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Monday, June 20, 2011
Antenna Loading Coil
An antenna loading coil is an inductor placed in series with an antenna element in order to lower the antenna's resonant frequency.
A standard dipole antenna is resonant if constructed with a length of one-half wavelength. A vertical antenna (effectively half a dipole operating against a ground plane) is resonant at one-quarter wavelength. Resonance may also be observed at each of the odd multiples.
For most VHF and UHF antennas, a half or quarter wavelength is reasonable in size and can be readily accommodated in all but the smallest handheld transceiver antenna designs. The same is not true of HF antennas on the radioamateur bands or mobile antennas for 27MHz (11-metre) CB operation. Mobile antennas are inherently limited by the amount of available space, yet reducing an antenna's length increases its resonant frequency.
A loading coil may be used, on its own or in conjunction with a capacity hat, to tune the antenna to resonance at a lower frequency. One coil placed in the centre or at the base of a single-band vertical antenna is sufficient, but a single-band dipole will require two coils - one in each element of the dipole.
For multiband HF antennas, different loading coils (or adjustable coils) are needed for coverage of each band.
An antenna with a loading coil cannot be modelled electrically as an element in which the inductor has been replaced by the corresponding length of straight wire. An inductor will behave differently as current at one end of an inductor creates a magnetic field which immediately induces currents at the opposite end. An inductor also adds electrical resistance to an antenna, incurring a penalty in radiation efficiency compared to a full-size antenna.
An external antenna tuner is often used in conjunction with HF antennas to tune antennas to resonance within a band. In most cases, these are π or T networks in which a variable inductor and capacitors are used to tune for lowest standing-wave ratio as seen by the transmitter.
An antenna tuner must be rated for the full power of the transmitter and both manual and automatically-tuned versions of these units are commercially available. Some late-model transceivers may integrate this functionality, although for best results any tuner or loading coil needs to be at or near the antenna itself.