Why the wireless signal (RSSI) is a negative answer: in fact, the final analysis of why the wireless signal received is negative, this is not easy to understand more. Because the wireless signal is many MW level, so it has been polarized, converted to dbm, does not mean that the signal is negative. 1mW is 0dBm, less than 1mW is a negative number of dbm.
Figure out the definition of signal strength: RSSI (receiving signal strength) Received Signal Strength Indicator
RSS=10LOGP,
Only the incoming signal power P is received is the signal strength (sensitivity).
[Example 1] if the transmit power P is 1MW, the converted to dbm is 0dBm.
[Example 2] for 40W power, the value of the converted in dbm unit shall be:
10LG (40W/1MW) =10LG (40000) =10lg4+10lg10+10lg1000=46dbm.
Why the measured dbm values are negative.
A: The first thing we need to know is that the wireless signal dbm is negative, the maximum is 0. So the measured dbm values are definitely negative. Because the dbm value is only 0 in one case, that is the result of experimental measurements in the ideal state, generally we think that dbm is the maximum value of 0, meaning that the receiver of the transmitter launched all the wireless signal received, that is, the wireless router fired how much power, the reception of the wireless network card to obtain how much power. Of course, this is measured in an ideal state, in practice, even if the wireless network card next to the wireless router transmitter antenna will not achieve the effect of dbm 0. So the measured dbm value is negative, do not blindly think negative is bad signal. Power Unit DBm
DBM is a value of the absolute values of the test power, the formula is: 10lgP (power value/1MW).
[Example 1] if the transmit power P is 1MW, the converted to dbm is 0dBm.
[Example 2] for 40W power, the value of the converted in dbm unit shall be:
10LG (40W/1MW) =10LG (40000) =10lg4+10lg10+10lg1000=46dbm. Db
DB is a value that represents a relative value, and when considering the power of a is compared to the power of a large or small number of DB, the following calculation formula: 10LG (a power/b power)
[Example 6] The power of armour is one times larger than the second power, then the 10LG (A/b power) =10lg2=3db. In other words, the power of A is 3 DB larger than the power of B.
[Example 7] The 7/8-inch GSM900 Feeder's 100 m transmission loss is about 3.9dB.
[Example 8] If the power of a is 46dBm and the power of B is 40dBm, it can be said that A is 6 DB larger than B.
[Example 9] If a antenna is 12dBd and the B antenna is 14dBd, it can be said to be 2 DB smaller than B.
How to see the receiving power from the DBM value
DBM is a unit that represents the absolute value of the power, and his formula is 10LG power/1MW. For example, if the power received is 1MW, the value to be converted in dbm units should be 10LG 1mw/1mw=0dbm. Of course, in the actual transmission process, the receiver is difficult to achieve the receiving power of 1MW. So we can use this formula to reverse the value of the power received by the receiver from the DBM value. Myth: The smaller the dbm value, the better.
Since the dbm values mentioned earlier are negative, many people think that the dbm value is as small as possible. In fact, this perception is wrong. As stated previously, the DBM value is 0, and is ideal. So the closer to the ideal state of the dbm value, the more that the wireless router launch power is received by the wireless network card. So the dbm value should be larger the better, -50dbm that the wireless signal received is better than -70DBM. Myth: The greater the dbm value, the better.
And a lot of friends think that since the dbm value is 0 to send the signal to receive the best results, then we should let the enterprise wireless network each place in the DBM value as large as possible. In fact, this view is also wrong, although the greater dbm value to send the reception signal the better, but at the same time we need to install the enterprise's internal wireless network enough wireless signal relay equipment, this is not a small amount of money. The experimental results show that the wireless signal scanning component of XP system can meet the requirements of the network transmission, the speed and stability are not any problem, and the "very good" state corresponds to the dbm value of 0 to -50DBM. Therefore, we only need to ensure that the internal wireless network in various places of dbm value is not -50dbm can be, so the establishment of the wireless network is a high-speed and stable network. We have the highest price/performance ratio for wireless networks. Tip: Of course, sometimes, for the sake of funding, we cannot guarantee that the DBM value of each place in the intranet is between 0 and -50dbm, and that the measurement should be between 0 and -70dbm. Because when the XP system receives the wireless signal is less than -70dbm, the transmission is unstable, the slow speed phenomenon, so our wireless network can not use normally. Error: The reception power is small because the transmission is disturbed
It is well known that the transmitter power of a wireless router is generally 100mw and higher. So why is the power we receive is so small. is because the transmission process is relatively large interference. Below we take received signal for -50DBM namely 0.01μw as an example, if the wireless router launch power of 100MW, and received only for 0.01μw, the difference is 10 million times times. In fact this is normal transmission, as if only One-one hundred millonth of the energy emitted by the sun is received by the Earth. The receiving power must be much smaller than the transmit power. So the network administrator in the measurement than need to worry, as long as your signal strength is greater than -50dbm can not have any problems of wireless transmission data, and then step back even to the -70DBM can also guarantee the wireless speed of 54M transmission.
