Full size antennas occupy a lot of area making it difficult to keep them within a null region. Recently, I was plagued by a touch control lamp in my neighbor's house which radiated strong, wideband buzzing harmonics across the HF bands. Not knowing immediately what was causing the problem, I explored my house with a portable SW radio in search of the offending signal. 1 discovered a location in the attic above my garage where the interference could not be heard. The direct signal from the lamp and the signal radiated from the power wiring combined at this one spot and canceled each other out An active antenna at this spot would be immune to the interference.
Active antennas have at least two problems.
One is susceptibility to overload and inrermodulalion. The World Radio and TV Handbook did a review of active antennas a couple of years ago where they evaluated the various brands for overload performance. The other disadvantage, which is common to all physically small antennas, is that small antennas are more susceptible to fading than large antennas. HF waves propagated via the ionosphere can simultaneously arrive at the receiving antenna after having traveled different distances. When this happens, the waves can add to improve reception if the waves are in phase.