nRF24L01 Antenna dBm. dBi.

Hi all,

1st of all i wanna make a question and what i searched on wiki:

dBm

dB(mW) – power relative to 1 milliwatt. In audio and telephony, dBm is typically referenced relative to a 600 ohm impedance,[44] while in radio frequency work dBm is typically referenced relative to a 50 ohm impedance

dBi

dB(isotropic) – the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.

Aren't the dBm and dBi same antennas? especially when we talk about values like 20 dBi/m?

And we start the Topic

I got a nRF24L01 they mostly come with a 3 dB antenna, according to wiki and other websites
nRF24L01 can get :

Maximum output power +20 dBm

What we see on wiki:
Power lvl -- Power
21 dBm 125 mW Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles)
20 dBm 100 mW EIRP for IEEE 802.11b/g Wireless LAN 20 MHz-wide channels in the 2.4 GHz ISM band (5 mW/MHz).
Bluetooth Class 1 radio. Maximum output power from unlicensed AM transmitter per U.S. Federal Communications Commission (FCC) rules 15.219.[4]
*19 dBm 79 mW *
*18 dBm 63 mW *
*17 dBm 50 mW *
15 dBm 32 mW Typical Wireless LAN transmission power in laptops
*10 dBm 10 mW *
7 dBm 5.0 mW Common power level required to test the Automatic Gain Control circuitry in an AM receiver
*6 dBm 4.0 mW *
*5 dBm 3.2 mW *
4 dBm 2.5 mW Bluetooth Class 2 radio, 10 m range
3 dBm 2.0 mW More precisely (to 8 decimal places) 1.9952623 mW

So meaning that the nRF24 can get a +20dBm antenna we can get a new powerfull antenna than
the simple one 3 dBm. So according to wiki and the nRF24 the output of the antenna is 200mW which
means we can put a +20dBm antenna.

Questions:

  1. Then nRF24 comes with a 3dbm antenna with range 1300m changing and using a 20dBm antenna
    the range gonna increase?
  2. The nRF24 can handle 20dBm using a 20dB antenna it gonna increase the draw of the current?
  1. Then nRF24 comes with a 3dbm antenna with range 1300m changing and using a 20dBm antenna
    the range gonna increase?

Yes but at the price of being directional.

  1. The nRF24 can handle 20dBm using a 20dB antenna it gonna increase the draw of the current?

No you are just squeezing the same amount of RF power into a tighter beam.

Question:

  1. What's the difference between dBm and dBi ?

I done some reading and i found :

dBi is a measure of the increase in signal (gain) by your antenna

dBm is a measure of signal power.

Ok that but how i can pick a 20dBm antenna when there is only info about dBi.
Isn't dBm and dBi let's say the same? 20dBi ?

Isn't dBm and dBi let's say the same?

No dBm is a measure of power.
dBi is a measure of gain the two are not remotely the same thing.

Ok that but how i can pick a 20dBm antenna

There is no such thing that is why there is only info about dBi.

No dBm is a measure of power.
dBi is a measure of gain the two are not remotely the same thing.

Well i already said what they mean.

When i go to buy a antenna the info comes at dBi but all i know about the nRF its dBm.
How can i know what dBi when i know only the dBm ?

dBi is the antenna gain and dBm is the signal levels, both are not interchangeable.

Let say your receiver need receive level of -100dBm.
the Path loss (or path attenuation) about 110–140 dB
your transmitter have signal level of +20dBm
So your will need the antenna have gain of from 1 to 20 dBi to have the receiver able to receive the signal.

Domino60:
Well i already said what they mean.

But you do not seem to understand what they mean.

Domino60:
How can i know what dBi when i know only the dBm ?

You multiply the gain of the antenna by the output power of the transmitter to get the effected radiated power. The dBi is a property of the antenna which is totally independent of the output power of the transmitter.

Let me use an analogy to try and explain.

If you had a small light bulb, you could power it with 1W (=30dBm).
It would light your room, but not very well.

You could increase the overall illumination by raising the supply voltage, and increasing the power.

Another way of increasing the illumination, would be to put a parabolic reflector around the bulb - you've made a torch/flashlight.
The power of the bulb hasn't changed, but you have a higher light intensity in a certain direction, but at the expense of reduced illumination in other directions. This is the equivalent of an antenna's gain.

If the light intensity is say 100 times as high as it was before, you have a gain of 20dBi, This is a property of the reflector design, and is independent of the power delivered to the bulb.

In fact this isn't an analogy as light is also electromagnetic radiation!

Historical accident means for light we say luminosity for power and luminous intensity for effective radiated
power (per unit solid angle). Light emission could be measured in dBm and reflector gain in dBi if you wanted.

Note that dBi refers only to the birghtest "lobe" in the antenna (mirror/lens) radiation
pattern - averaged over all directions in space an antenna cannot have gain, it can
only focus that power in prefered directions, not create more power.

So dBm is something we measure in power like 20dBm = 100mW and dBi in something like
range diagram:


Fig. 1.1 360` Clear range signal (example 1km)


Fig. 1.2 Single way angle signal (example 3km only in one direction)

(The the data from the photos it's just for demonstration not real data)

Some questions:

  1. How can i know above what dBi the antenna start transmit in a single direction but not
    a 360 as the Fig. 1.1.

  2. I got on my nRF24 a 3 dBm antenna and the manufacture say that i can put a maximum 20 dBm
    antenna, how can i know is the 20dBm and 20dBi the same power?
    (When i go to buy a antenna there is only about dBi what i only know is dBm)

About the nRF's they gonna be transcivers, the gonna transmit in both directions.

How can i know above what dBi the antenna start transmit in a single direction but not
a 360 as the Fig. 1.1.

Because it will give you that diagram in the data sheet. That polar diagram will be different for every different antenna design even those with the maximum being the same.

I got on my nRF24 a 3 dBm antenna

No you haven’t, you have been repeatedly told there is no such thing as a 3dBm antenna, it is a meaningless concept please forget about this. You can have a 3dBi antenna, is that what you mean?

About the nRF’s they gonna be transcivers, the gonna transmit in both directions.

So?
The antenna gain applies equally for receiving and transmitting. So if you have a 3dBi antenna on both the transmitter and receiver the overall gain of the system is 6dB.