Welcome to the world of FPV. In this blog we will dive into the specifics of analogue video transmission. All drones that are flown via video link require two things onboard, a fpv camera and a fpv transmitter. We use some common terms for these in short hand like cam and vtx, tx being short for transmission/transmitter.
The simplest form of video transmission for FPV is analogue video, and its transmitted in a radio frequency band. There are legal bandwidths and power settings for amateur use and the common one used by FPV drones is 5.8 gigahertz, often written as 5.8GHz or 5G8 etc. There is a bunch of specific frequencies allocated for video transmission in this band, some more info on picking a frequency, particularly when flying with multiple pilots can be found here.
To put it simply, once your channel and Band has been selected your transmitter will attempt to broadcast a signal on that specific frequency. What it broadcasts is what it receives from the camera. So as long as your camera is working, connected to your video transmitter which has its antenna on and is also powered you should be broadcasting an image. There are various ways to troubleshoot problems here, but generally if you have the correct channel on your video receiving device and only have a black screen you may have a problem with your camera, or if you have static/snow you may have a problem with your video transmitter.
One very important aspect of video transmitters is that they don't like to run without an appropriate fpv antenna on. Its vital that the antenna is designed for the correct frequency, is continuous (not damaged or incorrectly connected) and for best reception it's best that it is positioned away from obstructions like the frame and the battery. If this is not adhered to your performance can be reduced significantly.
There are many different types of connector types, SMA, MMCX, UfL etc and these describe the different types of industry standard plugs that are used to connect the antennas to the vtx. Each vtx manufacturer will pick a specific type of connector depending on their design type/purpose, with the MMCX and Ufl types becoming more popular to reduce weight. It's also common to connect a "pigtail" to the vtx, which often has an MMCX or Ufl on one end and a SMA on the other. The advantage here is the SMA can be hard mounted on the frame somewhere relieving strain on the smaller, more delicate connector, and allowing for the quick removal or replacement of the antenna.
Antenna performance is measured in gain (decibels, or dbi) and is generally linked to directionality. As a general rule, as the gain increases the region in which it is active stretches. This has the effect of increasing its sensitivity or ability to receive/transmit in one axis while limiting it in another. The image below gives a good two dimensional illustration.
We refer to them as a unidirectional antenna (low gain, uniform field) and a directional antenna (high gain, directional field). The unidirectional antenna is best suited for use on the aircraft, since it is often changing its orientation and location while moving. The directional antenna is often incorporated on the receiving end in a diversity arrangement with another unidirectional antenna. With diversity on the receiver you can use both types of antennas to implement the best of both worlds, and this is the common way to increase range capabilities. High gain fpv antennas are often named according to their styles. Patch, helical, crosshair and yagi are a few different varieties.
Which brings us to the receiving end.
There are a few different ways to display the received image, they all depend on how you would like to view it. The overwhelming majority choose fatshark goggles for a complete immersive experience however displaying onto a screen is still useful in a variety of applications. Your video receiver is tuned into the correct channel and sends the analogue signal out to the display device (RCA). While it is possible to display it on laptops and mobile phones with an extra attachment it is generally a bad idea, can introduce latency and can suffer from the device freezing up. Ok for viewing as a spectator, but not usually for a pilot.
