Video Transmitter aka VTx.
It’s the component that transmit video signal as well as the sound. The more powerful, the more it will « yell » on radio frequency which it emits on.
VTx come in multiple frequencies, 800Mhz, 1.2Ghz, 2.4Ghz and 5.8Ghz. The lower the frequency, the larger the antennas used to transmit signal. 5.8Ghz is the one used for miniquads due to small size of antenna used. The reference is the ImmersionRC 600mw, but it is loud and the Aomway 200mw works really well. The newest one is the Tramp from ImmersionRC which has selectable power output up to 1mW. I strongly recommend to power your camera off of 5v output from VTx to get very clean signal, i.e. no horizontal lines displayed in video feed.
I no longer recommend using an SMA extension cable. You would be better off screwing your antenna directly to the VTx and securing it with zip ties to the top plate.
This is your eye in the cockpit of the craft. Selection criteria are latency, image quality, weight and size. The reference is now the Sony HS1177 after the old king Sony 600TVL. A good upgrade for your HS1177 is to swap the lens with a GoPro Hero1/2 lens.
Action camera or HD camera like GoPro or Mobius cannot be used as FPV camera due to their high latency between miniquad movement and displaying image on your FPV goggles.
On some occasions, putting a FPV filter between FPV camera and battery might help with cleaning video because of voltage spikes induced by ESCs changing motor throttle. It can be custom build for few dollars with a capacitor, a ferrite ring and some copper wire scavenged from old PC power supply or from a washing machine motor in my experience.
I strongly recommend to power your camera off VTx 5v output as it does filter the signal and spare you from any FPV filter need.
The OSD (On Screen Display).
This a kind of copilot which overlay useful information on your goggles screen like battery voltage or even altitude, speed and direction to come back home if it has GPS module.
The OSD super simple mini is the bare minimum to monitor your battery voltage while flying and the G-OSD 3 from HK is the most comprehensive one with its GPS module. Another compact model is the Micro minimOSD. Another OSD integrated with Power Distribution Board is the power OSD which looks interesting for compact builds. My choice is now the HUBOSD which is perfect on 4S and very economical.
Antennas and their shape play a major role in video signal feed quality. They come in pairs, one for the VTx on the quad and the other one to mount on the goggles. The reference used to be the SpiroNet but the TBS Triumph is now the best.
If you fly only in front of you, you can increase your range by swapping your clover leaf antenna with a patch antenna to mount directly on your goggles.
Video reception module aka VRx.
Action Camera or HD camera.