"Radium" wrote in message
oups.com...
Hi:

Can the Spin Exchange Relaxation Free Magnetometer be used to receiver
distant AM radio signals in which the carrier frequency is 150 KHz?
What if the carrier frequency is 44.1 KHz [for the same reason CDs use
a 44.1 KHz sample rate]? 40 KHz?


Thanks a bunch,

Radium

Radium,

The simplest answer is no. The reason is that you specify 'distant'. The
strength of a magnetic field decays rapidly with distance and for most
practical purposes, 800 metres would be a maximum typical detection range
for man made magnetic fields as opposed to natural fields like the earth's
magnetic field. Man made fields can be many times thousands of times
stronger than the earth's magnetic field at a local level, but the field
strength decays rapidly as you move away from the source. A friends father
experimented with a factory machinery control system using induction coils a
foot long and about 8 inches in diameter wound with thousands of turns of
thin copper wire. The maximum reliable communication distance with this set
up was only 200 yards. This was 35+ years ago and detection techniques have
improved considerably. The reason for the experiment was to try and develop
a control system that could work in the presence of very high strength RF
fields. This was superceded by the development of fibre optic cabling which
was not susceptible to RF pickup.

Communication using the magnetic field element of electromagnetic waves is
best demonstrated by the 'molephone' devices used to communicate with cavers
underground. See the following link for details and try googling cave
rescue, underground communications, and molephone for more links.

http://www.scavalon.be/avalonuk/technical/radio1.htm

Squids and other highly sensitive magnetic field detectors can be used to
detect the most minute distortions or changes in the earths magnetic field
but are not suitable for communications use. It is possible to build a
magnetometer at home that is so sensitive that it will pick up disturbances
in the earth's magnetic field caused by solar activity for less than $100.
This is not however going to be capable of receiving and demodulating 150kHz
radio signals.

It doesn't really matter what frequencies you want to receive, 44kHz,
150kHz, 150MHz or what the modulation method is, a diode (or two) and a
tuned circuit are essentially at the heart of all receiving and demodulating
systems. They are cost effective, reasonably efficient and they work. Why
would you want to spend a million dollars to receive signals that can be
picked up (probably a lot better) on a ten dollar consumer radio?

Modulation methods, frequencies and bandwidths are chosen by commercial
broadcasters to meet specific demands, e.g. limited bandwidth available for
broadcasting, high fidelity music broadcasts, high efficiency powerful
signals that can punch through interference, etc, etc. The basic rule is to
get the job done as cheaply, efficiently and as quickly as possible.

Mike G0ULI