This is the process
- Step 1 - Determine dB figure (71dB - 41dB) to get 30
- Step 2 - Determine power by dividing dB figure by 10 (30/10 ) to get 3
- Step 3 - Determine the multiplication using the power on 10 (10 to the power of 3) to get 1000
OK lets say the dB figure is 3dB (which we know equates to doubling)
- Step 1 3dB
- Step 2 0.3
- Step 3 10 to the power of 0.3 gives 1.9953 (which is usually taken as 2)
OK lets say the dB figure is -3dB (which we know equates to halving)
- Step 1 -3dB
- Step 2 -0.3
- Step 3 10 to the power of -0.3 gives 0.5012 (which is usually taken as 1/2)
what you have to do to understand logs is this
Be happy with the concept that any positive number can be expressed as a power of 10
0.01 === -2
0.1 === -1
1 === 0
3.16 === 0.5
10 === 1
31.6 === 1.5
100 === 2
1000 === 3
Be happy that once you have the “power” representation, then multiplication and division is really addition and subtraction of the powers
10 (power of 0.5) x 10(power of 1) = 10(power of 1.5)
This is why dB is so useful - because it is the log representation of gain or loss, so whilst you are in this domain, it is easy to calculate the gain/loss because you use addition/subtraction.
If one talks about dB, then it is just a way of talking about “gain”
If one talks about dBi, dBd or dBm then it is no longer a “gain” figure as such because it is referencing “something”
If one talks about dBi, then it is not a gain as such, but a “perceived gain in a certain direction” referenced to the condition of an isotropic antenna . It is a slightly meaningless idea really, because no SI units are referenced. dBi references the “fluffy” notion of how the power shape of an EIRP situation changes i.e. it describes the Effective power gain in a particular direction which of course ignores the Effective power loss in other directions.
Because this “fluffy” nature of dBi (and dBd) can be misunderstood, a lot of people may think that dBi is associated with WATTS (a scalar SI unit), but is not, dBi is more about describing the “focussing shape” of an antenna, but it does this in the LOG domain
Now dBm also references something, but this time it is a SI unit (milliWatts),
CONSIDER THIS… dBm IS NOT A “GAIN” VALUE AT ALL, IT IS AN ACTUAL TANGIBLE DISCRETE VALUE OF POWER (described in LOG form).
e.g the power value of 25mW is expressed in the log domain as 14 dBm (The formula is 10 x log (25/1) ).
The advantage of doing this transformation of the power value from the world of “Watts” to “dBm”, is that power is now expressed in the LOG domain (and this means it can be easily added/subtracted with other “non-SI unit” LOG values e.g. dB /dBi / dBd, but you should not add/subtract a “SI unit” LOG value with another “SI unit” LOG value e.g. adding 2 dBm figures together - if you are doing this, then chances are, something has gone wrong when you transferred values into the log world)
N.B. The value in dBm describes a power in Watts by comparing what the gain would have to be compared to 1 milliWatt (which is an SI unit).
The reason why things still work if people conceptually misunderstand what is going on, is because the domain of logs just works anyway (addition/subtraction) - but there is the catch that if you use dBi, then the gain value is always the same as dBd + 2.15.
It is reasonable to assume (if the math is not known) to think that the more expensive an antenna, the more it can magically increase distance (a bit like thinking a more expensive sports car gives you more speed).
The truth is that there is no free lunch - if you want twice the distance with your existing antenna, you need 4 times the power (erp), and using an expensive appropriate antenna does not usually give the increase in distance as much as people may think because for Lorawan, an omni directional antenna in the horizontal plane is prefered (think of the torch example in the video) and people usually disregard the need for proper placement (because it is not told to them enough! )
There is a mathematical limit to how “good” an antenna can be - and it is not really linked to size of antenna - Whilst a “long” antenna may be needed to house the folded loop dipoles of a collinear antenna array , there is a law of diminishing returns here, and with impedance and loss considerations, there is a natural limit beyond which there is no benefit to having a longer antenna.
If anything, putting a small dipole on a long stick would give most of the same antenna “distance increase” as a collinear (compared to previously having the antennae indoors)