating parts of the connectors.
I have just checked this out with a site master 332 and it looks good in terms of return loss.

### RE: Inexpensive 2.4GHz rotary joint?

Thanks to bogeyman for some real work!

Personally, I would be much happier putting SPI and power through slip rings rather than using a rotating RF connection.

### RE: Inexpensive 2.4GHz rotary joint?

You do not have to rotate the antenna.  Get a circular wavguide horn and position it so it is pointing straight up.  Then put a metal plate at a 45 degree angle to the centerline and rotate it around the circle.

If you really needed to make a rotating antenna, you could probably use a standard microstrip antenna, but use a coaxial capacitive feed, with non-contacting airgaps (or teflon sleaves) insulating the inner and outer conductor pairs, each quarterwave long.  Should be easy to design.

### RE: Inexpensive 2.4GHz rotary joint?

biff44, 2.4GHz is a pretty low frequency for a horn isn't it, meaning a pretty large antenna. Then the plate probably needs to be a fair way from the horn so as to not be in the near field. It just seems a rather large arrangement.

I would be happy with this at 24GHz but at 2.4GHz it is 10x larger.

### RE: Inexpensive 2.4GHz rotary joint?

(OP)
bogeyman - That's a really good idea.  I can keep the rotation speed as low as necessary, so I will order some N types on my next part order.  I also really like how inexpensive this solution would be.  Thanks!

logbook - I also really like the slip ring idea. I don't have much knowledge of electromechanical products like that, so I had no idea what terms to search for.  However, a cursory search of slip rings is showing me a lot of vendors like the original rotary joint, so I'm really worried that a 5-contact slip ring is going to result in another $400 quote that's just way beyond my budget. biff44 - I need to be able to alternate antennas. ### RE: Inexpensive 2.4GHz rotary joint? Ok, how about a telephone cord untangler from Radio Shack, http://www.radioshack.com/ Model: CU2050CL Catalog #: 279-001$6.99

This looks as though it will do what you need, with a bit of work. I found it when searching for "lego" and "slip" because I figured there would be rotating joints on kids robot toys etc.

### RE: Inexpensive 2.4GHz rotary joint?

(OP)
logbook, I think you may have found the perfect part there.

I need to send VCC, ground, data in, data out, clock, and maybe slave select.

I'm thinking that I can ride the clock signal on VCC and filter out one or the other as necessary since the clock will be at ~2MHz... and if I do need slave select, I can probably rig up something with a 555 watching the clock input.

I'll stop by a Radio Shack this week and see how the part handles a 2 MHz square wave on top of 5V while rotating.

Thanks again for the good product find!

### RE: Inexpensive 2.4GHz rotary joint?

How about an antenna to antenna coupler. Picture two spiral antennas (shaped like hockey pucks), one atop one another. Rotate the top one, keep the bottom one still. RF amplitude doesn't change, your phase will change of course. The coupler loss may be 1 or 2 dB, but can be minimized with proper design.
You have a very narrow frequency band so loss in the coupling can be small. Wide frequency band is more difficult to couple energy from one antenna to another.

The type N idea is quicker and cheaper, but if you want a long term rotation solution, (with slightly more loss) antenna to antenna coupling is a good solution.

You may want to use circularly polarized patch antennas as couplers, less loss in them as spirals tend to have absorber in their cavity. Plus you can probably buy a pair within your budget and try them out.

kch

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