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Tablets (iPad, Samsung) cellular signal blocked by aluminum

 
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spqr05
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Joined: 26 Sep 2015
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PostPosted: Sat Sep 26, 2015 9:54 pm    Post subject: Tablets (iPad, Samsung) cellular signal blocked by aluminum Reply with quote

Hi, I'm making a housing out of aluminum and running into cellular degradation due to the material as I'm using tablets inside these aluminum housings (aka case). AKA RF, Cellular antenna issues with the tablets and material.

Antennas are very new to me and I'm looking for some help with understanding the frequencies, wave lengths and overall what I can do to improve my cellular (RF) signal inside the aluminum enclosure, test, etc.. Any help is greatly appreciated.

Currently, we have made several holes / openings and I believe this maybe causing an issue. What is the correct equation / details I need to determine the exact width and size of holes (apertures) I will need to maximize the RF. Plus do I need to make openings on the top, front, sides and back of the case?

How do I test my RF in DBi from a tablet to the towers ??

(I can on my mobile phone connect to the closest towner and gauge DBi but cannot with a tablet as their is no way to enable field test mode with the carrier. )

Currently, testing Verizon iPad mini and Samsung Galaxy Tab 4 8". When using speedtest.net, depending on the location I can have 26ms to latency cannot connect and .1 mbps to 40 mbps. Notice approx 80% reduction in signal and when the signal is weakest the case blocks the signal and cannot upload .10 mbs if at all. when compared to a cell phone on Verizon that gets .9mbps. We have made holes in the face plate of the iPad Mini by the camera and front but still see the uploads being the biggest issue or problem. We have determined once placed inside the rear cover we immediately notice 2 bar reduction in signal, regardless tablet generation. Am I having the signal radiate thru the case due to having changed the case and now I have spacers in their or air? We made a bigger button on the back to open the power button could these be causing issues with the RF?

We have opened a 2 holes (1.25" x .5") on the back but still don't get good receptions, could be we are chasing a ghost at our office but I'd prefer to be able to test and have some advice for antenna experts that understand frequencies and if I'm causing more issues and need to make a few quick changes to mitigate signal loss and take it from there.

I know the Aluminum is a problem and will be but at this point I cannot change the material as easily as I would like and I'm making everything in the US now.

Thanks for your help in Advance.

J
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admin
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Joined: 03 Jan 2007
Posts: 210

PostPosted: Mon Sep 28, 2015 2:51 am    Post subject: Reply with quote

As for the rear of the iPad it is pretty clear where the cellular antenna window is. Are you covering this up with metal? If so it's not clear why you are surprised by your results.
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spqr05
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Joined: 26 Sep 2015
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PostPosted: Mon Sep 28, 2015 4:57 pm    Post subject: Reply with quote

Thanks for the comment I do agree, however, the rear window by the antenna is completely open and still causing interference issues (this is the surprise). With that said, I'm looking for help to calculate the wavelength aperture needed for cellular carriers to determine the size of the aperture and then do additional testing.

We have seen significant improvement in the wifi and cellular, however, I still notice additional interference and the ability for data to be uploaded very disrupted. The latency increases by approx 30-50 seconds to connect to 250 if at all. This means to me I'm still having additional interferences.

Does anyone have experience with the cellular antennas to provide any additional insight or experiences is what I'm looking for.
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helix
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Joined: 29 Jan 2015
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PostPosted: Wed Sep 30, 2015 4:24 am    Post subject: Reply with quote

You're not going to be able to effectively diagnose and fix your problem by looking at network throughput and latency. These are merely symptoms of the disease, which is almost certainly antenna detuning (as opposed to pattern problems). Detuning degrades the match between the antenna and the transceiver, reducing the signal strength.

Normally, at the tablet OEM, antenna engineers model the snot out the antenna AND the metal tablet case TOGETHER. By adding a second significant piece of metal you've changed their design. If you had a lot of design data for the tablet and its antennas you could model the effect of your added enclosure and perhaps come up with a workable way to adjust your enclosure. But since you almost certainly don't have that, the only approach I can think of is to crack open the tablet, disconnect the antenna from the transceiver (it probably has a U.FL or W.FL connector), connect it to a VNA, re-assemble, and then look at the effect of your added enclosure and cut on it until the S11 at the band of interest is acceptable (say less than maybe -3dB minimum, -6dB is better, -10dB is preferred).

You probably don't want to do that, either, so I'd either 1) remove as much metal where you know the tablet antennas are, 2) switch to a plastic case which will affect the tablet antennas far less. A long explanation for simple advice which you no doubt already know.

