Well, to be honest, this unfortunately exceeds my current knowledge of the physics involved. At least my comfort level in being able to succinctly and accurately attempt to explain it is exceeded.
A good portion of the explanation lies right in the Free Space Loss equations and models. *Initial* free space loss is considerable due to the logarithmic nature of the radiated power, even at _very_ close ranges.
I've dug up the following in hopes of explaining things a bit better (and refreshing the knowledge myself);
If anyone else has some additional 'physics' style explanation as to why initial free space losses are so extreme, I myself would like to hear it as well. I just know that it is.
In the link budget for Ptx=20dBm ,Transmitter Gain 1dBi, Receiver Gain 1dBi,Frequency =2.5Ghz,Distance =1Cm then recived power is equal to +19.60dBm!!!!
I'm not sure what to tell you. I can assure you however, that you will be very unlikely to register an RSSI value greater than -20dB in real life.
In fact, I wouldn't be surprised if most radio hardware out there now have some form of protection mechanisms to limit received radio power to 0, -10, or -20dBm. They really aren't designed for power levels that high in typical operation. Sensitivity at much smaller received power levels is of much greater importance and practical use.
"I'm not sure what to tell you" ??? Please ,control yourself by http://www.pasternack.com/t-calculator-friis.aspx
Second my opinion smartphone frontend receiver is automatically protect for overload and adc conversion is made for a precise range power values.When the receiver go near transmitter the received power increase and for a specific threshold (-30 dBmfor example) the receiver limites input power to prevent saturation problems.
... so what exactly is your question here, grigione?
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). For Wifi it's inacccurate for distances below a meter or so.
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). In reality you will be lucky to catch a ten-thousandth of the power (-40dB). 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. 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...).
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.
Someone above suggested that the positive/negative thing is just a random choice or that the sign might be wrong. It's not, it's a measurement of power. dB means a ratio measurement relative to *something* (ratio = 10^(dB/10)). +10 dB means 10 times bigger and -10 dB means 10 times smaller, so -40 dB means 10x10x10x10 = 10000 times smaller. dBm means dB relative to 1 mW, so 20dBm is 100mW, and -20dBm is 10uW. Wifi transmitters are usually ~100mW, and you will never get better than -30 to -40 dB of path loss with normal wifi antennas, so the received power is always less than 1mW, so is always negative when measured in dBm.
If you goto this site http://www.pasternack.com/t-calculator-fspl.aspx?gclid=CLGX46qW7MMCFW4Q7AoduH4AVg
this will allow you to calculate the free space path loss,
if you assume a frequency of 2437 (channel 6), a distance of 30ft., the transmitter antenna has a gain of 2.2 dBi, and the receive antenna is a typical laptop device which has a 0 dBi antenna then the free space path loss is equal to 57.20 dB
You can read more about free space path loss here
I don't have the time to provide you with a detailed explanation. I think some of the URLs provided by others can provide you with additional background information.
What I can say is that radio waves experience loss as they propagate. They experience more loss when propagating through more lossy media. Free space (vacuum) causes the least amount of loss. For more information, see the section labeled 'Free Space Propagation' on this page:
Basically, doubling the distance from the transmitting source will reduce the power density at the receiver by a factor of four. So, if you receive 0 dBm at 1 meter, you will receive -6 dBm at 2 meters. (3 dB loss equates to a loss of half the power).
When you add other factors into the path, such as solid materials (drywall, cinder block, etc), metallic surfaces (any conductive metal object, or even wire mesh embedded into walls), you encounter additional signal attenuation as the signal passes through these materials.
One other thing...EIRP, or Effective Isotropic Radiated Power is a theoretical value for a radiating signal source that radiates power equally in all directions (think of a point source in the middle of a soccer ball...at the surface of the soccer ball, at all points, the power will be equal). This is not something that can be achieved in the real world, since there is no ideal transmitting element. Every transmitting element transmits its' signal in a pattern, depending on the design of the element, where there are directions where the transmitted power is stronger, and other directions where the transmitted power is weaker, or even non-existent.
