In this post, we will talk about how we learned from extensive testing and meet-up races, to setup and manage video transmitters and frequency channels for quadcopter racing, making it possible to allow maximum of 8 people flying at the same time.
This article is written by Matthew from KQRC (Kent Quadcopter Racing Club). Edited by Oscar.
All the research has been done and tested over a few months at KQRC and we have had great results but there has been some learning curves. Please note that I will not touch on the legals of FPV as this is still very much a grey area. It should be noted that 25mW is the permitted power in the UK currently.
What is FPV Racing
FPV Racing has been covered here before.
Basically, FPV Racing (First Person View) is the racing between RC Models, the norm is multirotor or more commonly quadcopter (stop calling them drones LOL). There are a camera and a video link back to your screen or goggles (FPV glasses), so you have a live video feed into your goggles that you fly from.
why is VTX Channel Management So Important
So why is video transmitter (VTX) such a big thing? Well, if you can’t see then you can’t fly. Even though there are some 32 channels (even 40 channels now with race band) still are not that rosy.
To organise a smooth FPV event or FPV race, you need to consider the following regarding video transmitters:
1 – VTX Power
Here is an example of 3 VTX’s of the same brand make but different output power.
And depending on who has the most Powerful VTX, it can be a real problem as a 600mW will blat out a 25mW when only frequency separation is 20Mhz apart.
For most outdoor races, max 200mW should be allowed, and for indoor flying 25mW would be enough.
2- VTX Bands
There are a lot of channels, but not all VTX can run all these channels, some have Boscam channels, some have FatShark and now also race band is also available. Here is a table of all 5 bands, 40 channels in total.
For example here you can’t use band 1 CH1 with band 1 CH2 together in this chart. The frequency spacing is only 20Mhz and it’s just not enough. The two channels will bleed in to each other, causing frosting (snow) and Ghosting (the other person’s video) in your video feed.
Race band was designed to allow 8 pilots to fly at the same time using all 8 channels. But the channels are spaced at only 35Mhz, and we have had mixed results with this. So we moved to 40Mhz spacings and filtered out a great chart that works for most.
3 – RX Equipment
Some receivers can only get certain bands, for example the immersionRC ones normally only get FatShark channels, and others such as Boscam can get most if not all of them.
4 – Antenna
There are LHCP (left hand circularly polarized) and RHCP (right hand circularly polarized) antennas, using different aerials we have had good result but I will explain in more detail in a bit.
We ran a test when we alternate the LHCP and RHCP antennas on 25mW VTX we also have had great results. It is my understanding that a LHCP signal hits the ground or a object it becomes a RHCP signal, but when it does not bounce it stays the same.
Now by using a 25mw VTX the signal is weaker so less signal reflection, meaning less interference on the channels near by.
5 – Race Grouping
Group people to race together depends on their equipment and what channel they support.
The chart above was a original chart I used for the Boscam FatShark Channels. I took all the available channels and one by one worked out the most suitable frequency, and I also only wanted 4 FatShark and 4 Boscam at 40MHz spacing. By doing so we could maximise the number of racers to 8 and still keep a clean channel for all to race on.
However we had issue with cross interference when using the top and bottom channels such as where a blue meets 5771 to 5860 for example. So I set out the spacing as best at 40Mhz and this worked great.
Here is an improved version of the chart, we can see some channels I have red filled, which people are unable to use due to equipment limitations.
So as you can see I have 7 racers but 1 can only use 4 of the channels and 1 can use 6. So I start assigning channels based on this.
You can also see I have what antenna people are using, and again this is a deciding factor, such as Nick who has 4 channel he can use but with only a RHCP antenna, now he is limited to 2 channels, i.e. 5740 and 5820. David could use 6 channels but with only RHCP antenna, he also limited to 3 channels: 5740, 5820 and 5805.
This filtering goes on until we make a good VTX race team system, I wont even start on groupings just yet.
The best way is for everyone to have a set of LH and RH antennas. Also limit VTX power to 25mW if you are flying indoor, to prevent multipath signal issues. We use a VTX scanner that shows who is on what channel and what power VXT they are using.
As you can see I didn’t even use any Raceband channels in order to run an 8-people race, so is Raceband worth the cost to switch to?
Race band is a new thing released in 2015, and basically it’s made of 8 channels spaced at 35MHz apart, and the channels are on those frequencies that can not be received by Boscam or FatShark video receivers.
However I don’t think the 35Mhz channel spacing is enough, to allow running neighbouring channels at the same time without too much interference (e.g. ch1 with ch2), at least 40Mhz separation is needed IMO.