Flying FPV with other people requires a little extra effort. Our video transmitters must be set up to avoid interfering with each other, causing dropped video feeds. Here are two simple rules for everyday flying—and several strategies for groups—to keep multiple quads in the air all day long.
The physics explained in this article are simplified. Our goal is to provide a basic understanding of the factors involved and enough practical information for pilots to use at the field.
Understanding the Radio Spectrum
Frequencies, Bands, and Channels
Most FPV video equipment uses the 5.8GHz ISM band. A “band” in this context is a designated range of frequencies in the radio spectrum. In the United States, amateur usage in this part of the ISM band extends from 5650MHz to 5925MHz. Other countries designate the bands slightly differently, so be sure to check the regulations in your area. For most people, not every frequency on a video transmitter is legal to use! This is especially true for the newer 48- and 72-channel transmitters. Get your amateur radio operator’s license to better understand local regulations and the general theory of radio communication.
Your video transmitter (VTx) also uses the term “band”. There are a number of “bands” that video transmitters use, which are often labeled “A”, “B”, “E”, “F (Fat Shark)”, or “R (Raceband)”. In this context, “band” has a completely different meaning: a band is a collection of individual frequencies. These frequencies are called “channels”. All VTx equipment has a table of the frequency that each band and channel corresponds to. Most of the time, it looks like this:
Setting your VTx to “R1” is equivalent to broadcasting on 5658MHz. If you could tune your transmitter to broadcast on 5695MHz, you would be broadcasting on “R2”. Basically, VTx bands and channels are shortcuts to these frequencies.
The specific VTx bands and channels listed above are very common, but aren’t officially standardized. Manufacturers decide whether or not they want to follow what others are doing. It’s generally good for everyone when they do, but sometimes they take liberties to make equipment easier to manufacture or sell. We’ve seen equipment with alternate frequencies in some of the channels or bands listed by different letters. Because of this, it’s always best to understand the actual frequency value you’re using. Your “A1” may not be the same as the next person’s.
Power and Distance
Video transmitters use a certain amount of power to generate their signal. Common amounts are 25mW, 200mW, and 600mW. Higher-powered transmitters allow pilots to fly behind obstacles or further away, but also more easily cause interference with other signals. Use the lowest transmission power necessary. For an open field, a 25mW transmitter may be enough. Many race events limit transmitters to 250mW or less.
Another important factor in using video transmitters is physical distance. The distance between transmitters often makes little difference, but the distance from any transmitter to a receiver has a large impact. A transmitter that’s broadcasting near a receiver has a much higher strength than one that’s further away. This relationship generally follows the inverse square law: each time you increase the distance by about half, the signal strength is reduced by half. (Or, inverted: the signal strength is doubled when you reduce the distance by about a third.) If you carry your transmitter right past where a pilot is sitting, you will have increased your signal strength relative to their receiver several times over! Especially with high-power transmitters (600+ mW) this can easily overpower a receiver with interference and make them blind.
Separation and Harmonics
A broadcast signal makes use of other frequencies on either side of the one that’s selected. The number of frequencies that are covered are the signal’s bandwidth. It’s generally accepted that on typical equipment, our analog video bandwidth is roughly 30MHz. This means that in order to use two frequencies at the same time, you need roughly 30MHz difference between them. (30MHz is only a rough guide, but it’s a good number to remember as a safe starting point.) The difference between two frequencies is often called “separation”. You can reduce the amount of separation needed by using antennas with opposite polarity, but that’s a discussion for another article.
Frequencies with specific mathematic relationships to others can also cause harmonic interference. This type of interference occurs even with a large separation. Thankfully, most VTx bands/channels are set up in a way that avoids harmonic interference, but 5665MHz and 5905MHz (E3 and E6) are known to have this issue.
It should now be easy to see how the VTx bands and channels can conflict with each other. In some places, the separation between channels on different bands is as little as 1MHz! If you power up a video transmitter on A1 (5865MHz) while your friend is flying on B8 (5866MHz), your transmitter (which is right nearby) will surely overpower and interfere with theirs (which is much further away)—knocking out their video feed and potentially causing a crash.
Frequency Management for Pilots
It’s the responsibility of everyone with a video transmitter to be considerate of others using the spectrum. Just two simple things can make sure you’re not causing problems that make people upset.
One: Always Follow This Simple Rule
Commit this to memory: Do not power on a video transmitter when another pilot is already in the air. If nobody is flying, any interference you create won’t cause someone to crash. Following this rule prevents mishaps from forgetfulness, malfunctioning equipment, or any other reason. Everyone who wants to fly together can power on at once before takeoff, then check that their video feed is clear. Once everyone is clear, you are all ready to fly. This is a required step for racing but is easy to put into practice any time you are just out with friends. It may save you and your friends hundreds in crash repair costs. Just do it!
