When such a product is very close to
a mobile phone operating in its upper frequency band and at its high power level (about 600 mW) significant
currents can be induced into the product. This is because it is resonant near
the operating frequency of the mobile phone. Enough current can be induced to permanently damage the circuit in some cases.
In the car key case, when the key was in a pocket with a mobile phone
and the phone became active, the electronic circuit in the key could be damaged.
Although the key would still open the door, it would not start the car
as a result of the damaged circuitry in the key.
Even if the circuit is not damaged, temporary operational malfunction
can occur. I have observed this in a number of automotive and other
In my February 2006 Technical Tidbit, Construction of a Coaxial Antenna for Troubleshooting
a small homemade antenna and method is described for subjecting a small
product to an approximation of the very near field of a mobile phone.
If your product has an electrical length of about 6 cm (upper mobile
phone band) or about 12 cm (lower mobile phone band), I would highly
recommend testing it with the simple procedure outlined in that
February 2006 Technical Tidbit.
Even for products of other sizes that may be in close proximity to a
mobile phone, I would recommend looking for resonances using the method
in my June 2006 Technical Tidbit, Measuring Structural Resonances
If a resonance is found in one of the mobile phone bands, then go on to
test the product with the method in the February 2006 Technical Tidbit.
Even if no resonance is found in the mobile phone frequency bands,
testing the device for response to nearby mobile phones may be the
safest course of action.