Looking for a way to communicate with a cell phone confined inside a metal enclosure

[Dirtyfighter]

Hi everyone,

I am a young RF engineer with 3 years of experience in antenna/RF design, and I am currently struggling with what I think is an uncommon situation (well, from what I have seen doing a little search on the web :lol. I am being asked to find a way to communicate with a cell phone placed anywhere on the underside of a metal enclosure which is 200 mm in length, 110 mm in width and a little less than 30 mm in height. This metal enclosure opens by lifting the top cover; from an external point of view it can be seen as a perfect Faraday cage. Because of the dimensions of the metal enclosure, I think placing an antenna (which would have to operate over all of the four frequency bands used in cellular communications) on any of the inner face is not relevant because of the too small distance to the cell phone (comprised between a few mm and a few cm depending on where this one is located with regard to the hypothetical antenna). I might be wrong but from what I understand at such a small distance the antenna would only generate reactive near field, which the antennas embedded inside the cell phone could not "deal with".
Another idea I have had is to turn the enclosure into a TEM cell, however although I have never used one I understand that they need to be large compared to the object to be tested, and normally the coupling strip (septum) is located right in the centre section of the cell which is here impossible...
Last idea, someone told me that I could try to turn the top and lower faces of the enclosure into capacitor plates. This means that the cell phone would be sitting between the plates but I am battling to figure out how it could be implemented.
So if you guys have ideas, comments, whatever... Thank you in avance !

P.S : sorry if I made mistakes, I am not a native speaker
P.S 2 : I have access to CST MS

 

[andre_luis]

I've seen extension cord antennas used with GSM/GPRS modems, placed outside IP67 classified metalic industrial cabinets , having more than 1m lengh, therefore this sounds a reasonable solution for me. However I'm unaware if in your too small box what is the effect of the proximity of the metallic walls, such as some self interference due to reflexion of leaking EM fields.

 

[FvM]

"Communicate" is a rather vague term in this regard. It's quite easy to get a certain coupling to the cell phone antenna. A piece of wire fed into the enclosure will be usually sufficient.

There are coupler pads for smartphones to be used inside a car, they are small enough to fit your enclosure. Imagine it as some kind of multiband patch antenna.

 

[Warpspeed]

Its only a perfect Faraday cage if all the walls are electrically continuous.

A slot, especially a deliberately tuned slot will leak like mad.
Four frequency bands, four different tuned slot radiators cut into the box ?

If the box dimensions are too small for a slot, a tuned tank circuit half inside, and half outside might suffice to do a similar thing.

 

[FvM]

O.K., the nature of the metal enclosure is one more vague point in the post.

I was assuming a "perfect" faraday cage, as used in RF test equipment. Or to ask in a different way, why do you have an enclosure at all? It hasn't been said yet. Of course we can have all degrees of leaky enclosures.

 

[Warpspeed]

I was assuming this is not a real world project, but a problem posed like an "interview" type of question sprung on a job applicant.

 

[Dirtyfighter]

"Communicate" is a rather vague term in this regard. It's quite easy to get a certain coupling to the cell phone antenna. A piece of wire fed into the enclosure will be usually sufficient.

There are coupler pads for smartphones to be used inside a car, they are small enough to fit your enclosure. Imagine it as some kind of multiband patch antenna.

Are you talking about **broken link removed** ?
Ok but if it operates like a multiband patch, it's not likely to achieve decent performances within a metal box, is it ?

Well, let's say that I should have said "transfer as much energy as possible" rather than "communicate"... :grin:

- - - Updated - - -

O.K., the nature of the metal enclosure is one more vague point in the post.

I was assuming a "perfect" faraday cage, as used in RF test equipment. Or to ask in a different way, why do you have an enclosure at all? It hasn't been said yet. Of course we can have all degrees of leaky enclosures.

The enclosure is needed to isolate any cell phone that is placed inside it from cellular base stations. Actually there are RF circuits oustide of the box that emulate a base station. Simply put, the goal of this device (box+ circuits) is to send a SMS to any cell phone that is placed inside it.
I said that the enclosure can be seen as a perfect Faraday cage to simplify the problem, of course it is not. It is probably leaky because of the EMI gasket between the top cover and the rest of the box, however as long as it passes EMI tests it's OK. BTW, I am only allowed to drill one or two tiny holes for any cable/connector.

 

[mtwieg]

Well, let's say that I should have said "transfer as much energy as possible" rather than "communicate"... :grin:

Put the box in an oven, or drop it off the top of a building.

Seriously the term "communicate" means nothing here. Do you have to communicate via the phone's own antenna? Or would its USB port work?

 

[FvM]

Are you talking about this ?
Ok but if it operates like a multiband patch, it's not likely to achieve decent performances within a metal box, is it ?

O.K. the setup is quite similar to those that I know from a customer who makes SIM card and related terminal (mobile phone) test equipment. You'll probably need a double screened enclosure to isolate near by base stations.

