Loop Antenna Theory
Loop antennas can be analyzed as several separate types, with very different characteristics:
- small receiving loops, less than about 0.1 wavelengths, where the current is approximately constant around the loop. These have a broadside null and maximum radiation off the edges.
- small transmitting loops, where efficiency is an important factor.
- full wave loops, (generally between 2/3 and 3/2 wavelengths) which have maximum radiation broadside to the loop. This is probably the most common loop for ham radio use.
- larger loops, 2 wavelengths or more, which have maximum radiation off the sides of the loop, and may have a broadside null. These may simply be a full wave loop on one band used on other higher bands, which can make a very effective multiband antenna when installed horizontally.
We can also look at some of the intermediate cases, and some antennas that look like loops but technically aren’t:
- intermediate loops, around 1/4 wavelength or so
- half wavelength loops, an early directional antenna
- bisquare antenna, 2 wavelengths around but open at the top.