![]() dB m means dB relative to 1 mW, so 20dBm is 100mW, and -20dBm is 10uW. +10 dB means 10 times bigger and -10 dB means 10 times smaller, so -40 dB means 10x10x10x10 = 10000 times smaller. dB means a ratio measurement relative to *something* (ratio = 10^(dB/10)). Someone above suggested that the positive/negative thing is just a random choice or that the sign might be wrong. Looking at it another way, if what you're expecting to happen actually happened, when you put your receiving antenna close to the transmitting one it would prevent any other device from accessing your wifi by sucking up all the power. Also having another antenna in the near field will reduce the efficiency/gain of the antenna, and the EM field in the near field is not the right shape to be picked up efficiently (this is difficult to explain, the maths gets pretty complicated.). The rough explanation is that the power from the antenna is radiating in all directions (actually this is impossible due to the 'hairy ball theorem', but it's close enough) and only the part that passes through the receiving antenna is 'caught' by that antenna. In reality you will be lucky to catch a ten-thousandth of the power (-40dB). You will never get anywhere near the amount of power you put into one antenna out of another antenna nearby, unless the antennas are specifically designed for maximum efficiency in the near field (NFC, wireless chargers). For Wifi it's inacccurate for distances below a meter or so. The Friis transmission equation is only valid in certain conditions, one is which is that the distance between antennas must be much greater than the wavelength of the signal (see wikipedia for more info).
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