Small loop antennas (circumference, C, <0.085 ?) are often used for special applications, such as transmitter locating. These antennas respond differently to the incoming EM wave and have a peak response aligned with the plane of the loop, and a null perpendicular to the loop.
The more common form of loop antenna in amateur use is approximately 1 wavelength in circumference. These antennas have patterns similar to those of dipoles, and have the advantage of a DC short, providing an advantage in areas where static pickup is a problem. They also provide a large capture area, which has led to the conclusion that they should have more gain than their dipole cousins, but this is often offset by the fact that the average height of the antenna ends up being lower. The structure doesnt have to be the classic circular shape, but can be a polygon instead, as indicated in the examples above. Regular polygons are preferred to keep the fields on opposing wires as far apart from each other as possible, as this tends to reduce the efficiency of the antenna. Loops may be fed in the center of a horizontal or vertical side (generating horizontal and vertical polarizations respectively) or fed at a corner or apex. They may be paired together to form a directive antenna (see the next section) and this popular form is known as the Quad.
Loops may even be mounted horizontally. At typical residential heights (25-40 ft) this usually leads to antenna patterns that have a large lobe directly up. This is useful for short-range contacts using a propagation mode known as near vertical incidence, where the signal is reflected off the ionosphere. Such antennas also work well at harmonic frequencies, with multiple azimuth lobes and elevation lobes that are much lower down, and better suited to long haul contacts. This makes an 80 meter horizontal loop a tempting antenna
currents approximately ¼ wavelength away tend to make the
elements appear electrically shorter, much like in the case with
the inverted vee. Because of this effect, the rule of thumb for
the circumference of a loop is:
C (in feet) = 1005/frequency in MHz
In addition, the feedpoint impedance of the typical loop is approximately 100 W making the match to the common 50 W or 75 W coax cables a little problematic. For a single band loop, this is not a big problem as a 1/4 l (electrical) length of 75 W cable attached to the loop will make a good match to a 50 W feed system.