If you always fly your drone alone, you might not know what frequency management is. But as you start flying with other pilots, frequency management becomes very important for all pilots to enjoy the best video feed. So how do you manage VTX frequency to fly in a group?
To fly FPV drones in a group, list down the VTX frequencies that everyone can operate. Use a calculator to determine the frequencies to use and avoid, so that IMD is minimized. Split the pilots into smaller groups if required. Common combinations are “R1, R2, R6, R7”, “R1, R2, F2, F4, E5”, and “R1, R2, F2, F4, R7/F8, R8”.
In the field of FPV drone racing, we rely on the video transmitter (VTX) to feed the video receiver on our FPV goggles with signals that are transformed into what we see. However, the video transmitter and the video receiver may unexpectedly cause interference due to overlapping frequencies between drones.
Hence, the frequency used by different pilots needs to be managed if they want to fly simultaneously. This will reduce any static, interference, or complete loss of connection. But how is it exactly managed? What exactly causes this interference to begin with? Continue reading to understand more.
Understanding the Terminologies
Signal transmission in FPV setups is commonly adjusted using the “frequency” utilized by the drone. A frequency is a certain point within the radio spectrum used to send or receive signals to generate the video feed within your FPV goggles.
However, despite having several frequencies that a pilot can utilize to operate their FPV setup, the selection is limited by the Federal Communications Commission (FCC) regulations (or equivalent in other countries), possibilities of interference, and hardware limitations.
As such, available frequencies are grouped in certain sets known as “bands.” There are only 8 frequencies in each band that can be used by a pilot. These are known as “channels.”
The definition of band differs depending on the part of the FPV setup in mind. When talking about FPV video equipment in general, a “band” is known as the frequency wherein the equipment can efficiently function. Most, if not all setups, utilize the 5.8 GHz ISM band – making it a little harder to ensure that there will be no interference once all pilots are operating their FPV equipment.
On the other hand, a band in a VTX context refers to the set of frequencies or channels that a transmitter can utilize to manage the video feed. These are commonly labeled as “A,” “B,” “E,” “F (FatShark),” and “R (Raceband)”, and there are only eight channels available for each band. Recently released VTXs support multiple bands – providing a greater variety of options available for frequency management.
Available Frequencies and Limitations
Due to the FCC guideline, the only available frequencies for FPV drone racing are within the 5.8 GHz ISM band, which can then be defined as the frequencies around 5.65 to 5.95 GHz.
It was implemented as a regulatory restriction because other functionalities utilize the radio spectrum as well for their communication. This includes aeronautical navigation, mobile signals, fixed satellites, radiolocation, and other functionalities that might interfere or be interfered with by the signal from your drone.
The table shows the frequencies available for each band and channel within the 5.8 GHz band.
Some VTXs may offer bands labeled as “D”, “L,” “U,” and “O” as an additional configuration for your setup, increasing the number of available channels for you to use. However, its significantly lower frequencies are not necessarily legal to use in the United States or most countries for that matter.
To ensure that your selected VTX is qualified to participate in your preferred FPV race tournaments, it might be wise to make sure that your available frequencies are compliant with the legal band selection.
Why Manage the Frequencies when Flying in Group?
Each and every person has a different FPV drone setup. As mentioned earlier, due to the limitation of available or legal bands, there is a tendency for their frequencies to somehow overlap or interact with each other.
Obviously you should not use a frequency that is used by another pilot. Otherwise there will be interference which causes you to lose your video feed or see each others’ videos. You would want the spacing between the 2 frequencies to be at least 35-40 MHz.
Two different frequencies could form a new frequency that is unsupported by either one of the two drones causing it, resulting in static in the video feed transmission or occasional interferences that could impede the pilot’s vision during the race.
This phenomenon is also known as Intermodulation Distortion or IMD. It usually occurs in drone races due to the insufficiency or total absence of frequency management – a technique utilized to ensure that each pilot’s channel is sufficiently-spaced to avoid IMD.
Determining if An IMD will Occur
There is a formula used to determine if an IMD will occur between different channels. Do note that calculating these probabilities will only be efficient in a small group of either 3 or 4 pilots. Operating with more pilots could make this method confusing.
