A small loop antenna with a capacitor for adjusting the resonant frequency.
Any receiving or transmitting antenna, consisting of one or more turns of wire forming a dc short circuit, comprises a loop antenna.
A small loop antenna has a circumference of less than 0.1λ (for each turn) and is suitable for receiving, but generally not for transmitting. It is the least responsive to signals coming from along its axis, and most responsive in the plane perpendicular to its axis. A capacitor can be connected in series or parallel with the loop to provide a resonant response. An example of such an arrangement is shown in above figure. Small loops are useful for radio direction finding (RDF), and also for reducing interference caused by human-made noise or strong local signals. The null along the axis is sharp and deep, and can be pointed in the direction of an offending signal or noise source.
For receiving applications at frequencies up to approximately 20 MHz, a loopstick antenna is sometimes used. This antenna, a variant of the small loop, consists of a coil wound on a solenoidal (rodshaped), powdered-iron core. A series or parallel capacitor, in conjunction with the coil, forms a tuned circuit. A loopstick displays directional characteristics similar to those of the small loop antenna shown in Fig. 27-4. The sensitivity is maximum off the sides of the coil, and a sharp null occurs off the ends.
A large loop antenna usually has a circumference of either λ/2 or λ (a full wavelength), is circular or square in shape, and lies entirely in a single plane. It can work well for transmitting or receiving. The λ/2 loop presents a high impedance at the feed point, and maximum radiation/response occurs in the plane of the loop. The λ loop presents an impedance of about 100 Ω at the feed point, and the maximum radiation/response occurs along the axis (that is, perpendicular to the plane containing the loop).
The λ/2 loop exhibits a slight power loss relative to a λ/2 dipole in its favored directions. The λ loop shows a slight gain over a λ/2 dipole in its favored directions. These properties hold for loops up to several percent larger or smaller than exact λ/2 or λ circumferences (as determined for the wavelength in free space). Resonance can be obtained by means of an antenna tuner if the loop is fed with open-wire transmission line.
Sometimes, loop antennas measuring several wavelengths in circumference are strung up horizontally among multiple supports. These are technically large loops, but their gain and directional characteristics are hard to predict. If fed with open-wire line and an antenna tuner, and if placed at least λ/4 above the surface, such an antenna can be exceptionally effective.