# How much current can you draw from a resonant loop antenna

1. Aug 4, 2010

### htg

In the case of a loop antenna with uncompensated inductance, the magnetic field generated by the current in the antenna is in antiphase to the incoming magnetic field of the EM wave. So it is believable that the current which causes cancellation of the flux through the antenna is the maximum current that you can draw.
But in the case of a resonant antenna, there is 90 degrees phase shift between the magnetic field generated by the current in the antenna and the magnetic field of the incoming EM wave. In the idealized situation (no resistance, neglecting radiation by the receiving antenna) is there any limit on the current?
(Or does it behave like I0*t*sin(2*Pi*f*t) if the induced voltage is U0*sin(2*Pi*f*t) ?).

2. Aug 5, 2010

### Born2bwire

There is always going to be some implied limit on the current. For example, there is only a finite amount of energy contained in the fields that can be extracted to generate the currents. One can consider the equivalent sources of the fields to be in the form of currents and charges and given that the total energy contained in the fields is finite then the equivalent energy from the sources must also be finite. This ignores the fact that any receiving antenna can only capture the fields incident upon their effective aperture which further reduces the amount of energy that can be extracted out of the fields.

3. Aug 6, 2010

### Bob S

You could use the equation for a (parallel or series) resistive LRC circuit, and use R as the resistance (load) of the external circuit. The circulating current for a series R in the loop is then I = Vout/R.

Bob S

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