Yes, the modems are protected against damage during operation without antennas though this can significantly increase the heating of the modems and is not recommended for long periods particularly at high output power settings.
One is the minimum. Two allows for use of diversity and is recommended. Note that the antennas do not have to be the same if using two antennas on the modem.
RF Design sell a variety of antennas on our store that are compatible with the various modem products. If you wish to use a different antenna it should meet the following requirements:
The black cable of the FTDI (pin 1) should connect to pin 1 on the modem as shown in Figure 1-1.
Figure 1- 1: An FTDI cable connected to the RFD900x modem
If using the RFD900u or RFD900ux you will need to align the red wire of the 8-way adapter cables with the red wire of the FTDI (pin 3) and the black cable of the FTDI (pin 1) should connect to pin 1 on 8-way cable as shown in Figure 1-2. They can then be connected using the male-to-male pin bridge that was supplied with the 8-way cable.
Figure 1- 2: An FTDI cable connected to the RFD900ux modem via 8-way adapter cable
What do I need to upload the firmware or to change the modem configuration? The best system for changing modem settings or firmware is the RFD900x Modem Tools (see “Useful Links”). After downloading and installing the tools connect the FTDI cable to the modem and to a computer. Open the RFD900x Modem Tools to upload the desired firmware or to change the modem configuration. For full instructions on the modem tools see the RFD900x Modem Tools User Manual in “Useful Links”.
Make sure the FTDI cable is plugged in to a USB port then open the Device Manager. Check for a Ports (COM & LPT) tab, then expand the tab to look at the list of devices. To identify the COM port of your cable you can disconnect the cable and see which listing disappears or appears when you reconnect the cable. If there are no COM ports listed or are unknown devices listed in the Device Manger you most likely need to install the FTDI chip driver that is available from the FTDI website, see “Useful Links”.
The RF signal from the modems can interfere with servo operation. To minimise this:
The default settings and operations of the alternative firmware are not the same as the SiK, so it is recommended that you consult the relevant firmware manual for detailed operation. Some simple troubleshooting is outlined below.
Due to the nature of the RFD modem communications, there may be a noticeable jumpy nature to servo movement and a perceivable latency in the R/C signal.
Depending on the telemetry data requirements you can help to reduce these issues somewhat by reducing the MAX_WINDOW parameter from the default of 131 to as low as 40 depending on the air rate. This can reduce the data throughput somewhat so the user may have to experiment to find the settings that best fit their use case.
Providing a dedicated 5V and at least 1A power supply: The modem operating at full power can sometimes exceed the supply capabilities of some flight controllers or the FTDI cable, particularly if connected to a USB extension lead, unpowered hub or a low spec device with power delivery limitations. This can cause issues ranging from limiting amplifier performance to intermittent resets of the modem causing loss of link.
Line of Sight (LOS): The RFD modems are LOS devices with limited ability to get around obstacles, primarily due to the wavelength of operation. To this end operating with minimal obstructions, such as buildings, terrain, trees, etc. obscuring the LOS will improve the quality of the link.
Elevation: The modems are line of sight devices and the elevation of the antennas at both ends affects the radio horizon distance and therefore the line-of-sight range. It can also help mitigate the problems of obstructions in the line of sight and reduce the effects of multipathing. Generally speaking the more antenna elevation the better the performance.
Antenna placement: The placement of the antennas can have a large impact on performance. Antennas placed near to or against metal or carbon fibre can experience distortions to resonant frequency, gain, radiation pattern etc. which can impact performance. Further operating close to metal or carbon fibre can block signal from some directions causing connection issues.
Antenna polarity: The polarity of the antennas can have a large impact on performance. Most antennas have linear polarisation, which should be the same at both ends of the link. If the modem tilt cannot be guaranteed diversity is recommended, with the antennas at 90 degrees to each other, so that the modem can automatically select the antenna which has the best polarisation.
Coaxial Cable: Coaxial cable and adaptors can be a cause of significant losses between antenna and receiver. These losses can be minimised by using cable with an appropriate design frequency, using lower loss variants of cables such as LMR195, avoiding damaged and pinched cables, minimising the number of connector adaptors used as each adaptor will have losses and minimising the overall length of coaxial cable used.
RF Noise: The RFD modems have band filtering to reduce the noise at the radio receiver, despite this noise from other RF devices on the same band, or from electronics such as switching regulators or electronic speed controllers, can reduce the ability of the modem to receive a signal. Therefore, where possible reducing the noise in the area, can improve range. This can be achieved in many ways such as shielding noise sources, considering antenna placement, turning off interfering devices, using directional antennas etc.
It is possible to operate 2 pairs of modems in proximity, but there are trade-offs in transmit power, data loss and effective distance between antennas. The effective distance between antennas can be increased by using linearly polarised antennas at 90 degrees.
Firstly, the NetID of the two pairs should be different so that the modem pairs only receive the correct data.
Secondly, try to use non-overlapping frequency bands / RF channels.
The data loss will depend on the modem RSSI (Received Signal Strength Indicator). Typical numbers are:
|Configuration||Accepted Loss||Required RSSI|
|Overlapping channels||<10% data loss||<-30dBm RSSI|
|Non-overlapping channels||<10% data loss||<-10dBm RSSI|
|Non-overlapping channels||<1% data loss||<-25dBm RSSI|
Typically, two 3dBi antennas 1m apart will have ~26dB attenuation. If one antenna is rotated by 90 degrees, the attenuation will increase to ~36dB.
Antennas with different gain with have a proportional change in attenuation e.g. two 2 dBi antennas would have ~28dB of attenuation while two 6dBi antennas would have ~20dB attenuation and one 6dBi and one 3dBi antenna would have ~23dBi of attenuation.
Reducing Tx power by 6dB will halve the distance required between antennas for the same overall attenuation.
To get <10% data loss with non-overlapping channels (-10dBm RSSI) with antennas 1m apart and the same polarisation, we can set the Tx power to a maximum:
-10dBm + 26dB = +16dBm
+26dBm. (-10dBm + 36dB = +26dBm).
So, the user should carefully check the intended antenna set-up, the Tx power required to achieve the desired range, and the data loss permissible to use pairs of modems in proximity.
Note：The preceding guide only applies to omnidirectional type antennas. Directional antennas separation requirements will depend radiation patterns and other factors.
This indicates that the modem is stuck in bootloader mode.
RDF900x/RFD868x modem Firmware
The firmware is the same for both the 868x/900x modems and can be found at. http://files.rfdesign.com.au/firmware/
RFD TOOLS ：http://files.rfdesign.com.au/tools/
FTDI Cable documentation http://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_TTL- 232R_CABLES.pdf