See this URL for a quick explanation and diagram:
So, in your situation, it's very possible that, due to the radiation pattern of the antenna used by your access point, the signal strength in the direction you are testing is off of the primary lobe of the antenna gain path, as designed by the engineer. Most access points utilize an omnidirectional antenna, which has a 'theoretical' transmitting pattern that looks like a donut:
So, if you are directly above the access point, you're in the path of the donut 'hole', a region of low transmitted signal.
Also, keep in mind that the access point also needs to receive the signal that your device transmits. So, even if the access point receives signals in the same omnidirectional path that it transmits signals, the quality of the signal it receives from your device is highly dependent on the transmitting pattern and signal strength of your device. If your device transmits a weak signal, or is transmitting a stronger signal in the direction opposite where the access point is located, the received signal at the access point will be reduced greatly.
Hope this helps.
Thanks for the detailed explanations- I understood what you were trying to convey there. I also understood what Grigione2015 was asking- it seems his question was not really answered rigorously. I have the same question he had. Which is as follows: why do we switch the sign of the Tx power from +22.5/23 dB to -22.5/23 dB when we look at it in-terms of RSSI (lets assume an isotropic RX, so 0 dB Rx antenna gain)?
I realize you mentioned the answer to the question as something to do with the IEEE specifications- could you please elaborate on that point?Thanks!
Also, I noticed a couple of others tried answering his counter questions with the path loss equation - I think they may have misunderstood what he was asking or maybe they didnt and I havent caught up to them yet. His counter-question was as follows: okay, so you tell me near the antenna, we switch this +20 dBm to -20 dBm, so conversely, this -20dBm RSSI was from this 20 dBm Tx...so when I'm far away what prevents me from saying- when my RSSI reads -60 dBm the Tx power is 60 dBm. Going back to the main argument, his main concern was at the "so called " 40 dbm loss between the Tx and Rx when they were close to each other (20 dbm(Tx output) - -20dBm(RSSI)=40 dBm)-to this point some people tried explaining that as path-loss without understanding that that's not really pathloss but what you earlier mentioned as a IEEE unit convention switch.
Also, now that I revisited this, you had mentioned
802.11a EIRP = 17db (40mw) - 0db + 6dbi = 23db = 200mw of actual output power
802.11bg EIRP = 20db (100mw) - 0db + 2.5dbi = 22.5db = 150mw (approx) of actual output power
based on the example above in theory if you were to measure it right at the antenna you could get an RSSI of -23 or -22.5 respectively
Going by what you said a Tx with higher power would measure lower RSSI (I thought -23 is less than -22.5 )? Why? That seems to be counter intuitive. So if my Tx was giving a power of 28 dBm then RSSI would be -28 ...correct?
Basically the problem here is there are number of measurements and values that people refer to that are all treated different.
So for example
SNR = Signal to Noise Ratio should always be represented as a positive value number and is based on a scale of 0 (zero) to 100+ where the higher the number the stronger (better) the SNR is.
RSSI (Received Signal Strength Indicator) = RSSI is always (in regards to WiFi atleast) as a Negative number because of the Scale developed for dB measurments. The RSSI scale is based on a 0 (zero) to -120 values where the closer the number is to 0 (Zero) the stronger the RSSI is.
Noise = Noise is always (in regards to WiFi atleast) as a Negative number because of the Scale developed for dB measurments. The Noise scale is based on a 0 (zero) to -120 values where the closer the number is to 0 (Zero) the stronger the Noise is.
Antenna Gain = Gain is the increase in RSSI that the antenna provides which is why it is always expressed in a Positive number so that when it's added to an RSSI it it brings it closer to 0 (Zero)
Transmit Power = TX Power is always expressed in a Positive value, because that is the actual dB level of signal produced. If you were to connect a spectrum analyzer directly to the antenna port you would see an RSSI of 20 dB (roughly speaking) as there would be minimal loss.
EIRP is the effective power transmitted at the element and is not a measure of the RSSI that a client device would receive and that is where a lot of people get confused.
Hope this helps clarify it.
The easiest way to explain this .. A radio that is transmitting is a positive like 20dbm . A radio receiving this transmission isn't putting anything on the medium he is listing so he is a negative ..