DB,DBI, DBd, DBC,DBM,DBW
1, DB
DB is a value that represents a relative value, a pure ratio, only indicates the relative size relationship of two quantities, no units, when considering the power of a is compared to the power of a large or small number of db, according to the following formula: 10log (a power/b power), if the use of both the voltage ratio calculation, to use 20log (methyl voltage/ b voltage).
[Example] a power is one times greater than the power of B, then 10LG (a power/b power) =10lg2=3db. In other words, the power of A is 3 DB larger than the power of B. Conversely, if the power of a is half of the power of B, then the power of a is smaller than the power of B 3 DB.
2, DBi and DBD
DBI and DBD represent the amount of antenna power gain, both of which are a relative value, but the reference datum are different. The reference datum of dbi is omni directional antenna, the reference datum of dbd is dipole, so the two are slightly different. It is generally believed that the expression of the same gain, expressed by DBI, is 2.15 larger than that of DBD.
[For example] for an antenna with a gain of 16dBd, the gain is converted to a unit of DBI, then 18.15dBi (generally ignoring the decimal digit, 18dBi).
[Example] 0dbd=2.15dbi.
3, DBc
DBC is also a unit representing power relative values, exactly as the DB method is calculated. Generally speaking, dBc is relative to the carrier (Carrier) power, in many cases, to measure the relative value of the carrier power, such as the measurement of interference (same-frequency interference, intermodulation interference, alternating interference, out-of-band interference, etc.) and the relative values of coupling, spurious, etc. In the place where DBC is used, db substitution can also be used in principle.
4, DBm
DBM is a value that represents the absolute values of the power (which can also be considered a ratio based on 1mW power), and the formula is: 10log (power value/1MW).
[Example] if the power p is 1MW, the converted to dbm is 0dBm.
[For example] for 40W power, the value of the converted unit in dbm shall be:
10log (40W/1MW) =10log (40000) =10log4+10log10000=46dbm.
5, DBw
Like DBM, DBW is a unit that represents the absolute value of the power (which can also be considered a ratio based on 1W power), and the formula is: 10log (power value/1w). The conversion relationship between DBw and DBm is: 0 dBw = 10log1 W = 10log1000 mw = dbm.
[Example] if the power p is 1w, the conversion to DBW is 0dBw.
In short, db,dbi, DBd, DBC is the ratio between two quantities, indicating the relative size of two quantities, and dbm, DBW is the absolute size of the value of power. In DB,DBM,DBW calculations, pay attention to the basic concept of a dbm (or dBw) minus another dbm (dBw), the result is db, such as: 30DBM-0DBM = 30dB.
In general, in engineering, there is only addition and subtraction between dbm (or DBW) and dbm (or DBW), without multiplication and division. And the most used is subtraction: dbm is actually divided into two power, and the signal power and noise power Division is the signal-to-noise ratio (SNR). DBM Plus dbm is actually two power multiplication
Why the antenna gain unit sometimes with dbi, and sometimes with dbd, the difference between the two.
Answer: DBI and dbd are antenna gain units, we are accustomed to the ideal power supply radiator as a reference
To specify the gain of omnidirectional antennas, and to consider directional antenna with ideal half wave symmetric oscillator as reference
The gain. DBi and DBD can be converted to each other, G (DBD) =g (dBi)-2.15
Source of information:
Http://www.cnblogs.com/haitao-fan/archive/2012/06/29/2568975.html
http://blog.csdn.net/achejq/article/details/6576822
