All About Wireless: Wave Propagation
Welcome to the second installment about wireless. In this issue, we will focus on Electromagnetic wave propagation, covering the concepts of wavelength, reflection, diffraction, absorption and polarization.
Unlike acoustic waves, electromagnetic waves do not require a physical medium such as air in which to propagate. In fact, they propagate most efficiently through the vacuum of space. Propagation velocity in space is the speed of light, or 3 x 10^8 meters per second. This velocity value is important to remember in wireless microphone and IEM engineering applications because we can use this to derive wavelength at any given frequency.
Wave Equation
According to the wave equation, which states that wavelength is equal to velocity divided by frequency, one wavelength at 500MHz equates to 0.6m. Similarly, one wavelength at 600MHz equates to 0.5m. You will notice that that higher the frequency, the shorter the wavelength, and this point is significant when we come to consider the effect of obstacles in the propagation path.
As Electromagnetic waves are propagated away from the transmitting antenna, they are influenced by the physical environment through which they travel. In a similar fashion to acoustic waves, Electromagnetic waves are subject to free space propagation losses at a rate determined by inverse-square law. The inverse-square law states that for a perfect omnidirectional point source, the signal power present twice the distance from the source will have decreased by a factor of four. This is the major factor limiting the range of a transmitter in ideal 'line of sight' conditions.
Electromagnetic waves may also be affected by the size and composition of obstacles in their path. They are particularly sensitive to the presence of metallic obstacles, as a reflection of the wave will occur if the dimensions of the obstacle are longer than the wavelength of the signal. The angle of reflection is equal to the angle of incidence, and the phase of the reflected wave is shifted by 180 degrees.
Possible Obstacles for Electromagnetic Waves
Reflecting obstacles will usually cause an RF shadow behind them, which can create 'dead-zones' in the coverage area. Interestingly, if a reflecting metal object is porous, and the pore dimensions are smaller than the signal wavelength, the surface will act as if it were solid. Screens, grids, bars and other metallic arrays can therefore reflect Electromagnetic waves even though there may be a clear line of sight through the obstacle itself.
If the obstacle pore dimensions are larger than the signal wavelength, the electromagnetic waves will pass through the array unaffected. This is a key reason why it's important to have an understanding of the approximate wavelength of the frequencies you are using – this knowledge will help you to assess the RF environment, identify potential points of signal reflection, and minimize the creation of 'dead-zones' in the intended coverage area.
Non-metallic substances do not reflect electromagnetic waves, but tend to absorb a portion of their energy as the wave passes through them. The amount of signal attenuation is a function of both the thickness and composition of the material, as well as signal wavelength. Dense materials tend to absorb more energy, and high frequencies tend to suffer greater signal absorption. In a similar fashion to reflective metallic obstacles, very dense absorptive materials can attenuate signals severely enough to create an RF shadow that degrades reception of the signal in the area beyond the obstacle.
The human body, for example, made up primarily of salt water, is an effective absorber of RF energy. Positioning a human body between the transmit and receive antennas can cause degradation in RF performance due to absorption of the RF signal.
This graph depicts the difference in RF signal strength measured in a 360-degree range around a human body at two different frequencies transmitted by a belt pack transmitter. Transmitted signal strength is reasonably consistent from 100 to 260 degrees as the signal propagating in this direction does not pass through the human body. Transmitted signal strength is far less consistent from 260 to 100 degrees due to the influence of the human body. There is a general attenuation of approximately 6dB caused by the body's absorption of the signal.
Additionally, strong nulls exist at approximately 355 and 40 degrees in this test. It is important to note that, in practice, the beamwidth and total attenuation of these nulls would vary depending on the person, and where the belt pack transmitter is positioned. Interestingly, the more body fat a person has, the more RF energy is absorbed.
The strength of an Electromagnetic wave arriving at a given location is therefore equal to the strength of the original source transmission minus the total amount of attenuation caused by propagation distance and environmental influences. Because propagation losses vary wildly as transmitters are moved around a space, dynamic range of the signal can be as much as 50dB. It's therefore crucial to ensure the signal detected by the receive antenna is as strong as possible. This is why it's also important to consider the Polarization of the Electromagnetic wave.
Polarization of the Electromagnetic Wave
Polarization refers to the electric field orientation of the Electromagnetic wave. A vertically polarized Electromagnetic wave is produced when the transmitting antenna elements are positioned perpendicular to the Earth. If the antenna elements are positioned parallel to the Earth, the Electromagnetic wave will be horizontally polarized. The received signal will be strongest when the polarization of the transmit and receive antennas matched.
In practice, if the polarization of the transmit and receive antennas is orthogonal, or offset by 90-degrees, the voltage induced in the receive antenna may be attenuated by as much as 20dB. Considering this polarization loss is in addition to propagation losses, and the fact that antenna polarization mismatch is easily avoidable, the importance of matching the physical orientation of transmit and receive antennas is clear. We will discuss antenna positioning best practices in a later issue.
Next month we will look at the design characteristics and performance features of the various antennas most commonly utilized in wireless microphone and IEM applications.
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