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During a range testing for Ra-02 Ai SX1278 LORA, I noticed a strange behavior. Following are the settings that I used. Tested distance is 1200m line of sight. We tested the setup for different antenna combinations. All antennas are same type and have a 2dbi gain.

Tx Antenna. Rx Antenna RSSI. SNR
433 MHz 433 MHz -130 -10
915 MHz 915 MHz -110 4
2.4 GHz 2.4 GHz -108 8

These are the configuration settings.

   433.0,                     // freq
   31.25,                     // bw  
      9,                      // sf  
      7,                     // cr   
      RADIOLIB_SX127X_SYNC_WORD, // syncWord
      17,                        // power   2 - 17dbm
      8,                         // preambleLength 
      0);                        // gain  

As you can understand, our modulation frequency is 433MHz so we should get a higher RSSI and SNR with the 433MHz antenna but we get a higher gain for high frequency antennas. Does anyone has a proper explanation for this?

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  • I have fixed the formatting a bit, but I’m not sure if the 2.4 GHz SNR should be 8 or .8. Doesn’t change things much though.
    – jcaron
    Commented May 2 at 18:20
  • Can you provide the antenna specifications or at least the references? Does your line of sight include no obstructions in the Fresnel zone? The line of sight should be at least 14 meters above ground at the midpoint for instance. Were the tests performed at the same time? No differences in weather or other ambient conditions? Given the gain, I suppose the antennas are dipoles, did they all use the same orientation? No differences in cabling either?
    – jcaron
    Commented May 2 at 18:25
  • Antenna type - passion-radio.com/vhf-uhf/mini-433-straight-323.html Yes, all the tests were performed at the same time. An additional fact, is we conducted tests for a shorter distance of 4m, at an indoor area and we noticed the same effect. Commented May 4 at 7:55

1 Answer 1

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I doubt the three antennas are equivalent (though you only gave a link to one of the three): the quarter-wavelengths for the 3 frequencies are around 17.3, 8.2 and 3.1 cm respectively, so while the latter two can be regular straight dipoles and stay around 10 cm, a 433 MHz antenna of that size necessarily uses a coil or a helix structure, which probably results in different gains and/or radiation patterns.

I couldn't find a real data sheet for that antenna (with radiation patterns), but a similar antenna shows an average gain of -4.8 dB, very far from 2 dB. I think someone just thought “this looks like a dipole antenna so the gain should be 2.2 dB”, but without a detailed radiation pattern to support it you can’t really trust it.

I think you'd be better off with a real quarter-wavelength antenna (would be quite a bit larger, though, but still manageable). Make sure when you buy an antenna that it comes with a real datasheet like the one above with real figures and radiation patterns.

With that antenna, you can try to make sure the two antennas are exactly within the same perpendicular plane (i.e. if they are exactly vertical, that they are exactly at the same height), that may help, depending on the actual radiation patterns, but don’t expect anything magical: to get the same performance, lower frequencies need larger antennas.

Depending on your use case (point-to-point or point-to-multipoint, fixed or mobile...) and your region (which has an influence of max EIRP) directional antennas may be another option.

Also don't forget that at 433 MHz the Fresnel zone is quite large (14 m radius at midpoint), and you need that much space without obstacles around your line of sight for optimal results.

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