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 dB, dBi, and dBd
I have a loose grasp of these terms and their relationships. Is there any way to rule of thumb the gain of any antenna over a dipole over earth?
Never do anything you don't want to explain to the ambulance technicians.
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Rule #1 -- don't trust the manufacturer's claims -- especially if it is some secret / special / oddball design.
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dB is a measure of power. There are lots of reference points, as you have found. Others you'll see are dBm and dBw (in reference to a milliwatt and a watt, you'll see these referenced in receivers and transmitters).
Add 3dB and your power doubles. 30dBm = 1W. 33dBm = 2W.
dBi is gain in reference to an isotropic radiator (theoretical antenna that doesn't really exist - its a sphere of radiation = equal radiation in all directions). Since an isotropic radiator doesn't really exist in practice, the more "realistic" reference point is a dipole. This is a dBd measurement. There is 2.15dB difference between a dBi and a dBd, dBi is always higher since a dipole is a real antenna and has a real radiation pattern = not equal in all directions = has "gain".
The same principle applies to a dBd as a dBm - add 3dB and the "power" doubles. Note that this is the power at the antenna, not the received signal strength. If you have 50 watts in to the antenna and start with a dipole then go to a 2 element beam with 3dBd gain you are effectively radiating 100 watts.
If you ever operate on 60 meters, or used to, the power/antenna/gain issue is really at play. The regulations were that 50 watts ERP was the max from a dipole. That isn't "tune your radio to 50 watts and run a 4 element beam with 8.5dBd gain". That is 50 watts in to a dipole.
Lastly, the reason behind using a dB is it is a logarithmic scale. The power density of a radiated signal drops off quite substantially with distance from an antenna. Think of a light bulb and how bright it is at 10 feet from it vs 100 feet from it... If you used a direct relationship between distance and density the numbers would be ridiculous. The decibel puts things down to Earth. Also note that a lot of receiver measurements are in the - 100's of dB, noise floors near -130 to -140dBm... The energy those represent is so minute it isn't realistic to represent it outside of a dB. Microvolts can be used for the smaller measurements too, but that is still a direct relationship and doesn't hold up in scaling power like a decibel does.
Steve
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dB=decibel, unit used in measuring AF/RF power
dBi=db gain over an isotropic radiator
dbd=db gain over a dipole at the same height above ground
I'm certainly no expert but I believe these are reasonably accurate definitions (sure someone will help me out if I'm wrong)
73, Jeff
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dB is just a ratio. It isn't ever a quantity.
dBi and dBd are related by a fixed quantity; dB over Isotropic (dBi) is 2.2 dB higher than dBd (dB over a dipole in free space).
Those are measurable parameters, and also quite well predictable by modeling if all the variables are known.
But "dB" is only a ratio.
A government which robs Peter to pay Paul can always depend on the support of Paul.
-- George Bernard Shaw
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 Originally Posted by WB2WIK
dB is just a ratio. It isn't ever a quantity.
dBi and dBd are related by a fixed quantity; dB over Isotropic (dBi) is 2.2 dB higher than dBd (dB over a dipole in free space).
Those are measurable parameters, and also quite well predictable by modeling if all the variables are known.
But "dB" is only a ratio.
I agree for the most part. Even dBi and dBd aren't "values". You still have to add wattage to have a "value". The dBi and dBd are just "power factors" with the same scale, different references.
Conversely, dBm and dBw are values. Their reference point is a milliwatt and watt, respectively. So 30dBm = the real value 1 watt, 30dBw = the real value 1000 watts. Without a real value reference dBi and dBd measurements don't give you a "value", just a scale.
In my first post I should have said "representation of power" instead of "measure of power". It isn't a direct measurement in and of itself, just a different way to represent it in a way that doesn't give you ridiculous numbers.
Steve
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Originally Posted by KB5HAB
I have a loose grasp of these terms and their relationships. Is there any way to rule of thumb the gain of any antenna over a dipole over earth?
No, unless you measure the two or compare the two in a model. dB is always just a ratio between two things, so we always have to know the reference.
A dipole at 5/8th wave over medium soil has a gain of about 8.5 dBi. If you subtracted that from the dBi gain of a model antenna over earth, you would have the dBd gain.
I explain that many places on my website, because so many people are fooled by dBi. Look at the gain here:
http://www.w8ji.com/rhombic_antennas.htm
73 Tom
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Originally Posted by K9STH
It's important to remember that if you use the ratio of two voltages to calculate a dB figure (using the expression dB=20.log(V1/V2)), those voltages must be measured across the same value of impedance.
If we don't do that we get meaningless figures; for example a 10:1 passive step-up transformer would have a "gain" of 20dB.
Steve G3TXQ
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"dB is a measure of power. "
Ratio - it's specifying a ratio logarithmically ... and when used with a suffix, as a ratio to that unit (e.g. milliwatt, Watt etc.)
Jim de WB5WPA
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