(F1 x 2) – F2 = Fx
This formula utilizes 2 frequencies: F1 and F2 being your reference frequencies. The resulting Fx is the frequency that you should avoid if you have F1 and F2 currently running. Then, reverse F1 and F2 in the formula to calculate the frequency to avoid from another end.
For instance, you plan to fly with 2 more pilots. Say your friends are using Fatshark channel 1 (5740 MHz) and 4 (5800 MHz).
5800 x 2 – 5740 = 5860
5740 x 2 – 5800 = 5680
Both 5860 and 5680 MHz (which corresponds to Fatshark channel 7) are channels to avoid.
Let’s say you want to use 5840 MHz. Do the same calculation comparing the frequencies of your 2 friends.
5840 x 2 – 5740 = 5940
5740 x 2 – 5840 = 5640
5840 x 2 – 5800 = 5880
5800 x 2 – 5840 = 5760
None of the resulting frequencies correspond to the frequencies used by your friend so you are good to go.
However, note that 5740 used by one of your friends is only 20 MHz away from the 5760 (the channel to avoid) and your channel (5840) is only 20 MHz away from 5860 (the other channel to avoid). Hence, there might still be some level of IMD because of this. Ideally, you want to be at least 35 MHz away for the channels to avoid.
Manual calculation for frequency management takes a lot of time and can be prone to error. I recommend using this tool instead. Simply key in the available frequencies (separated by comma or space) and you can find the best frequency combinations that cause the least IMD.
As a general rule of thumb, evenly spaced frequencies will likely cause problems and should be avoided. For instance, there are 4 pilots, each using Raceband at channel 1, 3, 5, and 7 respectively. The channels are considered evenly spaced. In contrast, channels 1, 3, 6, 7 are not evenly spaced, and result in the least IMD compared to a group of evenly spaced channels.
VTX Frequency Management: How to Do It?
Considering all the factors at hand, vtx frequency management should be performed using the following steps.
Before you start flying in a group, everyone should provide the details of available frequencies and bands their drones can operate on. Doing so will allow you to group the pilots accordingly and prevent interference during the flight.
The most basic rule in a drone race or group flight is not to turn on your drone when you are not flying. This will ensure that ongoing races or flights will not encounter any problems that could potentially affect the drones of the participants.
In races, the event organizer may use “channel cards’ that could determine the available frequencies a pilot can operate. This will ensure that certain frequencies will be locked once it is being utilized, making it easier for the organizer to track which are being used and which are still idle.
Avoid Residential Areas
Residential areas, apart from being a hazardous place to hold a racing in, will undoubtedly contain Wi-Fi routers. Certain Wi-Fi routers may also operate under the 5.8 GHz band – causing interference to a drone’s video feed when it is positioned at a relatively close area.
Consequently, a drone’s VTX may also affect a Wi-Fi router’s speed and could probably be the cause of any signal drops when you are operating your drone at home. Considering how operating far from residential areas is beneficial for all, it is better to utilize a venue that is a significant distance away from highly populated areas.
When there are more than 6 pilots flying together, it is increasingly difficult to completely avoid interference. It is still doable though, by changing the antennas.
Mixing left-handed and right-handed circularly polarized antennas in a flying group can reduce interference. Left handed signals do not interfere with right handed signals. If half of the pilots are using LHCP while another half are using RHCP, you have the least interference.
Having said that, it is unlikely that anyone would own both LHCP and RHCP antennas. What you can do is to pay attention to the handedness of antennas that other pilots use. In your next upgrade, buy the one that is least popular.
In case this is not obvious, you should use only 25 mW output power when flying in groups. 25 mW is the factory setting for all VTXs. Using anything higher would requires a ham radio license in most countries.
While higher power output gives you clearer video feed, it also increases interference. Use higher power output if you are flying alone.
Proper management of the frequencies being utilized in a certain race is essentially the holy grail of all drone races. As such, knowing the concept behind these interferences may be more than beneficial for you, your drone, and your safety in the entirety of the competition. Think of it as mutual responsibility, a need that weighs on both you and the team organizing the race. VTX frequency management should, and should always be, a combination of personal discipline and good management.