This phenomenon can be especially frustrating when maintaining good reception requires the listener or viewer to remain posed in an awkward position. It is not limited to hands but can involve the entire body as well as other objects located nearby. In fact, even an airplane flying overhead can influence the reception of radio and TV signals, giving rise to a sort of raspy interference. In general, the locations and properties of materials in the vicinity of an antenna influence its performance, a property which is readily noticed when those materials are in motion.
Antennas are made of electrical conductors and, although they come in a wide variety of configurations, they often take the form of straight metal wires or wire coils. Examples are the "rabbit ears" and wire loops found on the backs of television sets (at least before cable came along), which are antennas tuned for VHF and UHF operation, respectively. When a transmitter drives an oscillating current in an antenna electromagnetic radiation--which carries both power and signals away from the source--is emitted. Anything that can carry a current can act like an antenna, although engineers work to design antennas that are optimized for efficiency, directionality, and steerability, among other things. A glance at an urban skyline reveals the wide range of designs engineers have developed for different applications.
When an antenna picks up electromagnetic radiation emitted elsewhere, an electrical current is induced in it. This current can be tapped and amplified by the radio or television receiver, allowing power and information to be communicated over large distances without direct electrical connection. In addition, the current in the receiving antenna is itself a secondary source of radiation. Not all of the received power is transferred to the receiving apparatus and the remainder is mainly emitted back into space. If no receiving apparatus is connected to the antenna, then most of the power intercepted will be re-radiated, with a small portion dissipated as heat. This entire reception and radiation process is termed "scattering" and the details depend on the size and shape of the antenna and the wavelength of the radiation in a complicated manner that is beyond the scope of this discussion.
Currents are also induced in tissue (as well as any other conducting or dielectric materials) upon exposure to electromagnetic fields. Although hands and bodies are not particularly efficient radiators, they will scatter a fraction of the power incident on them, depending on the RF frequency involved. Back at the receiver, the scattered signal will compete with the original signal from the transmitter, combining either constructively or destructively depending on the relative distances to the two signal sources. At the same time, the proximity of the scatterer to the antenna can influence how efficiently it transfers power to the receiver. Thus reception can be either improved or degraded, depending on the placement of one's body.
The effect the reader asked about will be most noticeable when the hands are close to the receiver, although very good scatterers (such as aircraft) can exert an effect at great distances. It will be most obvious when the size of the scatterer is at least comparable to the wavelength of the signal. For example, hand movements may influence the reception of VHF and UHF signals, but they will do little to AM radio signals having wavelengths of hundreds of meters, for example. Finally, it is worth noting that all of the bodies near the antenna will scatter the radiation that was already scattered by the others. The coupling between all the bodies involved can quickly become very complicated, and if there are many bodies present, the influence of any one is unlikely to be very great.
Antenna engineers make use of this coupling effect by placing parasitic antenna elements in close proximity to the part of the antenna, known as the driven element, that is attached to the appliance. All but one of the dipole wire elements in a television aerial, for example, are parasitic. They are intended to scatter incident radiation in such a way that all the contributions add constructively at the site of the driven element when the antenna is pointed toward the transmitter. The design of such antennas can still be a largely trial-and-error affair, with technicians moving elements from place to place in the laboratory as you might move your own body in order to pull in a weak broadcast.
Answer originally published November 10, 2003.