Without getting into any debates about wether electromagnetic radiation is bad for you or not, and without metioning the fact that we're exposed to it from a lot of other sources .... I was wondering if someone could help me with a quick question.
Wireless routers and cards seem to mention two items when it comes to their signal power levels. The signal power in dBm (usually around
15dBm) and an antenna measurement in dBi (around 5-10dBi). My question is, if two different routers both mention 15dBm as their transmitting power, but one has a 5dBi anetanna, the other a 10dBi antenna, are they producing the same strength/amount of radiation or is the one with the
10dBi antenna producing twice the strength/amount of radiation?
It's the sum of the power output and antenna gains (in decibels). In your example, the router 1 radiates: +15dBm + 5dBi gain = +20dBm while router 2 radiates: +15dBm + 10dBi gain = +25dBm The radiated power is 5dB more for router 2 than router 1.
However, that doesn't directly give you field strength and exposure. Field strength is in milliwatts per square centimeter, while exposure is a complex mess involving SAR (specific absorption rate) which include frequency and near-far field effects.
Things to think about - that signal power in dBm (dB = deciBels - a ratio and m = milliwatt - the reference level) can be likened to the size of a light bulb - lets say 100 watts for hand waving.
the gain of the antenna measured in decibels referenced to an isotropic radiator. That light bulb is more or less an isotropic radiator, as it radiates (more or less) equally in all directions. Now, antenna gain is obtained in a similar mode to those lightbulbs, using reflectors and lenses - to _concentrate_ the radiation in a desired direction. Think of the difference between a bare incandescent lamp and a flood light and a spot light - all with (example) 100 watts. Which one is brighter? Well, the spot is going to be brighter in one direction compared to the flood or bare bulb, but the bare bulb will light up in all directions not just one.
That depends on how are you measuring the power. If you sum up all of the power radiated in all the directions because you are four inches from the antenna, then both may be more or less equal. If you are looking at the peak power in the ideal direction, then the 10 dBi antenna is producing 5 dB (or 3.16 times - decibels are logarithmic) more signal strength. And if you are not in the direct line of fire (think about that spot light again), then "it depends".
Then, as Jeff notes, there are all kinds of other dependencies. Bottom line - there is no simple answer without making some assumptions.
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