Hi there.
I have been away from this list for some time,
as I have been waiting to bump into a nice problem for a comeback...
Here it goes.
We installed recently a new DSX400 for solids.
Now, it happens that we accidentally put the magnet basically aligned
with the most powerful FM transmitter of the region, about 3 miles
away on the top of a mountain. There are 30 FM radio stations kindly
covering the band 88 to 105MHz, with their antennas pointed my way. A
quick check with a spectrum analyzer revealed that particular room to
be flooded with about -70dBm of radiofrequency power at the probehead
location. This is some 12-20dB more then observed almost anywhere
else on the same floor, due to the directionality of the antennas. On
a 400MHz machine, this blanks most useful heteronuclei. To give you
an idea how bad it is, the 'intereference' on 27Al is as intense as
the signal from a 20% alumina sample. And that is FM...
Now, I have been trying to come up with possible solutions.
The most obvious ones would be :
1. move the spectrometer to another room
unfeasible for the time being (no room available)
2. Faraday cage.
possible, but expensive
3. Shutting down the transmitter (...)
I called the national agency of frequencies, and they say
-70dBm is within the emission limit, although there is no specific
law in France
There would be another option, which may twist a little bit the physics.
I was wondering whether just shielding the wall (actually the window)
in the 'incoming' direction of the beam could do the trick, seen the
directionality of the effect (this is not a close or 'spherical'
source, the beam appears to have a spread of about 15m).
I know this may be an atypical problem, but I still hope I can gather
some input from you guys.
Many thanks in advance
Received on Tue May 22 2001 - 14:52:20 MST