# Confusion about transmit power, EIRP, ERP etc

Hello, I am doing a dissertation on LoRaWAN, using the Arduino MKRWAN 1300 to conduct a few experiments. I am using the Arduino GSM dipole antenna which came bundled with the board.

I believe the “transmit power”/Effective Radiated Power (ERP) is limited to 14 dBm (25mW) by ETSI regulations. The MKRWAN 1300 uses the SX1276 transceiver which is capable of 14 dBm by default.

On the other hand, in the LoRaWAN regional parameters doc, I have seen the table that defines different TXPower indexes:

BUT, I have seen the equation that relates EIRP and ERP as: EIRP (dBm) = ERP (dBm) + 2.15
(unless this is incorrect?..)
The antenna I am using is the standard crappy Arduino one (Dipole Pentaband Waterproof Antenna — Arduino Official Store) which allegedly has a gain of <0 dBi according to the datasheet.

So my question is: what is the ‘Max EIRP’ as shown in the table above. Is it 16.15 dB? Or perhaps just 16 dBm, as I have seen someone else say online?

Second question: if the maximum allowed antenna gain is +2.15 dBi, why is the Arduino antenna they sell you 0 dBi?

I am using ADR, but I still want to understand what the transmit power actually is. This appears to be an area of confusion for me.

Thanks for any help anyone can provide.

The maximum EIRP is 16.15dBm, but this is a theoretical value because a real isotropic antenna doesn’t exist.
A dipol has a gain of 2.15 dBi, therefore only 14 dBm output-power is allowed. But you are allowed to take the attenuation of plugs and cable into account.

And: don’t care about 0.15 dB. This is more then the measurement uncertainty of many RF-meters.

If you go to a car showroom and buy a 1.6l car, why wouldn’t they give you the 2.0l model?

They probably found a nice balance between price, size and, er, price.

There is no limit to the antenna gain.
Max. ERP: 14 dBm (16 dBm EIRP) that means 14 dBm into a dipole with 2.15 dBi gain.
If you use an antenna with 10 dBi gain, you are allowed to output only 6 dBm.
If you use an antenna with 10 dBd gain, you are allowed to output 4 dBm.
You can increase the output-power by the cable loss.

Don’t use an antenna with to much gain. Why: look at the radiation diagram.

edit: 0 dBi means that this antenna is 2.15 dB worse than a simple dipole.

btw: on 869.525 MHz 27 dBm ERP are allowed in EU.

Thank you, that clears things up.

But also, surely that means I should have to define the antenna gain I am using, either in TTN somewhere or in my Arduino code? I am currently using the ‘Send and Receive’ example sketch in the MKRWAN library:

I am using ADR, which should set the transmit power automatically, but e.g. if I were to use an antenna with 10 dBi gain, how does it know not to exceed 6 dBm transmit power?

It doesn’t know this. It is your responsibility, not to exeed the legal values.
My recommendation is not to use antennas with high gain. 6 dBi is sufficient, use the remaining 4 dB to compensate cable and connector loss.

But in my case, since I am using 0 dBi antenna currently, I want to make sure I am actually using the 14 dBm max transmit power that the SX1276 is capable of. Do you know how I could do this?

With the MKRWAN - good question - it’s not a hugely accessible module for setting stuff up. And the more open code bases don’t have an obvious route to adjust power up or down to compensate for the antenna attached.

As you are on the Arduino supplied 0dBi antenna which is on a very short lead from the uFl, you are unlikely to see any losses worth caring about so power set will be power sent, within reasonable engineering limits.