FrSky DHT RSSI configuration

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The FrSky DHT is a flexible and relatively powerful 2.4 GHz transmitter. The kit comes as a DIY package with an RP-SMA connector which makes it easy to switch antennas. The communication is two-way allowing for both Tx and Rx signal strength indication (RSSI). The ACCST system shifts 80 channels around hundred times per second in the 2.4GHz band, similar to FSK.

I will use an D6FR receiver in this setup.

Serial adapter

To connect to the transmitter you need a USB-to-serial adapter. FrSky makes one with the necessary pin-header.

Frsky usb cable.jpg

Plug the black header into the corresponding four pins on the transmitter. Power will be provided by the USB-port.

RSSI configuration

The DHT provides a serial data port for configuration of the alarms for the RSSI and the two analog inputs. Download the 3rd-party configuration program and the required drivers.

Frsky config.png

Antenna testing

To get an idea of how well the different set of antennas would perform, I swapped the antennas one by one and recorded the Tx and Rx values from the configuration application.

All the tests where performed with a FPV VTX 1180 MHz without low pass filter running 20 cm away from the receiver and everything was done indoors with antennas about 12 meters apart in separate rooms.

  • Transmitter: FrSky 2dbi rubber duck antenna
    • Receiver: FrSky 2dbi rubber duck antenna - RSSI Tx: 37 - RSSI Rx: 66 - Without VTX Tx 62 Rx 70
    • Receiver: Generic Alfa 5dbi rubber duck antenna - RSSI Tx: 52 - RSSI Rx: 71 - Without VTX Tx 67 Rx 71
    • Receiver: Dipole 33 mm wire antenna - RSSI Tx: 41 - RSSI Rx: 74
    • Receiver: Dipole 32 mm wire antenna - RSSI Tx: 49 - RSSI Rx: 73
    • Receiver: Dipole 30 mm wire antenna - RSSI Tx: 62 - RSSI Rx: 78 - Without VTX Tx 59 Rx 70
    • Receiver: Dipole 29 mm wire antenna - RSSI Tx: 54 - RSSI Rx: 74
    • Receiver: Dipole 29 mm wire antenna - RSSI Tx: 54 - RSSI Rx: 74
    • Receiver: Dipole 27 mm wire antenna - RSSI Tx: 51 - RSSI Rx: 74
    • Receiver: Generic Alfa 2.4 GHz 14 dBi patch antenna - RSSI Tx: 73 - RSSI Rx: 79 - control
  • Transmitter: Generic Alfa 5dbi rubber duck antenna
    • Receiver: FrSky 2dbi rubber duck antenna - RSSI Tx: 37 - RSSI Rx: 54 - Without VTX Tx 62 Rx 71
    • Receiver: Dipole 33 mm wire antenna - RSSI Tx: 52 - RSSI Rx: 78
    • Receiver: Dipole 32 mm wire antenna - RSSI Tx: 41 - RSSI Rx: 74
    • Receiver: Dipole 30 mm wire antenna - RSSI Tx: 57 - RSSI Rx: 77 - Without VTX Tx 64 Rx 73
    • Receiver: Dipole 29 mm wire antenna - RSSI Tx: 49 - RSSI Rx: 74
    • Receiver: Dipole 27 mm wire antenna - RSSI Tx: 54 - RSSI Rx: 68
  • Transmitter: Generic Alfa 2.4 GHz 14 dBi patch antenna
    • Receiver: FrSky 2dbi rubber duck antenna - RSSI Tx: 73 - RSSI Rx: 83 - Without VTX Tx 75 Rx 82
    • Receiver: Generic 5dbi rubber duck antenna - RSSI Tx: 78 - RSSI Rx: 87 - Without VTX Tx 79 Rx 82
    • Receiver: Dipole 33 mm wire antenna - RSSI Tx: 74 - RSSI Rx: 83
    • Receiver: Dipole 32 mm wire antenna - RSSI Tx: 73 - RSSI Rx: 82
    • Receiver: Dipole 30 mm wire antenna - RSSI Tx: 74 - RSSI Rx: 81 - Without VTX Tx 67 Rx 77
    • Receiver: Dipole 29 mm wire antenna - RSSI Tx: 74 - RSSI Rx: 82
    • Receiver: Dipole 27 mm wire antenna - RSSI Tx: 74 - RSSI Rx: 82
  • Transmitter: Generic Dealextreme 2.4 GHz 14 dBi flat panel antenna
    • Receiver: FrSky 2dbi rubber duck antenna - RSSI Tx: 73 - RSSI Rx: 82 - Without VTX Tx 76 Rx 82
    • Receiver: Generic 5dbi rubber duck antenna - RSSI Tx: 78 - RSSI Rx: 85 - Without VTX Tx 79 Rx 85
    • Receiver: Dipole 33 mm wire antenna - RSSI Tx: 73 - RSSI Rx: 81 - holding the panel 90 degrees to the side to match the vertical dipole polarity
    • Receiver: Dipole 32 mm wire antenna - RSSI Tx: 73 - RSSI Rx: 82
    • Receiver: Dipole 30 mm wire antenna - RSSI Tx: 72 - RSSI Rx: 80 - Without VTX Tx 75 Rx 82
    • Receiver: Dipole 29 mm wire antenna - RSSI Tx: 76 - RSSI Rx: 83
    • Receiver: Dipole 27 mm wire antenna - RSSI Tx: 74 - RSSI Rx: 82

As a side-note, the dipole wire antenna should have an exposed core with the length: 299792458/1000/2440/4*0.95 = 29.18 mm (speed of light/1000/middle of band/quater wavelength*velocity in conductor)