mobile phone rf couple on to electronic circuits - RF Cafe Forums
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Post subject: mobile phone rf couple on to electronic circuits Posted: Tue Dec 13, 2005 2:14 am
IF THERE IS AN ELECTRONIC DEVICE THAT WORKS OFF OF A SPECIFIC FREQ. WHAT IS THE LIKEYHOOD THAT IT WOULD BE ENGAGED TO WORK BY ANOTHER TRANSMITTING DEVICE NOT EVEN WORKING ON THE SAME FREQ. ?
ALSO, CAN A REGULAR METAL DOOR PREVENT RF FROM PASSING THROUGH IT?
I KNOW THAT TO SOME OF YOU, THESE MAYBE VERY STUPID QUESTIONS AND IM PRETTY SURE IKNOW THE ANSWERS TO THESE BUT ID LIKE AN ENGINEERS OPINION.
Post subject: Posted: Tue Dec 13, 2005 2:20 am
...it depends on a lot of factors... Can't easily answer your question...
Post subject: RFPosted: Wed Dec 14, 2005 6:44 pm
Second question first:
The door won't let RF through very well - but the RF doesn't necessarily have to go through the door. That's why they make "screen rooms" - the doors and the walls are conductive, and connected together. RF can "sneak through cracks" - it can be really annoying to track down RF leakage.
Most receivers will respond to strong-enough signals on frequencies other than they are tuned to. To say any more really requires detailed knowledge of the situation. This is a problem which has been around since the beginning of radio.
Post subject: Posted: Thu Dec 15, 2005 9:05 am
The dielectric constant of wood, glass and cement ranges from 2-30, which menas that the RF energy will propogate through them. There are varying loss tangents ( how much the signal will dissipate per cm) which will attenuate the signal strength once the signal passes through. The signal can pass through the wall and you will receive it if the sensitivity of the receiver is low enough or the signal strength is strong enough. Try this experiment with your cell phone, go near a window and look at the signal strength, then move along the wall away from the window and look at the signal strength. The signal strength behind the wall will be lower since: 1) The wall is thicker, 2) The loss tangent is higher. Now if the wall has steel or aluminum studs, then you may not receive anythign since the steel studs may be setting up a faraday screen for that particular signal.
Post subject: Nasty RFPosted: Thu Dec 15, 2005 12:31 pm
Some general points.
1. RF can't be completely stopped, only attenuated down to a level thats determined by the amount of it your reflect or dissipate with the stuff you have between you and the RF source. Screened rooms can achieve upto 120 dB of Isolation (if your really lucky) but remember that many receivers have at least -120 dBm sensitivity!!!!!
When RF hits something that does not present the same impedance as free space, this causes some of the RF to reflect off away but some can pass through the 'something' especially when its not metal or if it has small gaps in it. How much that gets in depends on how big any gaps are relative to the wavelength of the signal your talking about.
2. When RF gets into electronic circuits it can wreak havoc even in well designed kit. It all depends on how big the signal is, its frequency and what sort of circuits your talking about. In general terms small levels of RF such as that from a cell phone at say 100 metres or so should not cause problems with most kit BUT if the kit in question has broadband recevier bits in it then these will receive the RF as well as the wanted stuff.
If the RF gets bigger it can get through the filtering thats normally present at the input of most radio kit and then it arrives at the amplifier and saturates it.
Another problem with RF is that it can also get into anything non linear and rectify which then upsets bias conditions etc.
The subject is huge............I have only scratched at the surface!