Metal objects can reflect radio waves, creating new versions of the signal.Water in the air (humidity) can absorb RF energy.Obstacles such as walls, trees, and hills can cause significant signal loss.In the real world, however, the range also can be degraded by other factors: In free space (an ideal condition), the inverse square law is the only factor affecting range. The goal is to make the received power greater than the receiver sensitivity. Received power = transmit power + gains – losses This relationship can also be stated as a link budget, which is the accounting of all gains and losses of a system to measure the signal strength at the receiver: Losses include any filter or cable attenuation or known environmental conditions. Antenna gains are usually expressed in dBi referenced to an isotropic antenna. Gains include any gains resulting from directional transmit and/or receive antennas. Maximum path loss = transmit power – receiver sensitivity + gains – losses Since both output power and receiver sensitivity are stated in dBm, you can use simple addition and subtraction to calculate the maximum path loss that a system can incur: This means that every 6-dBm increase in output power doubles the possible distance that is achievable.īesides transmitter power, another factor affecting range is receiver sensitivity. Every time you double the distance, you receive only one-fourth the power. Radio waves follow an inverse square law for power density: the power density is proportional to the inverse square of the distance. The primary factor in path loss is the decrease in signal strength over distance of the radio waves themselves. Path loss is the reduction in power density that occurs as a radio wave propagates over a distance. You can convert between mW and dBm using the following formulas:įor example, a power of 2.5 mW in dBm is: 10 dBm (10 mW) is 10 times more powerful than 0 dBm (1 mW), and 20 dBm (100 mW) is 10 times more powerful than 10 dBm. For every increase of 3 dBm there is roughly twice the output power, and every increase of 10 dBm represents a tenfold increase in power. Since dBm is based on a logarithmic scale, it is an absolute power measurement. Decibel-milliwatt is the output power in decibels referenced to 1 mW. A decibel value of 0 is equivalent to a ratio of 1. A decibel is a logarithmic unit that is a ratio of the power of the system to some reference. RF power is most commonly expressed and measured in decibels with a milliwatt reference, or dBm. An empirical solution, however, may reveal real-world situations that calculations don’t address.īefore we compare the approaches, let’s define a few terms to understand a manufacturer’s numbers or relevant variables for range. Either approach is fine as long as you account for all variables. Suppliers usually determine range by deriving it empirically from real-world tests or by using a calculation. Have you ever purchased a wireless radio for an embedded project and discovered that you didn’t achieve the radio frequency (RF) range stated in the datasheet? Why is that? It’s probably due to differences between how the supplier measured the range and how you are using the radio. Why Actual Range May Not Equal Stated Range This article identifies the factors involved in calculating range and shows how to estimate range to ensure a reliable communications link. One of the key calculations in any wireless design is range, the maximum distance between transmitter and receiver for normal operation.
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