Intro to FPV - Antennas
Like almost everything in this hobby, antennas have their own set of jargon and the deeper you dig, the more acronyms you’ll find. But you don’t need to be an antenna expert to make good choices. In this article we’ll break down what you need to know to get the right antennas for your set up.
There are two types of polarisation for video antennas in FPV, namely linear and circular. Once upon a time linear was the only option, but Alex Greve, aka IBCrazy popularised circular polarisation or CP. The two main benefits of CP are the way it ignores reflected signals and that it gives a solid signal no matter the orientation of your quad. Linear is still used on really small or super light builds where every gram counts.
A CP antenna sends out its signal as a spiral the direction of which can be clockwise or counterclockwise depending on whether your antenna is right hand (RHCP) or left hand (LHCP) polarised. You can choose either direction, neither is better than the other, but your receiving antenna must match the one on your video transmitter because an RHCP antenna is designed to tune out a LHCP signal. This is the magic of the spiral.
When your quad is beaming out it’s omnidirectional signal it will bounce off any objects nearby and as those signals travel a less direct route they come in slightly behind the main signal. We call this multipathing and it does horrible things to your picture quality. Add in a few trees and a wall or two and your video quality really takes a dive as static and breakups over take the actually picture. When a RHCP signal bounces off an object it becomes an LHCP one and vice verse so all the bounced signals and naturally ignored. No clever electronics needed, just science!
So that’s RHCP and LHCP covered. Other terms you may have read when looking at antennas are “omni” and “patch”. An omni is what we were talking about above - it transmits and listens in all directions and is the most common video antenna type in FPV. It looks a little like a mushroom and it’s the type you will always have on your quad as during a flight your quad is going to be pointing all over the place.
When used on your goggles or ground stations the omni antenna “listens” equally all round you so whether you fly in front or behind, you can go the same distance away before you signal starts to break up. (Technically when flying behind yourself your fat head blocks a lot of the signal which is why MenaceRC make the Periscope antenna).
Typically, when you fly, you’ll be heading in mainly one direction, whether that’s the track, a interesting clump of trees or that abandoned warehouse and this is where a patch antenna comes in.
A patch has more gain, meaning it’s more sensitive but everything is a trade-off and in this case, you sacrifice sensitively for coverage. A patch has a narrow field of view but it can “see farther” in that one direction. Where an omni gives you 360 degrees of coverage, a patch is closer to 120 degrees - so you can fly much further out in front of you but as soon as you go behind yourself your video will start to break up pretty quickly.
This is where diversity comes in. It’s takes the input from a patch and an omni antenna and monitors the signal from both, always sending the best signal to your goggles or scree and giving you the best of both worlds. Originally diversity was reserved for bulky ground stations but now a plethora of diversity modules are available for FatShark goggles.
And finally, there’s the connector type on your antenna. They can be SMA or RP-SMA (reversed polarity). Basically the same connector but with the male and female type switched over. If you mix and match you won’t get a physical connection - this means, at best, a very weak signal or, if theres no antenna properly connected at the transmitter end, a fried VTx. We try to limit confusion by only carrying SMA antennas but we do carry adapters that let you use an SMA antenna on a RP-SMA VTx or receiver. If you aren’t sure which you have, heck the image below. As a rule of thumb, cheaper Chinese VTx’s are typically RP-SMA while transmitters from TBS and ImmersionRC are SMA.
As always, if you have any questions or comments, leave the below.
Happy flying, Simon