Antenna_gain in dBi or dBd?


(LoRaTracker) #21

So if you increase the power level or the antenna gain by 6dB, how much extra distance do you get ?


#22

Stuart - if you read my linked post to the end, it tries to answer that question. I’m guessing you still think the math is wrong and that it should be x4 or something - because 6dB is mentioned - otherwise, why would you ask this question?

OK - this is off topic I know, but hopefully it is interesting for those that are interested in this sort of thing - Think of it like this:
If the distance is doubled, the field strength is halved but the power is quatered.

Consider a resistor (R) with voltage (V) across it which has a current (I) through the resistor.
everyone knows that power can be calculated as VI (or you can use V(squared) / R )

Now consider what happens if you halve the voltage across the resistor…You also find that the current is halved through the resistor and this is why the power is quartered.
Now turn this around and then you can see why it is the way it is.
Field strength is measured in volts per metre - it is NOT measured via power flux - look at the UOM

To put it another way. If you look at the link below, consider that all an RX antenna does, is “harvest” the alternating current that is induced by the alternating electrical field passing across it.
http://spiff.rit.edu/classes/ast613/lectures/radio_ii/radio_ii.html

The wierd thing is that there is no electrical charge moving between TX and RX antennae, so how is it all working and why do the calcs work even though there is no flow of charge? (its almost like thinking about how the transporter works in Star trek :slight_smile: )
If you want to blow your mind, consider that every antenna “system” is really just a very big capacitor with 2 “plates” (one to ground - the other to air), with the receiving antenna being in the vicinity of the alternating electrical field set up between the transmitting antenna and ground! (this is why and how RFID works in transmitting info back to the TX antenna - it varies the load that is “experienced” by the TX antenna - even though there is no direct connection between RFID tag and TX antenna)

No real “current” is flowing between the TX and RX antenna, but all the calcs work if you assume it does as per Maxwells equations regarding displacement current


(LoRaTracker) #23

I am not questioning the math or the detail of you post, but just asking a question, if you you increase the power level or the antenna gain by 6dB, how much extra distance do you get ?

Most of us would assume you get two times the distance.


#24

Ah OK - nice “trick” question :wink:

  • Increasing the antenna gain by 6dB should double the distance
  • Increasing the power level by 6dB is unlikely to double the distance, unless you increase the power at the node accordingly

edit - definition of trick question


“…or because the answer that seems obvious is not the correct one.”

I would say this phrase from the questioner indicates a hidden difficulty really - but hey ho :smile:
“Most of us would assume you get two times the distance.”


(LoRaTracker) #25

No trick, by ‘power level’ I did mean increading the transmit power by 6dB, from 10dBm to 16dBm for instance.

However, what I was really commenting on was your comment (which I cannot see any more) about people ‘missusing’ the inverse square law.


(Harald Naumann) #26

Robert, in US and Asia the gain of the antenna is will indicate the antenna gain in dBi. In Germany, some manufacturers quote the gain in dBd. 0 dBd corresponds to 2.15 dBi. Monopole antennas usually do not offer antenna gain. External antenna gain is always associated with a directional effect. An antenna is a passive component and cannot amplify. Antennas for the LoRaWAN gateway with 5 dBi are common. At 5 dBi the opening angle is already smaller. 5 dBi <> 5 dBi. The 5 dBi refer to the peak values and the beam can look very unequal. Therefore you should always consider the directional diagram of the antenna. In the LoRaWAN Node, one generally does not reach more than 2.15 dBi. You don’t want a directivity but radiation in all directions. Usually it is even less because losses occur in the enclosure, in the matching network or in large batteries nearby. With GSM 900 with 33 dBm one is glad if one achieves 27 dBm radiated power. We are usually at 30 dBm. With LoRaWAN it is the same game. 6 dB are quickly lost. With a loss of only 3 dB we are well served.


(Tony Smith) #28

@robertlie, Hi Robert, just watched your latest LoraWan tutorial on YouTube. Can highly recommend as it is a really good deep dive on the parameter to set up a gateway. Can now see where the discussion in this thread ended up being used. https://www.youtube.com/watch?v=WaWca5QfFp0


#29

@TonySmith, I am still not 100% sure about it. It all depends what information is send by the network server. I am studying the loraserver.io code and hopefully that might answer my question.

In my presentation page 73:


I said the following:
“…In this tutorial the ASSUMPTION is made that the
network server sends EIRP values and not ERP
values…”

I use the word ASSUMPTION intentionally because I am not 100% sure.


(Tony Smith) #30

Even if you find dBi quoted in the code, its common for people to get the units of measure wrong when using dB. Its not uncommon to see an antenna’s parameter in dB instead of dBi or dBd. Didn’t realise you also publish a pdf of the tutorial. Well done.