Author Topic: Maximum RFM9x LoRa Tx power? [is 23dbm actually feasible?]  (Read 2073 times)

WhiteHare

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Maximum RFM9x LoRa Tx power? [is 23dbm actually feasible?]
« on: December 24, 2015, 03:17:54 PM »
The RFM9x datasheet seems to imply that 20dbm at 1% duty cycle is the maximum Tx power setting for RFM9x, yet the documentation in the RadioHead .h file suggests (with caveats) it may be possible to get as high as 23dbm:

/// We have made some actual power measurements against
/// programmed power for Anarduino MiniWirelessLoRa (which has RFM96W-433Mhz installed)
/// - MiniWirelessLoRa RFM96W-433Mhz, USB power
/// - 30cm RG316 soldered direct to RFM96W module ANT and GND
/// - SMA connector
/// - 12db attenuator
/// - SMA connector
/// - MiniKits AD8307 HF/VHF Power Head (calibrated against Rohde&Schwartz 806.2020 test set)
/// - Tektronix TDS220 scope to measure the Vout from power head
/// \code
/// Program power           Measured Power
///    dBm                         dBm
///      5                           5
///      7                           7
///      9                           8
///     11                          11
///     13                          13
///     15                          15
///     17                          16
///     19                          18
///     20                          20
///     21                          21
///     22                          22
///     23                          23
/// \endcode
/// (Caution: we dont claim laboratory accuracy for these measurements)
/// You would not expect to get anywhere near these powers to air with a simple 1/4 wavelength wire antenna.

I'm leary to try it, so I thought I'd first inquire: is it possible to get any meaningful extra range  from it at those higher settings?

Also, what sort of antenna would be a lot better than "a simple 1/4 wavelength wire antenna" at getting "these powers to air"?

hatchett

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Re: Maximum RFM9x LoRa Tx power? [is 23dbm actually feasible?]
« Reply #1 on: February 04, 2016, 06:25:46 PM »
Im not sure what they mean by duty cycle, the Lora signal is a constant envelope signal. I simply pass the word "lora" back and fourth every 10 seconds. because its low and slow it takes a few hundred milliseconds to send the complete message. One of the things i did do was measure the input impedance on the SMA connector i have installed on the board, as you would imagine it is not 50ohms and it is reactive. If i can figure out how to use some of the non-canned modem settings i might see just how sensitive it can be. As it stands right now the range is not really close to the FSK radios we make that run at around 500bps at 20dbm. i suspect i am doing something wrong, i just have not figured it out yet.

WhiteHare

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Re: Maximum RFM9x LoRa Tx power? [is 23dbm actually feasible?]
« Reply #2 on: February 05, 2016, 05:37:55 AM »
Try inserting this line at the end of your setup procedure:

Code: [Select]
  rf95.printRegisters();  //Print all the RFM95 register values
That way you can check your assumption that you actually did change the Tx power amount.

I think the 1% duty cycle is just a crude proxy for chip temperature, which is maybe why Semtech doesn't elaborate.  If you're running at the highest power levels, your chip will definitely get hot if the duty cycle is too high.