Plain-language answer: A horizontal loop places most or all of a closed radiator parallel to the ground.
Why it matters in the antenna
At low electrical height it can favor high-angle regional coverage; at greater height and on harmonics it develops multiple lower-angle lobes and nulls.
What to verify
State perimeter, shape, height in wavelengths, feedpoint, band, and ground before using a pattern.
The common mistake
Calling a horizontal loop uniformly omnidirectional on all bands.
Operating boundary
Use the exact documented component, geometry, frequency, power, duty cycle, clearances, and feed/return system. Begin tests at low power and stop for heating, arcing, damaged insulation, or unstable supports.
