![]() Some antennas (such as those accompanied by a parabolic dish) have significant amounts of gain, and thus they can make a nontrivial contribution to the range or performance of an RF system. As always, we need a ratio when we’re working with dB, and in the case of dBi, the antenna gain is given with reference to the gain of an isotropic antenna. The dBi unit allows antenna manufacturers to specify a “gain” figure that uses the ever-popular dB scale. Here you can see the unequal distribution of radiated energy that results in gain in the forward direction (i.e., 0 °). Other antennas, however, can be designed to concentrate radiated energy in certain directions, and in this sense an antenna can have “gain.” The antenna is not actually adding power to the signal, but it effectively increases the transmitted power by concentrating electromagnetic radiation according to the orientation of the communication system (obviously this is more practical when the antenna designer knows the spatial relationship between the transmitter and receiver). ![]() These “isotropic” antennas are considered to have zero gain and zero loss. For example, phase noise (discussed in page 2 of this chapter) is reported in units of dBc/Hz the first part of this unit indicates that the phase noise power at a specific frequency is being measured with respect to the power of the carrier (in this case “carrier” refers to the signal strength at the nominal frequency).Īn idealized point-source antenna receives a certain amount of energy from the transmitter circuit and radiates it equally in all directions. Instead of a fixed value such as 1 mW, dBc uses the strength of the carrier signal as the reference. Two other dB-based units are dBc and dBi. Nowadays most RF engineers are working with relatively low-power systems, and this probably explains why dBm is more common. There is also a dBW unit this uses 1 W for the reference value instead of 1 mW. This is a standard unit used in real-life RF system development, and it’s very convenient when, for example, you are calculating a link budget, because gains and losses expressed in dB can simply be added to or subtracted from the output power expressed in dBm. You definitely want to familiarize yourself with the concept of dBm. A straightforward example is amplifier gain: If the power of the input signal is 1 W and the power of the output signal is 5 W, we have a ratio of 5: In contrast, dB is a unit that involves the logarithm of a ratio between two numbers. Current is also an absolute measurement because the unit (amperes) involves a specific amount of charge with respect to a specific amount of time. Voltage is an absolute measurement because we always speak of a potential difference, i.e., the difference in potential between two points usually we are referring to the potential of one node with respect to a 0 V ground node. You cannot say, “The output power is 10 dB.” It is easy to forget that dB is a relative unit. Instead, we will focus on practical aspects of the decibel in the specific context of RF systems. We won’t cover the generic details of decibels because they are already available on this page of the AAC Electric Circuits textbook. It’s a logarithmic unit that provides a convenient way of referring to ratios, such as the ratio between the amplitudes of an input signal and an output signal. If you become deeply entrenched in an RF project, you may find that the word “dB” becomes as familiar to you as your own name.Īs you probably know, dB stands for decibel. One of the most important words that you will need when working in the world of RF is “dB” (and some variants thereof). ![]() RF engineering, like all scientific disciplines and subdisciplines, involves quite a bit of specialized terminology. Learn about the decibel and its variants in the context of RF design and testing. ![]()
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