Everyone seems to have a good FPV signal during practice, but when it comes time to race someone always seems to have a problem. Does this sound familiar? Even if you’ve carefully made sure there’s plenty of frequency separation, you might be having problems with Intermodulation Distortion.

This article aims to provide practical information and not scientific precision. This simplified version doesn’t fully cover the physics involved, but looks at the effect only as relevant to FPV video. If you’re just getting started with multiple pilots, a better place to begin is Video Frequency Management: Keeping Multiple Quads in the Air.

What is Intermodulation Distortion?

Two waves combine to create a complex waveform that exhibits different frequencies.

When two waves (blue & green) combine, a complex waveform (red) is created that exhibits different frequencies from the original sources.

Intermodulation Distortion, or IMD, is a phenomenon where two signals broadcast together cause interference at additional frequencies. When they combine, they mix and produce a new waveform. This new wave, being a product of the two frequencies that produce it, contains additional frequencies not present in its sources. (If you just want to know how to avoid it, skip down to the next section.)

You can calculate the most important additional frequencies for a set of source signals using a fairly simple formula: (F1*2) – F2. Calculate this twice, swapping the order of F1 and F2 You’ll find two frequencies for each pair, one above and one below the set.

Let’s look at an example. You and two friends decide to keep things simple by using “F” Band. You’ve decided to use F2, F4, and F6. These correspond to 5760, 5800, and 5840MHz. As there’s adequate separation, (40MHz,) any two of these can power on together—but power on all three and you’ll have problems.

Here are the two extra frequencies that will experience interference with F2 and F4 powered on:
(5760 * 2) – 5800 = 5720
(5800 * 2) – 5760 = 5840

5840MHz is F6, so that pilot will be getting interference from the F2+F4 combination. If you continue to calculate all of the additional interference frequencies for each pair, you’ll find that F4 and F6 together cause problems for F2 as well: (5800*2) – 5840 = 5760.

We’ve set up a visual graphing tool with the formulas in place. Even if you’re very comfortable working with formulas, I recommend that you try it out and experiment with different values to help you visualize where IMD occurs. You’ll see that the interference frequencies (dotted red) are evenly spaced on either side of the two input frequencies you select. You may also want to see an explanation and demonstration of the IMD effect in this informative video by Rob Hair.

How Do We Fix It?

Bad interference during a race

Interference is more likely when flying indoors, so “tiny whoop” race organizers need to be especially sensitive to IMD effects.

Any set of evenly-spaced frequencies is going to cause problems, and the only real solution is to avoid them. Unfortunately, “set of evenly-spaced frequencies” neatly describes every band we have available, including Raceband! This is unfortunate as it’s often easy to change channels within a band, but more complicated to switch between bands.

If you’re only flying with three pilots, it should be easy to avoid IMD. Start very far apart, and then select a third channel that is NOT directly in the middle of the other two. For example, let’s say you start with F1 and F7. If you add F4, it will be the same distance from both F1 and F7 (60MHz) and cause IMD effects in both outside channels. Instead, choose F3 or F5.

With four pilots, Raceband is helpful since all channels give adequate separation. You might choose R1, R3, R6, and R7. This particular set has been tested by groups and found to be reliable. Note how neither R3 nor R6 are the same distance from any two other channels in the set. Because the frequency spacing in Raceband is even, you can think of it in terms of channel spacing instead of frequency: R1 and R3 are separated by 2 channels, R3 and R6 by 3 channels, and R3 and R7 by 4 channels. R1-3-6-8 avoids IMD with even better spacing, though some groups find R8 operates at lower power.

Once you get above four pilots, things begin to get difficult. You can’t expect to come up with a good set while out at the field, so many groups use a predefined set of frequencies. Frequency sets generated with avoiding IMD in mind are often named “IMD” followed by the number of pilots they support. For example, the standard currently used by MultiGP Championship events is known as “IMD6c”.

IMD5c is: R1 (5658), R2 (5695), F2 (5760), F4 (5800), and E5 (5885)
IMD6c is: R1 (5658), R2 (5695), F2 (5760), F4 (5800), R7/F8 (5880), and R8 (5917)

For the best possible chance of avoiding interference, use a profile that matches the number of pilots you will have flying. (5-pilot sets have less chance of interference than 6-pilot sets.) However, some organizers prefer to always run IMD 6c so they have the flexibility to add more pilots to a heat without making any other pilots change.

Getting Deeper

You might have special circumstances that prevent the above sets from working for your group, such as lacking Raceband support. Hobbyist “ET” has dug deeply into the subject and produced some excellent resources. If you’re a race organizer, make a point of reading these pages! ET provides a tool for testing and scoring different frequency sets, and an exploratory tool for generating new sets. ET’s tools are free and open source, and he has another video demonstrating IMD effects.

Beyond 6 Pilots

IMD graph tool

Spend a little time with this IMD graphing tool we set up. You’ll very quickly understand the problem much better.

If you need to get even more pilots in the air, you simply can’t get away from IMD on today’s equipment. You’ll have to turn to other strategies to get the best possible use out of the spectrum we have available:

  • Use low-power transmitters
  • Use circular polarization
  • Have alternating pilots use left and right polarization
  • Don’t allow quads to fly near the pilot area
  • Upgrade your receiver to one that better rejects IMD
  • Place an attenuator on the receiver

Wrapping Up

IMD isn’t obvious unless you’re aware of it. To help identify it at the field, remember that it generally affects multiple pilots at once, and there will be at least one pilot in between the two that are having issues. Avoid using frequencies with equal separation, and use a predefined frequency profile, (such as IMD5c or IMD6c,) when you want to fly with 5 or 6 pilots. Finally, use good equipment and only as much power as you need. With the knowledge and tools at your disposal, you should be on your way to clearer video whether racing or just out flying with friends.

You might also be interested in Video Frequency Management: Keeping Multiple Quads in the Air.

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