Transmission power

Many transmitting devices are not very efficient when it comes to generating RF. The circuitry on the radio module needs to do quite a bit of housekeeping to make sure the transmissions stay on the same frequency, that amplifiers are not exceeding certain levels and that the signal is as clean as possible when it comes out of the final power amplifiers of the radio module chip.

Even the final power amplifiers are not very efficient, simply because they have to work on a relatively high frequency. In some cases you may only get 30% efficiency. Saying that 130mA @3.3V power consumption will generate (according to P = E x I) 429mW of RF power is simply not realistic.

Unless we have precision measuring equipment (e.g. Spectrum Analyzer) we will not be able to tell the exact output power of the radio module and we can only refer to the manufacturer’s datasheet with regards to power levels.

Below is an excerpt of the datasheet for the HopeRF RFM69 radio module:

Without going through all these items, take note of the second last line that indicates the ‘Programmable Pout’ (RF Power Out):

“-18 to +20 dBm in 1dB steps”.

Due to the way the RFM69 modules are implemented in hardware, they make different use of the built in PAs (power amplifiers) on the SX1231h transceiver chip. Hence the RFM69 modules come in two main variants:

  • the RFM69W/CW can output from -18 dBm to +13 dBm
  • the RFM69HW/HCW can output from -2 dBm to +20 dBm

For more details and discussion see this forum thread and this excellent blog article.