Antenna loading
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 in 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.
Design calculators
An antenna with a loading coil cannot be modelled electrically as an element in which the inductor has been replaced by the corresponding length of straight wire.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.
There are a number of online calculators which can determine a suitable value and position for inductive loading coils based on an antenna's dimensions and desired resonant frequency.
Verticals:
Dipoles:
- Horizontal Indoor Antenna #2, VE3KL
- Antenna Loading Coil Calculation, OA4/KA2OIG
Various antenna calculation programmes are also available for download.
External links
- Inductors and Loading Coil Current (Mobile and Loaded Antennas), W8JI
- Frank's N4SPP Ham Radio home-built mini yagi beam dipole antenna
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