Antenna_gain in dBi or dBd?


#1

I am making a tutorial where I explain almost every parameter in the global_conf.json file.
There is one parameter which I find confusing. That is antenna_gain.

Looking in the packet_forwarder code (https://github.com/Lora-net/packet_forwarder) and global_conf.json file it seems that the antenna_gain must be specified in dBi.

The antenna_gain is used to adjust the TX power:
txpkt.rf_power = (int8_t)json_value_get_number(val) - antenna_gain;

Normally I would say the code is the documentation and stick with antenna_gain is in dBi.

However I think it is in dBd, but I can be wrong.

I have gone thru all submitted topics about this subject in TTN forum.
To my surprise some say antenna_gain is in dBi and some say antenna_gain is in dBd.
Unfortunately I could not find any documentation about this which is of course not surprising :frowning:
I also searched the Semtech forum.
PS: I know what the difference is between dBi and dBd and also ERP and EIRP.

My question is: Is the antenna_gain in dBi or dBd.


The BIG and SMALL ANTENNA topic part 2
#2

Since the difference between dBi and dBd is only a mere 2 dB, which is almost negligible compared to all other effects on the power budget, I wouldn’t worry about it :wink:


(Jeff Uk) #3

A ‘mere 2dB’ can be the difference between regulatory compliance and a large fine or possibly jail time depending on jurisdiction and therefore I would not treat so lightly! :slight_smile: …just saying…


#4

@rharte, I am making a YouTube tutorial and I need to be accurate. If I do not explain this correctly my viewers will send me comments about it. They are very observant. And I am also very curious what the correct answer is.


#5

Ah, well but then there is antenna cable attenuation, antenna mismatch/reflectivity, power variation over frequency and temperature, etc. There are many factors not taken into consideration that can add up to much more that 2 dB. What if that brings you jail time? :wink:


(LoRaTracker) #6

Exactly.

No point in a tutorial if you point out the limits and then say; “but dont worry about them”.


(Jeff Uk) #7

No excuses! :wink: You are required to take this into account and adjust off chip Tx pwr vs EIRP accordingly. Here in Europe allowance is for +14dBm allowing for a 2dBi Ant (16dBm effective) but reality is cable and connector losses etc mean you invariably come in lower…but if Ant gain higher, set Tx pwr higher or losses less than expected then you may exceed so need to demonstrate reasonable care. And I guess this brings us back to @robertlie 's original question :slight_smile:


(Kastayou) #8

Here is a link that you might find useful for your question:


(Tony Smith) #9

@robertlie, I’d like to suggest its neither dBi or dBd but just dB. The variable is designed to reduce transmit power when required and this is done in the units of dB. Now I hear you ask, how do I work out how much reduction do I require? Well this is the combination of the transmitter board you have (in dBm), less the cable and connector losses plus the gain of the antenna. Now all of this must be below a regulatory limit. (30dBm EIRP in AU915, 14dBm EIRP on AS923 and 27dBm EIRP on EU868). So when you’ve performed the calculation, the power variable allows you to reduce Tx power to stay within the limit.


(Jac Kersing) #10

Did a quick google on the difference of the two and as dBd seems to be the gain compared to a standard dipole. And dBi = dBd + 2.15.

Then a look at the packet forwarder sources:
The antenna_gain is subtracted from the RF transmission power specified by the back-end in the packet forwarder.
The resulting value is used to program the transmission power of the SX1257.

Looking at it all:
I expect the circuit design of the specific gateway will have a huge bearing on the power output (any gain/loss in the gateway) so for some gateways it might well be dBd and for others dBi??
(I am not a HF specialist so I might very well be mistaken)

EDIT:
I think @TonySmith hit the nail on its head…


(Tony Smith) #11

Thanks for confirming, this is what I suspected the packet forwarder/back-end is doing. This also indicates there is no allowance for a default type of antenna in the code as well.


(Jac Kersing) #12

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.


#13
  1. I have done a google search “antenna_gain global_conf.json” and found that all is using antenna_gain in dBi (none in dBd).
  2. In a Semtech doc, also antenna_gain in dBi.
    https://www.semtech.com/uploads/documents/picocell_gateway_user_guide.pdf
  3. Here is an interesting one:
    “…antenna_gain – configure the installed antenna gain if the network server sends EIRP value with downlinks…”
    http://www.multitech.net/developer/software/lora/conduit-mlinux-convert-to-basic-packet-forwarder/

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.


(Jac Kersing) #14

I know of at least one gateway for which the vendor provides different settings.


(Tony Smith) #15

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.


(Remko) #16

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.


(Tony Smith) #17

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.


(LoRaTracker) #18

Your not suggesting that vendors of eBay\Aliexpress, who seem to use dBi with a marked sense of excess, are not ‘technical’


#19

Thanks, that is helpful.


#20

Hi Robert,
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