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I have a project for a small remote control in which I'll use a magnet instead of screws/etc. to close the case.

Basically it is a two-piece plastic case in which one of the pieces has a magnet glued to it, and the other has a matching metal band at the corresponding spot. When the two are put together, the magnet holds them closed.

I was thinking if I could also use that same metal band as the antenna for the remote. I'd connect the magnet to the circuit, and since when the case is closed the band is tightly attached to the magnet, it'd be electrically connected to the circuit.

Would the magnetic field in the magnet prevent this from working? If yes, is there a workaround?

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antenna for what? RF? what frequency? are you should the metal band is even the proper size,shape,impedance for what your trying to transmit? – Mark Oct 13 '10 at 21:08
Sorry, I didn't mention because assumed was implicit. Yes, RF, 2.4 GHz. No worries about the metal band size/shape: I can modify it to accomodate the antenna specs. My only concern is if the field coming from the magnet would affect the circuit. – fceconel Oct 13 '10 at 21:39

2 Answers

up vote 11 down vote accepted

The field from a permanent magnet is the magnetic equivalent of DC; zero frequency. It shouldn't bother a bit, in terms of receiving. Now if you were transmitting through it, you might degauss it, but the power levels in receiving are so low as to not be a concern.

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In fact it's a remote that just transmits, doesn't receive. How much necessary is degaussing in such case? What would you recommend for it? – fceconel Oct 13 '10 at 22:09
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unless you're running more than a watt, the RF probably won't bother the magnet. i'd actually be more concerned that the magnet is made of actual metal and not some ceramic that would possibly exhibit weird dielectric properties at those frequencies. – JustJeff Oct 13 '10 at 22:30
OK, no worries. In this case is a metallic magnet with very low impedance. – fceconel Oct 13 '10 at 22:35
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you're more likely to run into impedance matching issues with the geometry of the thing, than anything else. – JustJeff Oct 14 '10 at 0:45
up vote 5 down vote

For the metal band to act as an antenna, it needs to be the correct length for 2.4 GHz operation and matched to the transmitter with a suitable network. The field from the magnet is irrelevant, but it might have a detuning effect on the antenna because of its size and position, which will need to be checked. It should be insulated from the antenna, of course.

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Problem is, it can't be insulated since it is between the antenna and the circuit. – fceconel Oct 13 '10 at 22:36
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it'll be a bit of a pain to match the impedance of the antenna through the magnetic. The magnet to metal band contact connection will have resistive and inductive properties in addition to the affect of however you attach the signal to the magnet. – Mark Oct 13 '10 at 23:10
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Mark is correct, forget about connecting the magnet to the antenna. Think of another way to keep the enclosure shut. – Leon Heller Oct 13 '10 at 23:23
Well, then I could left the antenna as a PCB strip, totally disconnected from the magnet and the band. I was thinking about them as maybe being better to propagate the signal, but guess (given your thoughts) it's not worth the trouble. – fceconel Oct 14 '10 at 0:46

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