I saw a demonstration of an RF detector being brought close to a mobile phone. As the detector was moved away, the number on it dropped to zero. How can that be? Shouldn’t there be some RF signal from the phone all the way to the cell tower, so that the phone can communicate with the tower?
In: Technology
Electromagnetic radiation decreases at r² rate. So at 3 x distance from the antenna, it is 1/9th of the power, at 10 x the distance from the antenna is is 1/100th of the power.
If the RF detector was set to a range then it would fall out of that range very quickly.
You may have heard of the inverse square law. Each time you double the distance, the power drops by four times. The power isn’t actually zero. It’s just low enough that it’s below what the detector will indicate. (Cell phones work at incredibly low received power levels, both at the phone and from phone to tower)
A mobile phone doesn’t constantly transmit at the highest power available. It is mostly inert to save battery life and is only listening to transmission from the tower (which the detector would count as part of the “ambient” RF field and not show, like the tare function on a scale). It gets a lot livelier when it is trying to use its radio to transmit. In the receive-only state the RF detector probably only detects the small interference caused by the phone’s internal power system and the processor/other digital stuff.
Squelch. We are constantly bombarded by RF. Radio and TV stations broadcast thousands of times more powerful signals than phones. The detector is tuned to ignore a certain level of background rf, and only reports a level of energy above that threshold.
One thing to mention – the RF detector like any measurement device will have a finite sensitivity, and signals below some level will measure ‘zero’. But the signal may still exist. In the case you described the signal *does* exist, at a power level that is lower than what could be measured with that equipment.
Electronics technician here. The receiver in the handheld detector is not sensitive enough. Also, the general principles of the dissipation of energy through space is best understood by reading about “The Inverse Square Law”. It says that signal gets weaker the further you are away. In actual fact the signal continues to infinity, in an ever weakening wave.
Your RF “detector” is essentially a piece of shit.
Go on amazon, get a software defined radio dongle. They are less than $100, download some apps that has a spectrum analyzer. You’ll be able to see the different bands your cellphone uses, although you probably won’t see Bluetooth/wifi cause the ones I saw only go to 1.7GHz.
Your cellphone has a chip for the different GSM bands, probably a built in NFC reader, Bluetooth, wifi and the ability to receive GPS/GLONASS and if it’s newer the new EU Galileo positioning system.
A spectrum analyzer is a better tool for identifying RF radiation. Pair it with a high gain directional antenna and you could even locate the source of RF transmitters from far away.
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