Meshtastic - Calculate Max TX Power

​I have taken an interest in Meshtastic (An open source, off-grid, decentralized, mesh network built to run on affordable, low-power devices). For more information see: meshtastic.org/

I have a couple of device and have been playing around with swapping out antennas. One of the things to take into consideration is configuring the correct transmission power for the device, antenna and frequencies I am using.​

In this post I will provide a quick overview on how to calculate the Max TX Power in dBm.

Jargon buster:

• ​dBm (dB milliWatt) - A signal strength or power level
• dBW or dBW - The decibel watt is a unit for the measurement of the strength of a signal expressed in decibels relative to one watt. 
• EIRP - Effective Isotropic Radiated Power
• Attenuation  - Loss e.g. cables, connectors
• Gain - in a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction. In a receiving antenna, the gain describes how well the antenna converts radio waves arriving from a specified direction into electrical power.

For this example I will use the New Zealand regulations for General User Radio Licence for Short Range Devices: gazette.govt.nz/notice/id/2022-go3100

EIRP
It includes the following details:
Low (MHz):  915.0000
High (MHz): 928.0000
Maximum Power dBW e.i.r.p.: 0.0

As I prefer to calculate using EIRP in dBm, the EIRP requires a simple conversion.

dBm=dBW + 30
30dBm= 0.0dBw + 30
Total EIRP = 30 dBm

GAIN
To find the TX antenna gain it's easiest to get it from the antenna specification. 
For example I am using GIZONT 17cm antenna with a 10dBi gain.

​ATTENUATION
​As my devices have the antenna connected to the device motherboard I am going to assume the attenuation  (Loss e.g. cables, connectors) to be effectively 0dB.

To calculate the Max TX power:
Max TX power = Total EIRP - TX antenna gain + Attenuation
20dBm = 30dBm - 10dBi - 0dB

Maximum Effective Isotropic Radiated Power