# Using magnetic fields to pickup radio freguencys

1. Apr 15, 2015

### Paul Turner

a simple idea.

Maxwell’s theory of magnetic fields says there is two components ,an electrical field and a magnetic field. (put simply).

I have an idea to pick up radio waves using a magnetic beam. rather than the normal dipole set-up, just wondering if you think this idea is possible to make it work in reality.

2. Apr 15, 2015

### sophiecentaur

There are plenty of receiving antennas, which measure the magnetic field, rather than the electric field. Pretty well every mf portable radio uses a ferrite rod antenna, which consists of resonant coil, wrapped a round a ferrite. Many mobile phones use 'slot antennas' - which are, as the name suggests, cut outs in a metal sheet (often the ground plane of the circuitry). These slots respond to the varying magnetic field of the signal. There are many advantages in not using electric dipoles, in some circumstances.
I don't know what you mean by that.

3. Apr 16, 2015

### tech99

I am wondering if you are thinking that the magnetic wave can be treated in isolation? There are always the two waves together.

4. Apr 16, 2015

### sophiecentaur

Yes, there are two fields involved in an EM wave. The OP is aware of this and seems to be looking for the magnetic equivalent to a dipole. The whole business of how a piece of metal 'picks up' a signal from a passing em wave is very difficult. For instance, an 'infinitely thin' piece of wire has a very finite effective cross section, which allows it to extract a very finite amount of power from a radio wave. How can that be - taken at face value?
The following may or may not be relevant to the OP as he has not yet responded to my query about what he meant by 'magnetic beam".
The E and H fields in a plane wave in free space have a constant ratio E/H and is about 377Ω and is referred to as the characteristic impedance of free space. E/H in other situations can be very different and the 'local impedance' can be much higher or much lower than 377Ω. The actual amount of power transferred by an antenna (transmitting or receiving) will depend upon the degree of Matching between the local wave and the antenna but, for a receiving antenna, the signal power pickup is not as important (at mf, for instance, the main problem is often the levels of incident interference and noise and not actual signal level)
The E and H fields can be measured independently by using an appropriate form of probe. Take a short dipole (significantly shorter than half wave resonance). It can be looked upon as a capacitor (the two halves will have a capacitance between them) in series with a very small inductor (two short lengths of wire). It's a very high impedance and will respond to the E field around it. The H field will not induce any significant current into the short lengths of wire.
Likewise, a small coil (loop antenna) will be a dead short for any E field around it and it will just not respond. However, a current will be induced in it, due to the H field.
How is all this relevant? The local impedance can be very much affected by metal structures and, in particular, the E field can vary wildly. A magnetic antenna will ignore this because it is only looking at the H field.