Ali, a 25 year old male from Pakistan asks on May 27, 2008,In microwave transmission sometimes we use power in positive e.g +13db and sometimes in negative like -13 db. What is the difference? How can we choose the right power? Please inform.
viewed 13336 times
When we describe power in dB (decibels), it is relative to some reference level, like 1 Watt (W) or 1 milliWatt (mW). If the power in dBW is positive, it means it is greater than 1 W. If the power in dBW is negative, it means it is less than 1 W. And if the power in dBW is 0, it means the power is 1 W. Similarly for dBmW: if the power in dBm is positive, the power is more than 1 mW.
It's a bit like per cent in that respect. For example, 1.1 W is 10% up from 1 W, and 0.9 W is down 10% (i.e. it's -10%) from 1 W. However, decibels are not the same as per cent!
decibels are actually based on logarithms. The dB measure of power is 10 log (P/Pref), where P is the power and Pref is the reference power level. Suppose 1 W is the reference, and you have a 10 W signal. In dBW, that is 10 log(10/1) = 10 dBW. On the other hand, a signal of 0.1 W is 10 log(0.1/1) = -10 dBW.
A common rule of thumb is that doubling the power of a signal adds 3 dB, and that halving the power subtracts 3 dB. To see this, consider a signal of power P Watts, or 10 log(P) in dBW. Now double the power to 2P. In dBW, it is now 10 log(2P) = 10 log(2) + 10 log(P) = 3 + 10 log(P). This is approximate, but close.
Let's look at the specific values you mentioned. A power of 13 dB is 20 times the power of the reference level, so it's 20 W if you are using dBW, or 20 mW if you are using dBmW. Similarly, a power of -13 dB is 1/20 Watt (0.5 Watt) or 1/20 mW, depending on whether it's dBW or dBm.
Finally, an example. Some wireless personal area networks under development have restrictions on transmitter power. In any 3 kHz bandwidth, they are limited to +8 dBm. That's more than 1 mW and less than 10 mW. Taking the antilog, we see that the limitation is about 6.3 mW in any 3 kHz.
Note: All submissions are moderated prior to posting.
If you found this answer useful, please consider making a small donation to science.ca.