Not in the code, that is what al the tx_lut entries in the config are for, so to be compliant we should not be using an identical global config supplied by TTN for all hardware but one tuned to the specific hardware.
- I have done a google search “antenna_gain global_conf.json” and found that all is using antenna_gain in dBi (none in dBd).
- In a Semtech doc, also antenna_gain in dBi.
- Here is an interesting one:
“…antenna_gain – configure the installed antenna gain if the network server sends EIRP value with downlinks…”
Unfortunately I have no idea how a network server works. I am curious how the txpk JSON object is created by the network server which contains the key “powe” which is the requested TX output power.
For now my (temporally) conclusion is antenna_gain is in dBi, because the code is saying so as well as other gateway manufacturers and I have not found any manufacturer using dBd for antenna_gain.
@TonySmith, as far as I know the LUT (board dependant) and the antenna_gain makes sure that you stay below the regulatory limit.
@kersing, during my research I have noticed that concentrators based on the Semtech SX1301 reference designs uses the same LUT power settings. The global config supplied by TTN uses the same. So your remark confirms my suspicion. But the LUT settings should be provided by the board manufacturers and I have the feeling they just use the same settings. I have no idea if this is correct or not.
I know of at least one gateway for which the vendor provides different settings.
What I meant, there is no assumption the antenna has a specific gain, (say 4dBi) that was also included in the equation. eg if this example was the case then the equation would look something like (actual transmit power = transmitter power - 4dBi - “antenna gain”)
You are absolutely correct the config file should be make/model specific.
The decibel (symbol: dB) expresses two values of power to another on a logarithmic scale. Wikipedia
In telecommunications power is compared to a refrence that is normally 1 mW (in 50 ohm when talking about RF) the aditional letter at dB (e.g. dBm) points to the reference used for expressing the comparison number. In the case of 1 mW reference it is dBm.
When looking to antenna’s, antenna gain (a comparison between two antennas expressed on a logartihmic scale), gain can be compared to two references. These are the dipole- or the isotrope antenna. When the dipole is used as a reference this is noted as dBd when a Isotrope antenna is used as reference this is noted as Dbi.
The diploe antenna has a donut shape radiation pattern while the isotrope has a perfect sphere shaped radiation pattern. The isotrope is a mathematical model that only exists in theory and can never be reproduced.
The differences in gain between a isotrope and a dipole antenna in dB are:
- the gain of a diplole antenna is 2,15 dBi (see Wikipedia for quick reference)
- the gain of a isotrope antenna is -2,15 dBd
The isotrope gain is popular among manifacturers because the increases the gain of their antenna with 2,15 dB for free. It does look great for selling.From enigeering point of view it is common to use dBd because the engineer knows how the dipole behaves. Only trouble is that dBi and dBd are randomly used.
My advice is only compare antenna gain when i or d are the same in the dB. and preferrebly use dBd.
Why is dBi used in this case? That is because regulators prefer to use undisputed references like the (mathematical) isotrope antenna. Doing so they have a scientific reference and leave the calculation to the manufacturers, engineers and radioplanners.
In the case of gateways we have to fullfill the requirement that we emit no more power than the regulator allows. This is expressed in Eirp. “Effective radiated power while using an isotrope as a reference”.
Assume the limit at +14 dBm Eirp, a 5 dBd antenna and 1 dB cable loss.
- The 5 dBd antenna equals to 5 + 2,15 = 7,15 dBi gain.
The maximum power that can be applied to the antenna system is:
- 14 - 7,15 + 1 = 8,85 dB.
I hope this information helps.
Manufacturers of commercial antennas will publish data in dBd or dBi and others in both. The units they use seems to be specific to the company. Here they are marketing to RF designers who understand the difference. In the IoT market which is a combination of experienced and inexperienced, marketing in dBi makes the antenna look better (eg mine is bigger and better than yours). Therefore don’t read anything technical into people marketing in dBi.
Your not suggesting that vendors of eBay\Aliexpress, who seem to use dBi with a marked sense of excess, are not ‘technical’
Thanks, that is helpful.
Really appreciated your Dragino “how to”.
I posted some info a while back - hopefully, you can use this for your video.
N.B. Assume any antenna gain figures are dBi - it is the more “correct” way to look at things (as it is relative to a point source). People’s default choice of antenna is usually a GPA or dipole, and this is why so many instinctively base their outlook on why dBd should be used. If you like ,it could be because people are so used to their “favorite” antenna already being incorporated in the system, so the following question is usually processed “How much better is this new antenna compared to my GPA / Dipole” rather than more correctly “compared to a point source”.
In the end - its mostly about placing an antenna up high and in free space.
1) Field strength DOES NOT vary inversely per the square of the distance. So may people get this wrong. I’ve tried to explain the maths in the links._
2) Why you shouldn’t use dBm to describe antenna gain
3) How to calculate gain/loss
4) How to use antenna modelling software
5) Log math explained
So if you increase the power level or the antenna gain by 6dB, how much extra distance do you get ?
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.
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 )
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
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.
Ah OK - nice “trick” question
- 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
“Most of us would assume you get two times the distance.”
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.
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.
@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
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
I use the word ASSUMPTION intentionally because I am not 100% sure.
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.
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