What is common between the radio, TV, radio controlled cars, and cell phones? They all use invisible waves to transmit information. These waves, called radio waves, are a type of electromagnetic radiation. Radio waves are not harmful; in fact they are an extremely useful method of global communication. Being a simple form of electromagnetic radiation, they move in waves from place to place with limited change. However, the blocking of radio waves happens in the form of either deliberate or unintentional events. For instance, a military may opt to block radio waves of an enemy. Other situations result in the natural interruption of a broadcast due to a myriad of reasons. The challenging part with this form of communication is the fact that different naturally-occurring phenomena like mountains, along with certain materials such as copper and aluminum, can create a situation in which the radio waves might blocked.
Radio waves are electromagnetic waves and travel at the speed of light which is 186, 280 miles per second (983,558,400 feet per second). The voltage in a radio wave alternates back and forth between plus and minus many times per second and we call this the frequency of the radio wave in cycles per second. A radio frequency of 100 megahertz (100 MHz) means 100 million (100,000,000) cycles per second. This is usually marked 100 on the middle of FM (frequency modulation) radio dials.
Two essential components for global communication are a transmitter and a receiver.
The ability of a wave to travel through a material is called transmittance, and materials can be alienated into good transmitters and poor transmitters.
There are two general types of matter (substances) in the universe that affect electromagnetic waves, conductors and insulators which are called “dielectrics”. Majority of them except some conductors are metals, such as copper, aluminum, silver and gold. Nevertheless, salt water is also a rather poor conductor. Pure water is a good dielectric substance. Most, but not all, dielectrics are non metals. Examples of dielectrics are paper, plastic, Teflon, glass, ceramic and dry wood. As the radio wave travels through the dielectric material, some of the power is absorbed generating heat and some of the power travels through and comes out of the other side.
The attenuation coefficient is the level by which a material will block or interfere with radio waves. This coefficient depends heavily on the thickness and composition of the material. Cardboard, paper, many plastics, water, and glass are all substances with very low attenuation coefficients. In addition, wood, brick, and cement have a limited effect on making radio waves blocked. However, metallic compounds, steel-reinforced concrete and the Earth reflect signals, preventing radio signals from passing through.
One of the major components of figuring out whether radio waves will get blocked or find their way onto the other side of an obstacle, involves the notion of diffraction. This depends on the wavelength of the radiation and the size of the obstacle that it is attempting to cross. Low frequencies (or large wavelengths) have an easier time passing over large objects such as hills, while higher frequencies work better with small obstacles such as rooftops. This can be very useful in making a radio wave blocked using the knife-edge diffraction method. That is, if a wave does not have a line of sight over an object, a sharp edge can be placed in the path, which causes the radio wave to be diffracted and redirected to where the broadcast needs to go.
Radio waves can be blocked (reflected) or attenuated (partially reflected or absorbed) by anything that conducts electricity. A thin sheet of aluminum can completely block (reflect) radio waves. They simply cannot penetrate it. Salt water (as in the ocean) permits radio waves to penetrate only a very short distance. In the same way, living plant material contains water and dissolved ions (the plant's "blood" or the plants food). This makes the plant something of a "conductor" of electricity. When radio waves (electromagnetic energy) come across a conductor, the frequency of the radio waves, their amplitude, and the "nature" of the conductor will result in a varying degree of interaction. The radio waves induce micro currents within the conductive material (the foil, the salt water, the plant with its moisture and ions, whatever). This interaction reflects or absorbs the energy (to a greater or lesser degree) of the incoming radio wave. The micro currents do not destroy plant tissues because the power contained in the radio wave is very low (micro volts per meter, and usually less). Positively, if a plant is "right on top" of an electromagnetic source, it will take some heat from the radiated energy. However, as to the effect of "regular" radio waves, the damage is not obvious, and the signal will be damped or attenuated by interaction with the plant.