Flying a quadcopter by watching it from the ground is limited and soon dull. The transformation that made small multirotors a sport is first person view, FPV, in which a camera on the craft sends its picture to goggles on the pilot's face, so the pilot flies as though sitting in the cockpit. It is a second radio link, separate from the control link of the last lesson, and it is engineered around a priority that surprises newcomers: not picture quality, but latency, the delay between the world changing and the pilot seeing it. A gorgeous picture that arrives a fifth of a second late is unflyable, because the pilot is always reacting to where the craft was, not where it is, so every part of the FPV chain is chosen to shave delay first and improve the image second.
The camera, built for speed and contrast
An FPV camera is not a photography camera. It is small, light, and tuned for two things the pilot needs and a phone camera does not prioritise. The first is low latency: the sensor reads out and the picture leaves the camera in a handful of milliseconds, where a consumer camera may buffer and process for far longer. The second is dynamic range, the ability to show detail in both the bright sky and the shadowed ground at once, because a pilot bursting out of a dark doorway into sunlight must not be blinded by a white-out while the craft is still moving. A wide-angle lens gives the broad field of view that lets a pilot see where the craft is going and judge gaps, at the cost of a fish-eye distortion that pilots simply learn to read. Everything about the camera serves flying, not filming, and the cinematic footage a craft carries is usually shot on a separate, heavier camera that does not feed the goggles.
Analog and digital, a real choice
The video link comes in two families, and unlike most of the earlier either-or choices, both are alive and sensible for different pilots. Analog video, the older kind, sends the picture as a continuously varying signal much like old broadcast television. Its picture is soft and full of static, but it has two virtues that keep it in service. Its latency is almost nothing, a few milliseconds, because there is no digital processing to do, and it fails gracefully: as the signal weakens the picture gets grainier and rolls with static but stays readable, warning the pilot to turn back while the craft is still flyable. Digital video sends a compressed digital stream and gives a sharp, clean picture, but it costs a little more latency to encode and decode, and it fails at a cliff: it holds a perfect image right up to the edge of its range and then freezes or blacks out entirely, with far less warning. A racer near the edge of range often still prefers analog for its graceful failure and minimal delay; a pilot who values the clear picture and flies within range chooses digital. Neither is simply better, and knowing why is knowing what to build.
The video transmitter, power and channels
On the craft sits the video transmitter, the VTX, which broadcasts the camera's picture, almost always on the 5.8 GHz band. Two of its settings matter. Its power, from a few tens of milliwatts to over a watt, sets how far the picture reaches, and more is not always better: a high-power transmitter close to the craft's own receiver, or to another pilot flying nearby, drowns out weaker signals, so pilots run the least power that holds a clean picture. Its channel is the specific frequency within the band, and this is where flying with others becomes a coordination problem. Two craft on the same or neighbouring channels interfere and both pilots see static, so a group flying together assigns channels across the band like guests seated apart at a table, and a pilot who powers up on an occupied channel ruins another's flight. The 5.8 GHz band has room for a handful of pilots at once if they space out, and channel planning is simply the etiquette that makes that work.
Example. A pilot flying analog notices the picture breaking into static and rolling bands as they push to the far side of a field, well before it becomes unwatchable. What should they do, and how would a digital system have behaved?
The static is analog's graceful warning that the link budget is running out, so the pilot turns back while the picture is still readable and the craft controllable. A digital system might have shown a flawless picture in the same spot and then frozen without warning a little further out, giving the pilot no cue to turn until the view was simply gone.
Now you. Two friends arrive to fly together, both with 5.8 GHz analog gear, and both power up on the same channel. What will each see, and what is the fix?
Answer
Each sees the two pictures interfering as heavy static, often unflyable. The fix is to move to widely spaced channels, one near the low end of the band and one near the high end, so their signals do not overlap.
Antennas, and why they are circular
The antennas on an FPV link are usually not simple whips but oddly shaped circularly polarised ones, and the reason is a neat piece of physics with a real payoff. A radio wave has an orientation, its polarisation, and a straight antenna receives best when its orientation matches the transmitter's. A quadcopter banks and flips constantly, so a straight antenna on it would fade to nothing every time the craft rolled ninety degrees. A circularly polarised antenna twists the wave as it travels, and a matched pair keeps a steady link through any craft angle, which is why the picture does not black out when the craft rolls. The twist also helps with multipath, the ghosting caused by the signal arriving both directly and bounced off the ground, because a reflection reverses the twist and the receiving antenna rejects it. The strange corkscrew shapes on FPV gear are earning that steadiness.
The whole chain, measured end to end
What the pilot experiences is the sum of every delay from the lens to the goggle screen, called glass to glass latency: the camera's readout, the transmitter's encoding if digital, the flight through the air, the receiver and the goggle display. A good analog system runs this in well under twenty milliseconds and a good digital one somewhat more, and the number matters because it adds directly to the pilot's own reaction time. A craft that answers its controls in a few milliseconds, from the DShot and control lessons, is squandered behind a video link that shows the result a fifth of a second late. This is why the whole culture of FPV optimises latency so fiercely, and it completes the picture of the machine: a stiff quiet frame carrying a fast control loop, fed by a low-latency command link and seen through a low-latency video link. The parts are all present. The last lesson puts them together and gets the craft into the air.