Radio Frequency (RF) & Wireless Design

One of the things that the article does not discuss is that when adding coils to an antenna to achieve the same electrical length (in wavelengths - typically 1/4 wavelength), but shorten the actual physical length, there is added coil-to-coil inter-winding parasitic capacitance (also know as self capacitance) that will shift and de-tune the center operating frequency. This self resonance of the coil will have its own operating resonance frequency.

The added capacitance works against the inductance in trading trading off the electric field for the magnetic field which is associated with the inductance.

This will ultimately affect the NEAR-FIELD radiation pattern and may reduce the magnetic field, or at minimum distort the magnetic field distribution which is typically uniform across a single wire monopole.

Therefore radiation efficiency is also compromised in magnetic field distribution along the antenna, which may also contribute to poor antenna radiation efficiency.