I have a question about RSSI and EIRP. When I bring my smartphone very near my antenna wifi router (+20 dBm EIRP) the analyzer wifi application shows me an RSSI level about -20 to -25 dBm (a negative value). I don't understand this value! If I have +20 dBm EIRP (a positive value), why do I get -20 dBm when I bring my phone close to the router antenna? Why don't I get a value of +20dBm?
RSSI measurements near wifi router
RFwifi
Related Solutions
"How is it even possible for a single fixed measurer to determine both RSSI and Noise?" - very good question. The noise they are talking about is receiver noise and not interfering signal. At very low powers, the noise is mostly the thermal noise of the receiver: ie, if you were to disconnect the antenna and replace it with a 50 Ohm load (most RF systems are 50 Ohm) you will measure a certain level of noise. So, even if you had all the ideal components, your noise power would be P = k*T*B*G, where k is the Boltzmann's constant, T is the temperature in K, B is the bandwidth in Hz, and G is the gain of your system. In reality, every component adds noise as specified by its noise figure (listed in the datasheet of every RF component). If you look again at the noise power equation, you will see that by reducing bandwidth, you also reduce the noise. However, high bandwidth is necessary for high data rates, which explains why you need good SNR for high data rates.
"Why both values are negative and measured in dBm" - 0 dBm means the power is 1 mW. -20 dbm means the power is .01 mW. The minus indicates the number of dB below 0 dBm. Without the minus, it would have been above 0 dBm
"But who radiate that power?" - in case of noise, it is internal, in case of signal, the transmitter. However, fundamentally it doesn't matter.
"But why is its value so small then?" - it comes from what is called Friis transmission formula. So, with several simplifications, imagine that my transmit antenna radiates power isotropically in all directions. So, your power is uniformly distributed on the surface of a sphere of radius r (and surface area 4*pi*r^2), where r is the distance from the transmit antenna. In Imagine, that your receive antenna is about 1 m^2 and it can capture all the radiation that hits its surface. Now, it can only capture 1/(4*pi*r^2) of all the radiation, making the receive power very tiny and RF engineering a complex field :). This is a very hand wavy explanation but I hope it makes sense
Copying an antenna design is hard, it won't work first time even if you keep all dimensions the same. Variations in the PCB dielectric coefficient will change the tuning.
Tuning the transmitter is different from tuning the antenna - the transmit frequency you're measuring is fixed by the crystal oscillator. The antenna is a filter that's independently tuned, and with your test equipment you can't easily sweep it, so you need to tune for best performance at your chosen frequency.
Fortunately its easy to tune the antenna, by cutting and/or extending it. This is the normal process when porting an antenna design to a new PCB, or even just changing PCB manufacturers or board suppliers.
Cutting process Try cutting the tip of the antenna shorter, 0.5 mm at a time. If the results get better, keep going, otherwise extend the track with some wire or copper tape, and start trimming again.
Measuring your results Set up a measurement so you can tell how you are doing, either use the two modules talking to each other, measure the RSSI or measure the range, which is much less accurate. Or measure the transmitted power with a spectrum analyser and a standard antenna. Keep the board under test close to the receive antenna. Rotate the board to get the maximum signal. Then you can start trimming, always measuring in the same position.
If you're going into production with the board, you'll need to control the dielectric. Specify the PCB material carefully, insist that it's from the same batch, etc. You can also keep some spare pads for tuning, zero ohm jumpers, a second capacitor pad in the tuning network, or some other way of adjusting the impedance on an existing board.
Best Answer
The energy output by your transmitter radiates in all directions (perhaps more in some depending on antenna design, but still generally true).
Geometrically speaking, only a very tiny fraction of that outgoing energy is intercepted by the receive antenna.
Hence you have substantial path loss, and even more so if you allow for losses in the transmitter and receive circuitry/antennas that may not be calibrated out of your numbers.
Further, your receiver may have a maximum signal level, either due to limiting at some point in the front end, or the taking of the measurement itself may effectively impose a maximum. RSSI measurements on consumer gadgetry tend to be advisory at best, and there's little need for them to work in the kind of extreme circumstances you are trying.
If you want to take a good measurement, couple a similar antenna to a 2.4 GHz RF power meter, or a professional quality spectrum analyzer with a front end that can accept that much power (most tend to have nameplate limits in the 10-20 dBm range, but you can add a calibrated attenuator inline).