Will magnet affect a Bluetooth device's RF reception?

I came across a problem with my Bluetooth portable device design: the antenna reception performance is super crapy! For some reason, I have to stick a 10mm x 5mm magnet onto the back of the device, and the lithium battery is placed inside the device's housing... I tested various antenna parameters, fast

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Bluetooth 9.99
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Hello All,

I came across a problem with my Bluetooth portable device design: the antenna reception performance is super crapy! For some reason, I have to stick a 10mm x 5mm magnet onto the back of the device, and the lithium battery is placed inside the device's housing... I tested various antenna parameters, fast TIS, TRP, etc. but still could not identify the pitfall that made my RF link so damn poor! Now, in my wildest guess: will that piece of magnet deteriotate the Bluetooth RF link? Please shed your light on it. Thanks!

Bluetooth 9.99

Reply to
Bluetooth 9.99

Hello All,

I came across a problem with my Bluetooth portable device design: the antenna reception performance is super crapy! For some reason, I have to stick a 10mm x 5mm magnet onto the back of the device, and the lithium battery is placed inside the device's housing... I tested various antenna parameters, fast TIS, TRP, etc. but still could not identify the pitfall that made my RF link so damn poor! Now, in my wildest guess: will that piece of magnet deteriotate the Bluetooth RF link? Please shed your light on it. Thanks!

Bluetooth 9.99

Reply to
Bluetooth 9.99

Hey! Hey! Hey! We have a Bluetooth designer in our midst ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

The electromagnetic wave incident on nearby metal will cause electric current to flow on the metal. In the near field, these currents will likely not be in quadrature. These currents subsequenty re-radiate more electromagnetic waves. (aka: "scattering")

It is possible that the scattering itself is causing other problems (channel fades, etc..), or even simple ISI-type interference. More likely, it is detuning the Bluetooth transceiver, and its not making rated power (that a total guess?!) The receive path would likewise suffer, but here you're more concerned (generally) with max voltage, not max power. It may not suffer in direct proportion to the transmit side of the link. In fact, it could even be better than expected.

In either case, it may be possible to add RF antenna folds to reduce the electromagnetic interference and scattering. This de-tuning approach would likely be most effective if you can discover, and detune the assembly at the offending resonant / harmonic frequencies. (I am presuming you are unable to move either / both the magnet and battery.?) And because it's Bluetooth and fairly high frequency, I'll also wager you don't have the real estate to try out very many solutions...

I am not aware of any modeling software, but it "must" be out there somewhere?? Otherwise, this might be one of those trial-and-error situations... It sounds like you're going to require the services of a really accurate spectrum analyzer / calibrated antenna setup, and maybe a network analyzer too.

I would start by looking at the spectral mask and move things around to the extent you can. That will at least "confirm" if re-radiation is the culprit. (I think?) Hard to tell without much more info, etc.. But from what you say, it seems reasonable to focus on making the nearby metal transparent to the Bluetooth signal. Good luck. - mpm

Reply to
mpm

As mpm wrote, putting big lumps of conductive materials close to the antenna will change performance.

First thing to check would be reception and transmission if you place the Bluetooth part of the design in open air. If that works, try changing the device (place the antenna away from metal surfaces or re-design the antenna to radiate in that environment).

And this is novel, how? (500 million chips or so shipped by my employer, containing a few modest contributions from yours truly)

Best Regards

Jens

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    Key ID 0x09723C12, jensting@tingleff.org
        Analogue filtering / 5GHz RLAN / Mdk Linux / odds and ends
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Jens Tingleff

[snip]

It was meant tongue-in-cheek ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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