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Strange heating on a 2.4 Ghz circuit

Hi, I have very little knowledge of radios and microwaves so please forgive the simple question...

I had a couple of zigbee (2.4 Ghz) test boards and was trying to send data from these to a remote location. These boards come with a dipole antenna basically etched on to the PCB. Once without thinking I kept two of these boards on a metal wireframe surface, one of the boards was switched on and transmitting once every

2 minutes and the other one was switched off. The one that was on was in a plastic box on top of the wireframe bottom, and the one that was off was left directly touching the wireframe. I must have had the test run for almost 20 mins and when I was getting ready to move things around I noticed the one that was switched off was flashing an led intermittently, and the battery on that one was super hot...the one that was transmitting was just fine though. Anyway after cooling it down i picked it up and tried switching it on and it continues to work fine.

So my question is, what is that all about? I know that the wires on those wireframe trays happen to be around 12 cms apart which is approximately the wavelength of a 2.4 Ghz wave, and that when the other one was on, the waves might have been skimming the surface like the skineffect on a Faraday cage. But why did the other one heat up so? How can I prevent that from happening?

Thanks, Auto.

Reply to
webstuff
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webstuff skrev:

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you probably shorted the battery and that's what made it hot, 1mW isn't likely to heat anything much ...

-Lasse

Reply to
langwadt

hmm.. that does sound possible, after all metal pins on a metal wire... so no fancy radio magic huh?

Reply to
webstuff

Just because the receiver was switched off doesn't mean the extra RF from the transmitter can't cause parts of the receiver to switch on in a disastrous way. That signal is still getting into the receiver and being rectified by active stages in the front end, supplying dc current to other areas, probably not in places the designer wants it to go.

I always tested my radio designs for this sort of failure. I remember one case where a state highway patrol radio, that was switched off, went into a high power transmitter oscillation when an officer from a city PD used his UHF walkie-talkie near the SHP car. No one knew anything was wrong until smoke started coming from the car's trunk.

Germany's Bundespost (like the US FCC) actually has a specific test for this that catches a lot of poor designs. The FCC could give a hoot.

Reply to
L

one thats confusing though is that if the battery shorted out for a good 10 minutes or so, shouldn't it have been completely dead?? Its still working now.. it shows 7.9 volts on a meter (its a 9V)

Reply to
kiritb

Just Murphy's Law - it was inevitable that you'd short out the back-up battery. You should check/change that battery, by the way.

Cheers.

Ken

Reply to
Ken Taylor

Evidentally it has a lot of capacity. The battery itself being hot can be explained only by excessive battery drain. There is no magic in RF circuits, so conservation of energy still applies. Given the low power ZigBee transmiter, the receiver's only source of meaningful energy was that battery. It doesn't matter how efficient the wireframe or the dipole antenna may have been at RF absorbtion, there just wasn't enough RF energy present to cause any detectable heating (unless you had a microwave oven right next to it with the door removed while operating).

Reply to
Howard Henry Schlunder

The circuit board pin/wireframe (BTW, what's a "wireframe"? Hardware cloth? Window screen?) interface has non-zero resistance. If it's just lying there, with no wiping action, the oxides and stuff could have prevented a perfect short, and I'd certainly think that contact area would come into play.

If you mashed it down onto the "wireframe", and ground it in, I'm sure you could kill the battery in very short order, or possibly burn your hand, or burn stuff up.

Good Luck! Rich

Reply to
Rich Grise

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