At extremely low frequencies (ELF), which typically range from 3 Hz to 3 kHz, the dimensions of transmitting and receiving antennas can be quite large due to the long wavelengths involved. The size of an antenna is ideally a significant fraction of the wavelength it is designed to receive or transmit.
For ELF signals, which can have wavelengths ranging from hundreds to thousands of kilometers, practical antenna dimensions become challenging. To overcome this, researchers have developed alternative antenna designs that are more compact yet efficient for receiving ELF signals. One such design is the "small coil antenna," also known as a "ferrite rod antenna" or a "loop antenna."
A small coil antenna is typically a loop or coil of wire wound around a ferrite rod or a similar magnetic material. It functions based on the principle of magnetic induction. When an ELF electromagnetic wave passes through the loop, the changing magnetic field induces a current in the coil. This current can be extracted and processed as a received signal.
The small coil antenna's effectiveness despite the signal's large wavelength can be attributed to the following factors:
Magnetic Field Capture: The loop of wire in the antenna captures the changing magnetic field associated with the ELF signal. The larger the loop area, the more magnetic field lines it can intersect, enhancing the signal pickup.
Resonance: The coil's inductance and capacitance properties can be tuned to match the desired ELF frequency range, maximizing the antenna's sensitivity. By adjusting the coil's dimensions and the number of windings, resonance can be achieved despite the large wavelength.
Amplification: The received signal from the small coil antenna is typically weak due to the low power levels of ELF signals. The signal is subsequently amplified by electronic circuits to make it usable for further processing and analysis.
It's important to note that while small coil antennas are effective for receiving ELF signals, they are not as efficient for transmitting at these frequencies due to the limited area of the loop. For ELF transmission, larger antennas, such as grounded wire antennas or large vertical masts, are typically used to achieve better efficiency and radiation patterns.