What are Loaded Coil Dipole Antennas?

What are Loaded Coil Dipole Antennas?

Dipole antennas can be made physically shorter than the usual half-wave dipole antenna by adding loading coils. It is useful for single band operation with narrow bandwidth. Of course it is a trade off between performance and fitting a dipole into a shorter available space. The loading coils have resistive and reactive components of impedance which can cause loss of RF power in the form of heat. Though this may not be critical while transmitting, loss of received signal power may mean that weak signals may be totally lost.

For better efficiency, the loading coil should be nearer the far ends of the shortened dipole, but that would need a larger inductance which may be technically difficult to implement. Thinner wires for the loading coil would mean more resistive loss of signal. Coils wound on plastic formers are inferior to those wound on glass or ceramic in terms of loss of RF power. An ideal loading coil would be an air wound one without any former, which is practically difficult to implement.

Physically shorter antennas with respect to the wavelength will need larger inductance for the coils. Inductance required depends on the total length of the antenna, distance from the center to the loading coil, diameter of the radiating wire elements and the required operating frequency.  Ideal coil is one which is made of solid copper and having a length equal to its diameter.

Online calculator is available at: https://www.66pacific.com/calculators/coil-shortened-dipole-antenna-calculator.aspx