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.
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
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
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!