Posted: 11/15/2012 5:21:15 PM EDT
I know they relate to antennas, but I'm not sure how. Please expain. 
This should be a quick kill for someone. |
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Decibel's can be used as relative measurements where with antennas the gain is specified with respect to a dipole (dBd) or an isotropic radiator (dBi). dB can be used to express a power measurement when the impedance is known. (dBm) As a rule of thumb, a change of 3 dB may double or halve the power of a signal. A change of 10 dB is 10 times or 1/10th, 20 dB is 100 times and 30 dB is 1000 times. If these are reductions in power, 1/100 and 1/1000th respectively. 73, Rob |
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The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibels (dB)
http://en.wikipedia.org/wiki/S_meter |
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Quoted:
I know they relate to antennas, but I'm not sure how. Please expain.
This should be a quick kill for someone. killed: 
http://www.ar15.com/forums/t_10_22/646491_the_CAN_T_FAIL_thread_for_getting_a_ham_radio_license_and_learning_basic_VHF_UHF_operations.html&page=2#i11023677 ar-jedi |
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Quoted:
I know they relate to antennas, but I'm not sure how. Please expain.
This should be a quick kill for someone. Decibels do not just relate to antennas, but instead relate to many things in the engineering world, particularly in the radio and electronics areas. Decibels (dB) provide a convenient way to measure things that exist over very wide ranges. When used by itself, a dB is unit-less. So, for example, one antenna may be claimed to have 0dB gain, another 3dB gain. Using the 0dB gain antenna you might receive a signal with a strength of, for example, 1 billionth of a Milliwatt (a billionth of a thousandth of a Watt, otherwise known as 1 Picowatt). Yes, this is very typical value. However it is a lot easier to work with a number like -90dBm, i.e. -90dB referenced to 1 Milliwatt. The antenna with 3dB gain would make that signal -87dBm at the receiver input. Similarly, units of RF loss are measured in dB. One type of coax might have 2dB loss in a 100ft, another 4dB of loss. Again, this is dimensionless. Just like with antennas it needs to be dimensionless because the amount of loss (or gain) depends on the power level applied. Consider a 50W radio.Converting to dB this is 17dBW. At the other end of the coaxial cable you might have 15 or 13dBW remaining to go into the antenna. Converting back the other way one gets 32W or 20W respectively. So you can see how a very few dB can make a big difference. It may all seem like a huge pain in the butt, but once you start doing these calculations you'll realize that making all of the calculations in Decibel form is much, much easier in the radio world. To go from linear units to decibel units: dBx = 10log(x) where log is the base-10 logarithm (not to be confused with the natural log "ln") To go from decibels to linear units: x = 10^(dBx/10) |