There are no simple answers like "make a slot in the enclosure a half wavelength long and it will all work."
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spqr05
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Joined: 26 Sep 2015
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PostPosted: Mon Oct 26, 2015 10:25 pm    Post subject: Reply with quote

I appreciate your comments helix on this matter. However, I'm not able to crack open the devices as this would require additional configuration and is not cost effective for us.

To fix the first design, regarding this post that used an apple device, we had to cut the top off to prevent any cellular issues and to maximize performance. Overall we added an aperture and were able to get cellular but not 100%.

We are working with an android product right now and again we are running into the same issues, we are going to test the aperture. I'm looking at transitioning to plastic but right now I have aluminum, cannot change the material and cannot alter the tablet internal / external configurations. Out of the box is there any recommendations that you believe I can do on my enclosure that we make to help improve this?

Here's are my question:

1) is there a way to test the RF interference with an app?
2) Helix had great recommendations but I'm limited and cannot do some, any other recommendations are greatly appreciated.
3) any recommendations on how I can help amplify the antenna with my enclosure that goes around it?
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helix
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Joined: 29 Jan 2015
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PostPosted: Sat Oct 31, 2015 2:20 am    Post subject: Reply with quote

To be clear -- my suggestion was to crack _one_ tablet open to connect a VNA to the antenna port to get some engineering info on how to modify the enclosure, not to modify all tablets. Even then, it's not a very practical solution. If the tablet OEM changes the design then you might have problems.

Search for "RSSI" (Received Signal Strength Indicator) in your app store, this is a quantitative measure (in dBm) of the signal strength received from the tower or access point you're currently connected to. If the tower, tower configuration, or propagation conditions change significantly during or between tests, the data you get will be plenty confusing.


With out knowing anything else about your problem, and assuming you're trying to just "fix" just one frequency band, I could possibly suggest attempting to engineer your metallic enclosure to be a resonant cavity. Presumably the resonance will increase tower signal strength within the cavity and hopefully that received by the tablet antenna. By reciprocity, it should have the same effect on transmit. (Technically for a frequency duplex'ed link it will be a different frequency so reciprocity cannot possibly hold, but it ought to be close.) Like anything, there's no free lunch -- as you increase the cavity Q, the field (signal) strength increases, but the cavity BW is reduced. With high enough Q, the BW restriction will distort and ultimately ruin your wireless channel.

With respect to the cavity: There are some simple formulas (but perhaps not quite easy for beginners to understand) that predict resonant frequencies of cuboidal (as well as cylindrical and spherical) cavities. Check Balanis' book on E&M (not his antenna book) or really any decent book on E&M for details.

When you put stuff in your enclosure (and that's the whole point of having an enclosure!), the resonant frequency predicted by the formulas will be perturbed (changed). If you run wires and other conductors through the middle of the cavity, things will get hopelessly perturbed and this approach won't be useful. But assuming you are able to keep all the metal stuff (e.g., the tablet and any wires) close to the walls of the metal enclosure, your enclosure probably reasonably resembles a 5 sided metal box. What you probably want to do is use the formulas for a 6 sided metal box (which you'll find already worked out in the book) and then cut one of the dimensions in half (technically this is called inserting a perfect magnetic conductor or magnetic symmetry plane or boundary, and is a similar idea to image theory you may have read about) to get the box you want to build with one side missing (that's the aperture). You must be careful, as this approach will only be valid for cavity modes that have Htan=0 on the aperture (because that's what's required by the magnetic symmetry boundary). The extra-tough part is that your cavity is likely to be slightly over-moded, as PCS wavelengths (~16cm) are about the same size as your tablet/enclosure typical dimensions. Generally things will be the most well behaved when the cavity mode you're operating on is the lowest-order mode. You can always use a computer code such as HFSS or CST to help design your cavity.

NOW the hard/impossible-to-tell part.... coupling of your resonant mode with the antenna in the tablet. This is basically near-field coupling and will be pretty tough to even generalize about without knowing more about the tablet antenna type, orientation, position, cavity size, shape, mode, frequency, etc. HFSS or CST would no doubt be useful here as well.

In the above description, the missing wall (aperture) serves as the connection between inside and outside the cavity. A generalized idea is to have a properly designed antenna on the outside of a 6 sided metal box feeding a probe on the inside to excite the cavity.


You're in a tough spot, and what I've proposed above is difficult, but is about the only approach I can think of given your stated restrictions. Putting antennas exterior to your enclosure and cabling them directly into a modified tablet would be a much more sure thing. ...You'll be a hero if you can pull the cavity thing off.
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