What is the Difference Between a Trap, Loading Coil and Phasing Coil in an Antenna?

 My single band VHF only CP22E antenna has a phasing coil between the upper and lower elements. CP22E is a two-element 5/8λ stacked ground plane antenna with three radials. Upper and lower sections are assembled with the phase coil in between. Coil ensures that the phase of the waveform is same all along the antenna length so that both sections work together. This increases the efficiency of the antenna. Without a phasing coil an antenna with double the length will not work on the design frequency. As a simple example, 20m long center fed dipole antenna meant for 7MHz will not work for 14MHz, being an even harmonic, though it will work for 21MHz being an odd harmonic. In the same way, my CP22E VHF antenna works fairly well on the UHF amateur radio band as it is roughly the third harmonic.

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 a resistive component in addition to the inductance, 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. An antenna which is shorter than the required length for a given frequency is supposed to be capacitive and the loading coil cancels out this capacitive reactance with an equal and opposite inductive reactance.

Traps on the other hand are tuned circuits will prevent transmission of higher frequency beyond them. For example if you have an antenna for 40m, and you keep a tuned circuit in between for 20m, the 14MHz signal will not go beyond the trap, while 7MHz signal will travel along the full length of the antenna. So the full length of the antenna will work for 40m while only the initial part works for 20m. In effect same antenna can work for both the 40m band and 20m band. Just like in case of loading coils, the current within the trap with tuned circuit can cause loss of power because of the resistance of the coil which converts part of the RF energy into heat energy. Coils can have a capacitance because of the conductors being parallel to each other. If that capacitance is matched to the inductance, it is possible to have traps without adding an additional capacitor in parallel to produce a tuned circuit, depending of the frequency and design of the coil. In short, a trap functions as an open circuit at the higher frequency and a closed circuit at lower frequency. So the initial part of the antenna works for the higher band and full antenna functions for the lower band. Coils can function as traps only at their self resonant frequency.