An abbreviated version of an article by Lloyd Butler VK5BR

The Basic Concept

A major problem in receiving signals, especially at VLF and LF, is the high level of local noise generated by nearby power lines and electrical equipment. Received signal-to-noise can be improved by using a shielded loop antenna.

                         

•  Electric component of the induction (near) field decreases with the cube of the distance .
•  Magnetic component of the induction (near) field decreases with the square of the distance
.

•  Electric AND magnetic components of the radiation (far) field decrease directly with distance.

Comparison of Electric and Magnetic components in received (near) field

Reception of localized noise takes place in the induction or near field region. A shielded loop antenna is essentially sensitive only to the magnetic component of the near field. But because this is lower than the electric component in, the level of interference is reduced. Further, if the source of interference arrives from a different direction than the signal, the noise is further reduced. The loop has a very sharp null at right angles its plane and can therefore be rotated to null the noise.

The received voltage is directly proportional both to the loop area and the number of turns. The larger these are, the higher the induced voltage. However, increasing these also increases the series inductance and shunt capacitance. Depending on the frequency, these can have a profound effect on the voltage delivered to the load. Wire resistance is also in series with the load, but its value is normally low in comparison.

Resonance

Frequencies lower than the loop's natural resonance can easily be achieved by adding shunt capacitance in parallel with the loop. Higher frequencies are achieved by decreasing the number of turns or the diameter on the loop.

Loop Interface

The dynamic impedance of the tuned loop is high. Therefore, it is best connected to the high impedance (inverting) input of an operational amplifier or an FET amplifier.

Unshielded Loop Antenna

If localized noise is not a problem, sensitivity may be improved by using an un-shielded loop. This reduces loop self capacitance and the number of turns for a given upper frequency.

Conductor Size

The loop sensitivity at resonance is directly proportional to its Q factor, which is the ratio of its inductance to series resistance. The resistance is the sum of radiation resistance and the AC loss resistance in the loop. The latter is dominant in that the value of radiation resistance is very small due to the small loop size. Wire loss, however, can be reduced by increasing the diameter of the wire.