|Home||Reviews||Tools||Forums||FAQs||Find Service||ISP News||Maps||About|
how-to block ads
A dB is a RELATIVE measure of two different POWER levels. There's also dB relative to VOLTAGE levels, but I won't go into those, as we're mostly concerned with POWER levels in our discussions here. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, and so on. The formula for calculating gain or loss in dB is: 10log P1/P2. It's used for stating the gain or loss of one device (P1) IN RELATION to another (P2). Thus, I can say that an amplifier has “30 dB of gain”, or I have “6dB total feedline loss”. I CANNOT say, “My amp puts out 30 dB”, or “I have a 24dB antenna”, as you must state what you're referencing it to, which is where the subscript comes in. The dB by itself is not an absolute number, but a ratio.
For amplifiers, a common reference unit is the dBm, with 0dBm being equal to 1 milliwatt. Thus, an amp with an output of 30dBm puts out 1 Watt. How much gain it has is a different matter entirely, and you can have two different amps, each with an output of 30dBm (1Watt), that have different gains, and require different levels of drive power to achieve their outputs. You can also have two different amps with the same gain that have different output powers.
There's also dBW (Referenced to 1 WATT), but you generally only use those when dealing with “Big Stuff”, as 30dBW is 1000w, and way beyond what we deal with here!
For antennas, a common reference unit is the dBi, which states the gain of an antenna as referenced to an ISOTROPIC source. An Isotropic source is the perfect omnidirectional radiator, a true “Point Source”, and does not exist in nature. It's useful for comparing antennas, as since it’s theoretical, it’s always the same. It's also 2.41 dB BIGGER than the next common unit of antenna gain, the dBd, and makes your antennas sound better in advertising. The dBd is the amount of gain an antenna has referenced to a DIPOLE antenna. A simple dipole antenna has a gain of 2.41dBi, and a gain of 0dBd, since we're comparing it to itself. If I say I have a “24dB antenna”, it means nothing, as I haven't told you what I referenced it to. It could be a 26.41dBi antenna (24dBd), or a 21.59dBi (also 24dBd!) antenna, depending on what my original reference was. The difference is 4.81dB, a significant amount. Most antenna manufacturers have gotten away from playing this game, but the reference will be different in different fields.
Commercial antennas tend to be rated in dBi, as the people buying them understand it, and Amateur Radio antennas tend to be dBd, as Hams are very familiar with dipoles. Sorry to go on for so long, but as an Engineer, it bugs me a bit to see things like this!
Thank you to drjim for this information.
thanks for such a useful explanation..... now every thing is clear to me.... pramod
It is true that isotropic point sources do not exist in nature. However, it is possible to design an antenna with an almost isotropic radiation pattern. I used the trial version of this software www.antennasoftware.com.ar to model the isotropic antenna using wires.
it is 2.14 not 2.41!!
good explination thank you
thanks.. every time i was getting stuck what is i, m, w after dB
26.41dBi antenna (24dBd), or a 21.59dBi (also 24dBd!) <- mistake, no? 21.59dB"d" would be 24dBi?
thank you! i've been wondering why people keep putting all that on the ends... explained very well
Thanks..Useful info however need some more clarity over the same in case of addition of db & dbm
Great article! As a ham I always enjoy reading useful information like this ...Thanks..
This is a fine article, and a valid peeve. Please check the numbers in the "24 dB antenna" discussion - I think the correct presentation is: "It could be a 24dBd antenna (26.41dBi), or a 24dBi antenna ( 21.59dBd), depending on what the original reference was, isotropic or dipole. The difference is 2.41dB, a significant amount" -- Mike
Thanks for this noble effort to clearly explain the terms.
thanq 4 dis useful information
Thank you very much, it help me and cleared my points.... Awais from Pakistan
I asked my friend how is the received power at the terminal (say a cell phone) the same with or without using HPA whcih causes high gain at the transmitter output?...he said received input might be same but you have high gain..how is tyhat possible?
Thanks! I am about 300 meters LOS from my WI-FI source and was hoping to improve the quality of the reception. The reason I'm here is because I've seen one WI-FI antenna being rated at 44dBi and another one, rated at 44dBm. While the specs appeared similar, there was a marked difference in the price. So ... I "Goggled" to find out. While I'm still in the dark as far as choosing between the antennas, I have reached the conclusion that I'm the best I'll ever be in reference to myself. Refresing! Jerry
Thanks for this noble effort to clearly explain the terms.RaNa AsIm
Thank you very much for being so tenuous.
Thanks a lot. It's very useful.
thank u so much for this good explanation...its cleared many of my doubts...
why dbm is negative when it is raaceived by mobile
Thank you so much... I got the difference.....