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Will magnet affect a Bluetooth device's RF reception?


Bluetooth 9.99

Jan 1, 1970
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.

Bluetooth 9.99


Jan 1, 1970
Hello All,

I came across a problem with my Bluetooth portable device design: the
antenna reception performance is...

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

I am not aware of any modeling software, but it "must" be out there
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. -