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Topic: Detect RF from cell phone (Read 8090 times) previous topic - next topic


Sep 27, 2012, 06:52 am Last Edit: Sep 27, 2012, 06:53 am by wizdum Reason: 1

In my experience, almost anything with a speaker or earphone will detect a cellphone operating nearby :-)

Not here, I have lots of ham radio and other stuff with speakers and I wouldn't tolerate it if cell phone noise came through them. But then again I'm in the northern hemisphere, so perhaps things are different here, like with toilet swirl direction?  :D


Its typically only GSM phones that do this, may be why you havn't heard it in NA, we're mostly CDMA where everyone else is GSM.

Its my understanding that GSM phones have to "ping" the towers every once in a while, so the tower has an idea how many clients it has in range. This ping uses more power than normal, so speakers pick it up even when they don't pick up normal phone use. Whenever I had friends over in my shop, my speakers would buzz about 5 seconds before one of them would get a text message or call. It almost sounded like Morse code, a series of distorted beeps, then a pause, then more beeps.


Has anyone built any RF detector?  A different frequency?  Let's compare results of different methods!


I've been an Amateur Radio operator for 40 years (WA7EMS is my call sign, a "Vanity call" and yes, the phonetics DO mean what they say... <BFG>)
What you are intending to do is difficult because of the Average "noise" level where noise is defined as all detectable energy. Providing selectivity at 900 MHz (Approx) can be done by several different methods, the simplest is the antenna and a 1/4 wave antenna is usually the choice for several different reasons that any RF engineer can point out. There are several problems with that issue too. The principal one is the selectivity of a 1/4 wave antenna isn't the greatest. A "Hotcarrier" diode... HP 5082-28XX series diodes or an OLD 1N23 microwave diode will make a great detector... for anything from 400 Mhz and up, the "sorta' limit is the anrtenna length as it is the only selectivity in circuit. Convert the RF to DC with the detector and apply some DC gain with an op-amp... You could use an LM358 here but I am real partial to R-R I/O Op-Amps and I would use an LMC662. It is a single or dual power supply type of op-amp that can swimg from Vcc to ground in single ended types of circuits and can operate to ground on the inputs whare some types of bo-polar types will latch up. So now you have an amplified broad band noise detector with a small antenns (1/4 wave @ 900 MHz is about 3 inches) and even that is do-able but to differentiate between SMS and phone conversations is nearly impossible and totally so when you consider the other sources of EMI (a simple message might well be 100 mS for the phone to receive the information and respond to the cell site for the message before announcing the call to you as the data rates are rather high < 100K baud. IMO you will have to be real clever to accomplish your stated goal.

--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard


No, I'm not trying to differentiate between those 2, that's the easy part.  There is no other noise within 3ft.  The existing noise is weaker coming from broadcast bands most significantly FM in the city.  Has anyone tried anything related to this?  I mean actually built it?  It's only one passive component....

KB3SF Ham since 1978


Still nobody has tried this?  I'm headed home to test it myself...



I am also interested in making a mobile phone detector as mentioned above. Have you tried to build one yourself?

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