Anyway, a device like the linked antenna coupler seems to me the optimum of what can be implemented in a small box. Consider that the mobile phone antenna itself is generating most of the overall Sxx uncertainty and that it's expecting something like free space around it.

I must confess that I don't get your "multiband patch in a small enclosure" point. Placing the internal mobile phone antenna directly upon the pad is far from any regular patch antenna operation, isn't it? The pad is designed to capture the radiated energy almost quantitatively, you should give it a try.

 

[Dirtyfighter]

O.K. the setup is quite similar to those that I know from a customer who makes SIM card and related terminal (mobile phone) test equipment. You'll probably need a double screened enclosure to isolate near by base stations.

Anyway, a device like the linked antenna coupler seems to me the optimum of what can be implemented in a small box. Consider that the mobile phone antenna itself is generating most of the overall Sxx uncertainty and that it's expecting something like free space around it.

I must confess that I don't get your "multiband patch in a small enclosure" point. Placing the internal mobile phone antenna directly upon the pad is far from any regular patch antenna operation, isn't it? The pad is designed to capture the radiated energy almost quantitatively, you should give it a try.

There are coupler pads for smartphones to be used inside a car, they are small enough to fit your enclosure. Imagine it as some kind of multiband patch antenna.

However I still don't get how antenna couplers work. Are they some sort of NFC antennas ?

Put the box in an oven, or drop it off the top of a building.

Seriously the term "communicate" means nothing here. Do you have to communicate via the phone's own antenna? Or would its USB port work?

Via the phone's own antennas.

 

[skroz]

Cell phone closed in metal box together with antena is dead piece of hardware. Faraday cage kills all radio waves trying to enter it or exit. RF engineer would know that student maybe not.

 

[mike65535]

If you're serious about this testing I suggest you look at the equipment Rohde-Schwarz makes for RF testing - including RF-tight boxes. (I say serious because it's expensive!)

Their RF TE can be used to simulate realistic cellular communications - and that RF can be coupled into the enclosure using an antenna inside.

Search for their model CMW-Z10 to start.

 

[FvM]

However I still don't get how antenna couplers work. Are they some sort of NFC antennas ?

Not particularly. NFC (13.56 MHz) uses inductive couplers (planar coils), for GSM frequencies you have EM antennas.

Looking at the R&S box datasheet , it's obvious that it can't receive the power of the mobile device quantitatively, there's still a considerable VSWR.

 

[Dirtyfighter]

Not particularly. NFC (13.56 MHz) uses inductive couplers (planar coils), for GSM frequencies you have EM antennas.

Looking at the R&S box datasheet , it's obvious that it can't receive the power of the mobile device quantitatively, there's still a considerable VSWR.

Looking at different websites I have found references for different cell phone couplers, all seem to be inductive.

Concerning the CMW-Z10 I don't know how much it costs but I guess it's way above what my employer is willing to pay for...

 

[mike65535]

I don't think you'd want a CMW-Z10 - it's an RF enclosure. I take it you already have some kind of "enclosure" (though I don't quite picture what it is) and are trying to inject cellular RF into it.

The Rohde Schwarz stuff is crazy expensive, but look around to see if you don't get an idea of what's necessary and/or possible. IOW, what do they use to get cellular signals inside that box?

Maybe what you want is some kind of "cellular repeater"?

 

[Dirtyfighter]

I don't think you'd want a CMW-Z10 - it's an RF enclosure. I take it you already have some kind of "enclosure" (though I don't quite picture what it is) and are trying to inject cellular RF into it.

The Rohde Schwarz stuff is crazy expensive, but look around to see if you don't get an idea of what's necessary and/or possible. IOW, what do they use to get cellular signals inside that box?

Maybe what you want is some kind of "cellular repeater"?

Ok Tomorrow I'll take a pic of the box, I think it will help everyone unterstand the problems I am batlling with.

 

[Dirtyfighter]

I don't think you'd want a CMW-Z10 - it's an RF enclosure. I take it you already have some kind of "enclosure" (though I don't quite picture what it is) and are trying to inject cellular RF into it.

The Rohde Schwarz stuff is crazy expensive, but look around to see if you don't get an idea of what's necessary and/or possible. IOW, what do they use to get cellular signals inside that box?

Maybe what you want is some kind of "cellular repeater"?

Yes that's correct.

As promised yesterday, enclosed are some pics :









These are pics of one of the prototypes, ignore the SMA conectors which were installed for tests and won't appear in the final product.

 

[tony_lth]

easy solution, copied from the mobile phone EMI box:
coaxial cable into, then uses a printed antenna.

 

[FvM]

Yes. All the antenna couplers offered as mobile phone accessory or professional equipment are in fact "printed" (planar) antennas.

 

[Dirtyfighter]

easy solution, copied from the mobile phone EMI box:
coaxial cable into, then uses a printed antenna.

OK, but EMI test boxes are significantly larger than that I am "playing" with.
So you guys sugget I should go with some patch or something like that ?