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I have a UHF reader with integrated 7dBi and 9-30dBm output power.

Can you explain to me what do these 7dBi and 9-30dBm mean, in a way an ordinary people (like me, who don't really have knowledge in network) could understand. As far as I researched, there is no absolute formula to get the distance of a dBi, theoretically speaking dBi won't affect the range of an antenna? but I also found out that higher dBi means higher gain and more directional. What does it mean?

In other words. What could possibly the pros and cons of having a higher/lower dBi and dBm?

If possible, can you give a situational explanation.

Thanks in advance.

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AndrejaKo's answer looks good but you specifically requested a layman's answer so I'll try to make it even less technical.

Firstly you are correct, there is no simple way to go from dB to range. Anything to do with radio involves far to many variables to give a simple 1 number answer for anything even close to real world usage.

However all things being equal you can apply some approximate rules of thumb.
For conventional radio signals a 3dB increase will give you a ~40% increase in range, 6dB a doubling of range.
For RFID type applications where signals have to bounce back to the reader again this effect is halved, 6dB will give you 40% more range, 12dB a doubling.

dBm is simply a measure of the power output. It is possible to convert this directly into watts if needed. 9dBm = 8mW, 30dBm = 1 Watt.

dBi is used to measure the antenna gain which is also indirectly a measure of directionallity. This is always a trade off.
If we assume the antenna is perfect then it will transmit all of the energy it receives from the electronics (a reasonable rough approximation for a good antenna). If it sent that energy equally in all directions the antenna would have a gain of 0 dBi, this equates to a gain of 1.
If you want to get more signal energy to go in a specific direction then you need to reduce the energy going in the other directions. If an antenna sent energy in a perfect hemispherical pattern it would have a gain of 3dBi.
Real world antenna gain patterns are far messier than the ideal but if an antenna has a gain of 7dBi then it's probably got a region about 90 degrees wide in which it transmits most of it's energy. That doesn't mean it doesn't transmit anything in other directions but the signal will be a lot weaker outside of that main region.

Why use dB for this sort of thing when we could use Watts for power and a normal linear gain number for the antenna? Three reasons:

  • Firstly to confuse people, it's job security for engineers.
  • Secondly the power levels involved in RF can vary massively, using dB makes it a lot easier to deal with combinations of uW and kW in the same equation.
  • Thirdly engineers are lazy and it makes the maths easier. When working through total system power you can simply add the dB numbers up (which are normally only 2-3 digits long) rather than multiplying numbers which are different by orders of magnitude.

Some of those reasons may not be entirely true.

--- addition ---

For 900MHz RFID there are two different factors that can limit the range. Whether the tag is receiving enough power to switch on and if the tag is getting enough power whether the returning signal is strong enough for the receiver to pick up.

If getting sufficient power to the tag is the limit then the maximum range follows the inverse square law and a 3dB increase in signal will give a 40% increase in range, a 6dB increase will double the range.

If reading the reply is the limiting factor then the range limit is 1/power^4. A 3dB increase will give a ~19% increase, 6dB a ~40% increase and 12dB will double the range.

Which one of these two is limiting the range depends upon the tags and reader used. Older tags needed a fair amount of power and so the first of those two limits, the tag powering up, was often the limiting factor. Newer tag silicon is far lower power and so it's less certain what the limit will be these days.

When I indicate an increase in dB that can come from either source, if you change both then add the two changes together to give the total effect. If the increase comes from antenna changes then keep in mind the dB value is only valid for the direction with the maximum signal strength, that direction could change if the antenna changes.

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  • \$\begingroup\$ So what is the correlation of decibel to rfid? does it mean higher db increased reader range? Thanks in advance. \$\endgroup\$ – Black Nov 9 '16 at 9:31
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    \$\begingroup\$ I though the original answer already covered that. I've updated it to include some more RFID specific information on range. \$\endgroup\$ – Andrew Nov 9 '16 at 9:57
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The thing you need to start researching is called link budget. Your range depends directly on receiver sensitivity, your equivalent isotropic radiated power (EIRP) and the path loss.