Two: Come Prepared
This one is also pretty simple: You must know what frequencies you have access to and how to change to them. The best way to ensure this is to have the user manual for your VTx with you at the field. If you’re unfamiliar with your equipment, you’ll have everything you need to get help from someone. Even if you are familiar with it, it’s unlikely that you will remember the entire frequency table by heart. Solve this forever, right now: Find your manual and pack it in with your other gear.
We’ve already discussed how equipment might not have its bands labeled the same as others, or channels may map to different frequencies. This makes the frequency table in the manual invaluable: it might be unique to you. Many video transmitters are built to be extremely light weight, so the user interface is just one button or set of DIP switches. Sometimes there aren’t any labels. Only your user manual can tell you how to change frequencies. As a whole, miniquad transmitters are not very intuitive to learn and each one is different. Sometimes, equipment doesn’t even come with a printed manual—you must go online and download it.
To go one better, learn and know your own equipment well. You should be able to find out what setting you are already on and how to change it. Try making changes at home until you are familiar with the process. We highly recommend that you build your quad in a way that gives you access to change channels easily.
Frequency Management for Events
As an event organizer, it’s your job to make sure each pilot gets a channel assignment that doesn’t conflict with others. There are many different systems that have been developed to do this. Find one that works for your group, or come up with your own. Here are some of the popular options:
Software systems exist which can calculate (what they consider to be) the best frequency assignments. Some work entirely from one computer while others are used through apps or websites. This can be a very efficient method to make assignments and distribute them to pilots, and is often incorporated into race management software. Automated systems have a lot of overhead which makes them less desirable for informal meetings. In some cases, these services require an internet connection for every user—that can be a tough hurdle when some of your users don’t have smartphones or the flight area has no data coverage. (A frequent problem for us!) Two examples of automated systems are MultiGP‘s RaceSync app and FPV Race Tracker, a Google Chrome extension. Systems originally designed for R/C car racing can sometimes be used for video frequency management as well.
Channel Cards or Flags
These are physical flags or cards with a frequency written on each. A pilot is only allowed to fly when they possess one of these, and must use that specific frequency. By creating a physical representation of the frequency band, it’s very obvious who is assigned to what. With this approach, a channel isn’t made available again unless a pilot returns the card/flag to a central location—that means occasionally chasing people down to get them to return it.
A run group is a set of people that must always fly together on the same day. They may be free to decide how to manage frequencies by themselves. Because each group does not fly with others, they only need to resolve frequency use within their own group. If pilots want or need to fly with other people (for example, in different classes on a race day,) you may need to reorganize the groups. This erodes the simplicity of the system.
First Come, First Served
At very large events where communication between participants can be difficult, pilots might claim a frequency for a certain period of time. Once chosen, other participants must choose a different frequency or time. This puts more responsibility on the individual pilot to figure out how to fit themselves in, and requires having a system in place to keep track of the claimed frequencies and schedule.
With this method, assign each pilot a frequency before he or she arrives. Pilots can’t fly at the same time as someone sharing the same frequency, but can fly with anyone else. It allows quite a bit of freedom during the day. To use this method, pre-event planning is required. You need to know what each person’s equipment is capable of in order to make assignments. You also need to communicate what frequency each pilot is assigned to and who else is sharing it.
We use this system for our community races, because we’re already collecting information with the race registration form. Registration lets us know how many people to expect and what classes they are participating in, which is necessary to set up race heats. We use an online spreadsheet to translate information we get from the registration form into a quick list of which groups a user has available and what classes they will fly in. All this up-front work is rewarded by a smooth race day.
To simplify which bands/channels can be used, the spectrum can be broken up into groups which each cover a range of frequencies. For example, we used 7 groups last year. If you are assigned to “Group C”, you can set your equipment on F5, A3, or B6—your choice (or whatever your equipment can handle.) Grouping is often applied in addition to other management strategies.
Things to Watch Out For
If you’ve followed all of the above advice and are still having issues, there may be something else going on. Faulty equipment can distort a signal, causing it to broadcast on the wrong frequency, the wrong power, or cause other kinds of interference. There may be other users nearby (wireless network equipment often uses the 5.8GHz band) or something in the environment (high-voltage lines) that causes interference as well. Even when using a proven management strategy you may have to make adjustments before everyone is clear. Have everyone power on and check for a clear signal before you take off.
What About Control Interference?
The Spektrum, FrSky, Flysky, and other control systems we use in our hobby operate on a different frequency band (2.4GHz) so they don’t interfere with video. These sophisticated radio control systems do not interfere with each other because they employ “frequency-hopping spread spectrum”: a technique which uses a wide bandwidth to become very resistant to interference and allow others to use the same bandwidth at the same time.
Several companies are developing new methods to overcome video interference. It may not be too long before we start to see affordable, mainstream video equipment where frequency selection is no longer a challenge. Until then, it’s up to each of us to make sure flying with others remains a pleasant experience.