Added on: July 30, , in: RF Engineering Articles There is a great deal of confusion when engineers, technicians, and equipment salespersons talk about units of antenna gain and field strength. This FAQ discusses units of gain and field intensity and explains how to convert between some of these units when appropriate. Units of Antenna Gain While field strength at any location is independent of antenna gain, received voltage at the receiver is not. Therefore, let us first consider antenna gain.
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Added on: July 30, , in: RF Engineering Articles There is a great deal of confusion when engineers, technicians, and equipment salespersons talk about units of antenna gain and field strength. This FAQ discusses units of gain and field intensity and explains how to convert between some of these units when appropriate. Units of Antenna Gain While field strength at any location is independent of antenna gain, received voltage at the receiver is not.
Therefore, let us first consider antenna gain. Gain may be expressed as either a power multiplier or in dB. Antenna gain stated in dB is referenced to either isotropic or a half-wave dipole. The microwave industry has universally established the convention of reporting antenna gain in dBi referenced to isotropic. The land mobile industry has almost universally expressed antenna gain as dBd referenced to a half-wave dipole rather than isotropic.
When a manufacturer lists a gain as dB, you may generally assume that the referenced gain is dBd. Broadcast antenna manufacturers commonly refer to a multiplier gain where the antenna input power is multiplied by this gain to yield the effective radiated power. The simplest antenna is an isotropic radiator. This is a theoretical antenna that radiates the same level of energy in all directions when power is applied to the antenna.
Even though this type of an antenna cannot actually be constructed, the use of the concept provides a uniform standard against which the performance of all manufactured antennas can be calibrated and compared. Figure 1 Half wave dipole vs isotropic antenna An antenna which can be easily built is a half-wavelength dipole. A half-wavelength dipole antenna has a gain of 2. The dipole concentrates the energy in certain directions, so that the radiation in those directions is greater than the radiation from an isotropic source with the same input power.
Therefore, the gain of an antenna referenced to an isotropic radiator is the gain referenced to a half-wavelength dipole plus 2. The energy radiated in the desired direction s is increased by reducing the energy radiated in some other direction s. For example, a collinear array of four dipole antennas will typically have a gain of 6 dBd.
This same antenna will have a gain of 8. Figure 2 Gain in dBd vs dBi Directional antenna patterns are sometimes plotted as gain in dB above a half-wave dipole. Other patterns are shown as a relative field voltage. These are directly transferable as long as one knows the absolute gain in dBd or dBi of the major lobe of the antenna.
P is the effective radiated power in the major lobe max in the horizontal plane in W, kW, etc. Units of Field Intensity There is also a great deal of confusion in the vocabulary for field strength also called field intensity. Each unit has both merit and common usage in certain disciplines in the radio communications industry. However, the widespread confusion about how they relate to one another causes both frustration and misunderstandings about system design and actual performance.
The following terms will be discussed at length. Electric field intensity is independent of frequency, receiving antenna gain, receiving antenna impedance and receiving transmission line loss. Therefore, this measure can be used as an absolute measure for describing service areas and comparing different transmitting facilities independent of the many variables introduced by different receiver configurations.
When a path has unobstructed line of sight and no obstructions fall within 0. Correlation between electric field strength and voltage applied to the receiver input is impossible unless all of the above listed information is known and considered in the system design. When the exact same conditions path, frequency, effective radiated power, etc. There is also a fixed loss in the receiving antenna transmission line — often assumed to be lossless.
To determine the level of field strength necessary to adequately receive a transmitted signal, use Equation 6 above, taking into account the frequency, receiving antenna gain and required level of receiver voltage for the desired level of quieting in the receiver. These predictions are for the voltage at the antenna terminals. Actual voltage and power levels at the receiver input must take into account the additional loss present in the receiving transmission line. This loss in signal is particularly critical at high frequencies when cables are long.
Figure 3 Electric Field and received voltage and power Figure 3 summarizes the relationship between electric field strength and the voltage and power at the receiver input terminals.
The electric field strength in dBu is a function only of: Transmitter effective radiated power. Distance from the transmitter. Losses from terrain obstructions. Since the electric field strength is independent of any receiver characteristics, it is a useful standard for computing coverage areas. The electric field induces a voltage into the antenna, transferring power into the antenna.
The power dBm available at the antenna terminals is also a function of the antenna impedance usually 50 Ohms. The transmission line usually coaxial cable or waveguide connects the antenna terminals to the receiver input terminals. The voltage and power at the receiver input terminals are reduced by the loss in this transmission line.
Transmission line losses are a function of the size and type of the transmission line and the operating frequency. In addition, other losses affect the power transferred to the receiver input terminals. Conclusion The obvious conclusion from this information is that receiving systems with different antenna gains require significantly different electric field strength values for proper operation.
Based on the actual equipment proposed and the above equations, the system designer can now calculate the actual field strength necessary for any particular receiving system.
Operating the receivers in areas where the field strength meets or exceeds the design level for the equipment can be expected to produce satisfactory system performance. Kraus, Ph. Ellis, P.
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