My Bluetooth Works Inside a Metal Enclosure

[fifty60]

I have a bluetooth dongle mounted inside of a metal hoffman enclosure. The dongle connects a bluetooth signal to a barcode scanner outside of the enclosure.

My thoughts on this is that it would not work, the 2.4GHZ BT signal would not be able to penetrate the enclosure. But sure enough, I can stand 50 feet away from the enclosure and the bluetooth dongle still picks up the signal from the barcode scanner.

Why in the world does this work? I thought the 2.4GHz signal would only penetrate a few atoms deep into the metal and bounce off?

 

[Dennis Alwon]

It's sneaking thru the screw holes....LOL - To be honest, I have no idea

 

[Jraef]

There are now two forms of Bluetooth; the original BR (Basic Rate) and the newer LE (Low Energy). BR was good to about 10m, which was fine for things like cell phones in cars, keyboards, mice etc. LE was developed mostly for battery powered devices to reduce power consumption, but it also boosted range and is now good to about 30m. All RF signals can pass through metal boxes, but the signal is diminished. Sounds as though yours is probably LE and diminished to about 1/2 of its range, but is still getting through.

 

[GoldDigger]

The other possibility is that the signal is being conducted over wire(s) that pass out of the enclosure and are exposed somewhere within Bluetooth range.

Sent from my XT1585 using Tapatalk

 

[K8MHZ]

The other possibility is that the signal is being conducted over wire(s) that pass out of the enclosure and are exposed somewhere within Bluetooth range.

Sent from my XT1585 using Tapatalk

Yes, I agree. The conductors are acting like passive repeaters and that overcomes the Faraday effect produced by the metal enclosure.

 

[fifty60]

Thanks, it makes sense now. The signal is getting through, but it is attenuated to about 50%.

 

[GoldDigger]

Thanks, it makes sense now. The signal is getting through, but it is attenuated to about 50%.

Just do not have the misconception that it is actually passing through the walls of the enclosure itself.
Slot openings and wire penetrations are two actual possibilities.

 

[Bugman1400]

I guess the insinuation is that the enclosure acts like a Faraday cage but, I thought it had to be grounded to do that. Did the OP state that or was that assumed? Perhaps I misread.

 

[gar]

170610-1054 EDT

Bugman1400:

Why does a Faraday shield have to be grounded, and why would that make a difference relative to radiation outside the box from a radio frequency source from within the shield?

.

 

[K8MHZ]

I guess the insinuation is that the enclosure acts like a Faraday cage but, I thought it had to be grounded to do that. Did the OP state that or was that assumed? Perhaps I misread.

No, a Faraday Cage doesn't have to be grounded to work. They are usually grounded to prevent build up of static charge.

They work by producing reflection and re-radiation to cancel out most of the RF. Some of the RF is absorbed by the material in the cage, but the principle is to create a shield by cancellation effects.

In order for a cage to be 100% effective, it would need to be designed for the transmitter, and the transmitter would have to be placed exactly in the center. Most of the time, 100% is not needed, and cages are usually not perfect (like in a radio transmitter) because they produce an acceptable amount of attenuation at a reasonable cost.

Your microwave oven is a Faraday Cage. It blocks MOST of the RF, but not all. But it blocks enough to provide safety. The RF can still be detected with a radio receiver, though.

 

[fifty60]

I thought you were kidding when you said the signal was being radiated through the opening and wiring on the enclosure. I know that happens, and some signal gets radiated that way, but you think that the majority of the signal is being radiated that way?

One way to test it would be to put the transmitter in the box before making any penetrations. There are a lot of penetrations in the box, several Mencom connectors, an HMI, and several switches and indicator lights.

So the signal is getting out retransmitting from the enclosure, not passing through the enclosure...fascinating.

 

[gar]

170611-0921 EDT

fifty60:

Why would you expect your RF radiation to penetrate a fully conductive shield around your RF source.

Have you ever taken a radio receiver inside an RF screen room and observed what you can hear (RF wise)?

You do not even need a fully enclosed shield and you can detect signal attenuation from a partial shield. My breezeway does that because of the metal lath in the ceiling.

.

 

[K8MHZ]

I thought you were kidding when you said the signal was being radiated through the opening and wiring on the enclosure. I know that happens, and some signal gets radiated that way, but you think that the majority of the signal is being radiated that way?

One way to test it would be to put the transmitter in the box before making any penetrations. There are a lot of penetrations in the box, several Mencom connectors, an HMI, and several switches and indicator lights.

So the signal is getting out retransmitting from the enclosure, not passing through the enclosure...fascinating.

Think of how a multi element antenna works. RF is reflected and directed with metal element. It isn't passing through them, instead the elements 'vibrate' and radiate the RF directionally.

A metal box can do the same thing. The RF may not pass through it, but the box will re-radiate a modulated signal to the outside. Not nearly as efficient as an antenna, but that is how a signal can be picked up outside the box. It will be very weak, but present nonetheless.

The dimensions of the cage and the placement of the transmitter inside the cage will affect the amount of re-radiation.

 

[macmikeman]

Use a lead lined box. That will properly block any signals and make your equipment useless. I guess then the benefit will be ''taking the rest of the day off...."

 

[GoldDigger]

Use a lead lined box. That will properly block any signals and make your equipment useless. I guess then the benefit will be ''taking the rest of the day off...."

Lead is useful for particle radiation and high energy gamma rays that can blast their way through conductive metal, but for RF emissions it is not as good as copper or any other higher conductivity metal.
It does have the advantage that you can mold it into virtually seamless enclosures.

 

[macmikeman]

Lead is useful for particle radiation and high energy gamma rays that can blast their way through conductive metal, but for RF emissions it is not as good as copper or any other higher conductivity metal.
It does have the advantage that you can mold it into virtually seamless enclosures.

That's useful to know, thank you. So now I have to add a layer of copper to my lead lined box where all my top secret electronics stuff goes in the vehicle while I drive from job to job............

 

[GoldDigger]

That's useful to know, thank you. So now I have to add a layer of copper to my lead lined box where all my top secret electronics stuff goes in the vehicle while I drive from job to job............

Or to your helmet. :angel: