To give a cemented idea on the conversions, a table of linear to decibel levels is presented.Īnother nice feature of decibels is they make multiplication problems become additions. Hence, the above quoted value of 1.149 -16 Watts is equal to -159.4 decibels, written -159.4 dB. To work around this, we use the decibel system.īy definition a decibel is a simple logarithmic transformation - units in linear are easily converted via: Its real tough to picture in your head the difference between 10 17 and 10 18. Incidentally, this might seem frighteningly small, but yes, antenna systems can operatte with this less power.Įngineers don't like to use linear units when the quantities can vary by such large amounts. Of the 50 Watts transmitted, about 2.3 -16 % of that power gets through. Using the Friis Transmission Equation and the fact that wavelength equals c/f, we can calculate the received power to be: The GPS satellites operate at a frequency of 1.575 GHz. Suppose the antennas are separated by 20,000 kilometers (as is the typical orbit of a gps satellite). Suppose we have a GPS satellite transmitting 50 Watts of power, with a gain of 2 in the direction of a receive antenna with a gain of 2. The Richter scale for earthquakes is also on a scale similar to decibels, because the magnitude of earthquakes can vary from barely perceptible with sophisticated equipment to earthquakes that demolish vast amounts of land.Īs an example, lets look at power received by an antenna relative to power transmitted by a tower or Satelitte. The decibel system is used when quantities can vary by massive amounts. Why is this? Why don't they just tell you how much power is transmitted in Watts or the gain of an antenna in regular (linear) units? Or the transmit power of your cell phone is given as -3 dB. Or maybe the minimum received power for an antenna system to work is specified as -70 dBm (decibels relative to a milliWatt). , are measured in decibels for instance, gain or loss are often specified in decibels, written as dB. Many parameters related to antennas, modems. An important thing to note about logarithms is that the logarithm of a negative number or of zero does not exist. Notice that our logarithm, 3, is the exponent. If we are given the number 1000 and asked to find the logarithm (log), we find that log 1000 = 3 because 10^3 = 1000. Little knowledge on logarithmic calculations and scales.Ī logarithm is the exponent to which the number 10 must be raised to reach some given value. This article applies for all devices and accessories dealing with very low power signals (antennas, Wifi, 4G.
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