What is small loop antenna?
Small loop antenna defined.
Large loop antennas are those with overall wire lengths of 0.5λ to more than 2λ. Small loop antennas, on the other hand, have an overall wire length that is much less than one wavelength (1λ).
According to a Second World War US Navy training manual such antennas are those with an overall length of ≤0.22λ. Jasik’s classic 1961 text on radio antennas uses the figure ≤0.17λ, while John Kraus (1950) used the figure ≤0.10λ.
An amateur radio source, The ARRL Antenna Book, recommends ≤0.085λ for small loop antennas. For the purposes of our discussion we will use Kraus’s figure of ≤0.10λ.
A defining characteristic of small loops versus large loops is seen in the current distribution. In the small loop antenna the current flowing in the loop is uniform in all portions of the loop. In the large loop, however, the current varies along the length of the conductor, i.e. there are current nodes and antinodes.
The small loop antenna also differs from the large loop in the manner of its response to the radio signal. A radio signal is a transverse electromagnetic (TEM) wave, in which magnetic and electrical fields alternate with each other along the direction of travel.
The large loop, like most large wire antennas, respond primarily to the electrical field component of the TEM, while small loops respond mostly to the magnetic field component. The importance of this fact is that it means the small loop antenna is less sensitive to local electromagnetic interference
sources such as power lines and appliances.
Local EMI consists largely of electrical fields, while radio signals have both magnetic and electrical fields. With proper shielding, the electrical response can be reduced even further.
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