The basic formula looks something like this: \$P_{rx}=P_{tx}+G_{tx}+G_{rx}-L_{path}-L_{system}\$, where the \$P_{rx}\$ needs to be greater than a certain value.

When you're calculating EIRP, you add the transmit power (in dBm) to the antenna gain (in dBi) and then subtract system losses, such as coaxial cable loss, connector loss etc. So the formula for EIRP is:\$EIRP=P_{tx}+G_{tx}-L_{system_{tx}}\$

Then you subtract the path loss in dB (there are calculators for that on the Internet, but as a beginner, don't expect to know enough to be able to get realistic results) and then you get the power available at the receiver's location.
To get the actual power at the receiver, you need to add the receiver's antenna gain (in dBi) to the power at receiver's location and also subtract the system losses at the receiver's site.

So how do transmitter power (in dBm) and the antenna gain (in dBi) work?

Well, antenna radiates power out and also focuses power in certain direction. The dBi value is the "gain" you get in the direction in which the antenna has maximum radiation compared to a "reference" antenna that does not focus power and instead just radiates equally in all directions. So the higher antenna gain is, the longer range you'll get, if the antenna is pointing in the direction in which you want to have long range. Also, if you go to some extremely high gains, it can get a bit complicated to actually aim antenna properly.

On the other hand, the transmit power is just that: power of the transmitter. You can compensate for a weak antenna by increasing transmit power or you can compensate for weak transmitter by increasing antenna gain. Of course, more transmit power needs more electricity, which can be important for small, portable devices.

It's a bit difficult to go into more details when explaining things to someone who has no background, but just keep in mind that different combinations of antenna gain and transmit power can work well in different situations. For example, if you're building a short-range system, where you have a fixed base-station communicating with small mobile stations, then it's a good idea to have large antenna gain a the base station, so that you can hear the mobile stations well, and also have low transmit power, so that you do not create too much interference for others.

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  • \$\begingroup\$ Will there be a difference if it is a patch antenna? \$\endgroup\$ – Black Nov 7 '16 at 14:47
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    \$\begingroup\$ @user7040804 All of the above stands for patch antennas as well. Basically, I talked about rules that apply to all antennas. The radiation diagram Andy put in his answer is for a dipole antenna, so it will look different for a patch antenna. You should take that into account when aiming the antenna. \$\endgroup\$ – AndrejaKo Nov 7 '16 at 22:56
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Can you explain to me what do these 7dBi and 9-30dBm mean

The output power pushed into the antenna when transmitting is 9 to 30 dBm. Using the 9 dBm figure, this means a power level that is 9 dB higher than 1 milli watt (notice the "m" after dB in "dBm" - this means the 9 dB number is relative to 1 milli watt).

So, how much is 9 dB you might ask?

To convert to a real ratio (relative to 1 milli watt), 9 is divided by ten to get 0.9 and then the antilog to the base of ten is taken hence, 9 dBm is 7.94 mW. 30 dBm is exactly 1 watt. So your transmitter is capable of outputtinng power levels to your antenna of between about 8 mW to 1000 mW (1 watt).

7 dBi is a bit more tricky conceptually. It would be nice if an antenna could be made that transmits (and receives) radio waves in all directions at equally. Then it wouldn't matter if a receiver were facing the wrong direction; you'd still pick up a transmission with the same signal level.

This idealized antenna is called an "isotropic" antenna and this is where the "i" comes in when we say "dBi" because, what this means, is that your antenna is capable of receiving a bigger signal in one particular direction compared to the theoretical isotropic antenna.

It's also capable of concentrating the power transmitted in one direction compared to that which an isotropic antenna would produce. Look at this simple dipole antenna: -

enter image description here

The antenna in the picture is vertical and in the vertical plane there is no transmitted signal nor can it receive in this direction. However, in the horizontal plane, it concentrates its transmitted power and, assuming no antenna losses relative to the theoretical isotropic antenna, that power transmitted in the horizontal plane MUST have a higher level compared to the isotropic antenna.

So, your antenna has 7 dB more gain in its optimum point relative to an isotropic antenna.

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