Bluetooth dongle inside a waveguide 3

[olismo]

Good morning to everyone.
For my test I need to strongly reduce the range of a bluetooth dongle (It is the classic USB bluetooth dongle working at 2.45GHz).
I read online that all attempts to shield the bluetooth dongle (e.g by wrapping it with aluminum foil) fail because the screen is floating (because for my applications I've need to be free from ground connection).
So I thought to put the bluetooth dongle inside a (rectangular or circular) waveguide, whose geometries determines a cutoff frequency higher than 2.45GHz.
In this way the signal should decay exponentially with distance (evanescent field).
Before proceeding to the physical realization I would need to know if my reasoning is correct and if you have tips for the realization of the above.
Thanks in advance for any your suggestion,

 

[biff44]

If it did work, it would be highly directional.

I met a nice lady at a trade show recently that showed me something that would be perfect to try. Her company makes heat shrink tubing that is made out of microwave absorber. Call these guys up:

http://www.arc-tech.com/

And ask for a sample of their lossiest heat shrink at 2.4 GHz, and put a layer or two around the dongle and see how it works.

Absorber material will not be directional, and will not have any floating ground type re-radiation weirdness.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[olismo]

Dear biff44, thanks a lot for your fast and useful answer!
I will investigate also in this direction.
Best regards.

 

[Higgler]

Aluminum foil should work.
I don't see why it doesn't. I've used it alot the past 20 years.

Or get metal tape at your local hardware store and tape up the item.

The absorber might shift the antenna frequency a little, but unless it's very thick, probably won't reduce the antenna efficiency by more than 5-10 dB.

 

[olismo]

Hi Higgler, thanks for your answer.
I tried the alluminium foil but its effect is limited because it does not work as a Faraday cage as it is floating and it is not connected to ground.
In other words, paradoxically, the foil re-transmits the signal emitted by the antenna embedded into the dongle.

 

[biff44]

If you think about it, there is no way to make a faraday cage around a usb dongle as it has to have 4 high speed wires coming into it that would break up the cage. Those 4 wires act as very nice antennas.

Now, if you were willing to provide 4 lowpass filters, each returning to the foil shield, with the cuttoff frequency maybe at 500 MHz, THAT plus the foil would work.

A marginaly acceptable alternative would be putting the dongle at the end of a usb cable, and putting 3 or so ferrite beads clamped around the cable (near the dongle) PLUS the aluminum foil around the dongle itself.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[biff44]

Heat absorptive shrink tubing PLUS the aluminum foil might be an option. The foil attenuates the signal, and the lossy heat shrink will de-Q any frequency resonances caused by the foil/antenna interaction.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[VEBill]

...does not work as a Faraday cage as it is floating and it is not connected to ground.

Shielding and Faraday cages can be perfectly effective even without being "grounded", no matter what you mean by "ground". E.g. An airplane fuselage makes an excellent Faraday cage, protecting the passengers from lightning strikes, even when it is airborne.

 

[olismo]

Dear Biff44 and VE1BLL, I've appreciate very much your post because now the problem is more clear for me.
Summarizing, the problem isn't the aluminum foil but the USB cable that works as an antenna.
Then there are three possible solutions given by Biff44:
1) 4 lowpass filters, each returning to the foil shield
2) Put the dongle at the end of a usb cable, and putting 3 or so ferrite beads clamped around the cable (near the dongle) PLUS the aluminum foil around the dongle itself.
3) Heat absorptive shrink tubing PLUS the aluminum foil

The first two solutions are clear to me while the third one not: Biff44 could you kindly explain to me how I've to use the Heat absorptive shrink tubing? (also link are appreciated)
Thanks in advance

 

[VEBill]

Foil is cheap. Shield the entire USB cable right up to the PC.

Ferrite beads on the cable would probably help, but keep in mind that the RF in the case of Bluetooth is 2.4 GHz.

 

[Higgler]

A bag of salt water is the cheapest absorber you can buy. if you add the extension cable as suggested and put the antenna in a bucket of water (insulate the USB dongle of course from the water by another bag), it won't radiate.
Salt is lossier, add some to the fresh water if needed. I measured fresh water at 10 dB per inch attenuation at 2.45 ghz, which is what you're working at.

 

[olismo]

Thanks a lot Higgler for your tip.
I will try this solution too.
I've planned to experiment all the solutions this week but I've to posticipate them for a job trip.
Anyway, after that I will try all the solutions I will report their results, in order to be useful for those that need to do this one.

 

[MikeHalloran]

Black conductive antistatic foam,

ex:

http://www.correctproducts.com/ESD-Packaging/High-Density-Black-Conductive-Polyurethane-Foam

absorbs microwaves like crazy.



Mike Halloran
Pembroke Pines, FL, USA