Thursday, 18 October 2012

FPV Basics

For anyone wanting to get into FPV, my advice is always to start simple and then build from there. Too many people expect too much from their first system and it almost always ends badly when newcomers set out with the goal of achieving too much with their first system. If it's your first time trying FPV then it's all about the KISS principle and simple is definitely better.

Before we get to the nitty-gritty of putting together a basic FPV system I just want to mention the 'S' word. As with all aspects of this hobby, it is important to always think about safety and with the addition of the FPV equipment the are additional safety considerations to take into account. Certain things like range checking become a lot more crucial, there are a lot more batteries to check before flight and you also have to plan your flight carefully before setting off. Think before you fly!

The rules for FPV vary from country to country, so check what they are in your country so that you stay on the right side of the law. Some countries require that you fly using a buddy cable, some countries require you to have a spotter and almost all countries don't allow beyond visual line of sight flying.

Which Frequency?
Not all frequencies are legal in every country, so it is important to check that whatever frequency you decide to go for is legal to use in your country. There are 4 frequency bands that are currently used for FPV and depending on where you live in the world and what frequency you already use for flying, will decide which frequency will work best for you.

In a lot of countries 900MHz is used for mobile telecoms, so is not available, but it is quite a nice option if it is available where you live. The pros of 900MHz is that because of the laws of physics, it will in theory provide the best range for the lowest power output and is much better at penetrating objects such as trees and buildings. The downside of 900MHz is that the antennas are huge, the receivers aren't very sensitive and the picture quality is the worst because it has the lowest video bandwidth.

In many countries there is a legal frequency at 1280MHz that can be used for FPV. The good thing with 1.3GHz is that like 900MHz, it has good range at low power and relatively good penetration of objects. The antennas are a bit smaller, but still relatively large and the receivers are reasonably sensitive and the picture quality is ok. The cons with 1.3GHz is that the band can be a bit congested and the second harmonic is at 2.4GHz, so it can interfere with 2.4GHz radio systems and requires a low pass filter in order to work with a 2.4GHz radio system. Also, the third harmonic of UHF long range systems sits right on the band and can drastically reduce video range. Be careful when purchasing systems advertised as 1.2GHz as many transmit on frequencies that are used for civilian aircraft DME (distance measuring equipment) equipment and you can get into big trouble for using these!

2.4GHz is perhaps the frequency best suited for FPV use as it is legal just about all over the world and thanks mainly to wifi, the receivers are very sensitive and the antenna designs well advanced. The video bandwidth is pretty good, so should allow a good quality of picture. The negatives are that you can't use a 2.4GHz video transmitter with a 2.4GHz radio control system, which are currently the most popular systems for flying RC aircraft. In urban areas 2.4GHz is very congested due to wifi networks and 2.4GHz doesn't have the same penetration as 1.3GHz or 900MHz.

Apart from one quite important thing, 5.8GHz is a great frequency for FPV. The antennas are nice and small, so easy to fit on the smallest of models, it's legal in most countries, has great video bandwidth for the best picture quality and has sensitive receivers and it works perfectly with 2.4GHz radio systems. The one very big downside however is that it really suffers from video drop-outs if you don't have line of sight all the time and also suffers quite badly from multipath fading effect, so circularly polarized antennas are a must. It will also give the shortest range of all the frequencies.

Here is a link to a great video from Alex AKA IBCrazy, who invented a few of the most popular antennas that people currently use for FPV, where he explains pretty much all of what I've just said above.

How to be successful in FPV - Frequency selection from C. Alex Greve on Vimeo.

How much power?
So you've decided which frequency to go for, now you need to decide what sort of power transmitter you are going to buy. Again, there are different rules in different countries and the legal maximum power limits are usually very low. You can use high gain directional antennas and still achieve reasonable range whilst keeping within the legal power limits. If you do decide to go for higher power transmitters then in my opinion, regardless of frequency, with modern antenna design anything over 500mW is overkill for FPV. It is far better to use the lowest power possible for what you want to achieve.

Gain and polarisation
These are 2 words that you will come across a lot in FPV and it's important to have a basic understanding of what they mean and how they affect video range. When we talk about gain, which is typically expressed as dBi, describes how well the antenna converts input power into radio waves headed in a specified direction. As a receiving antenna, the figure describes how well the antenna converts radio waves into electrical power.  The higher the gain, the more efficient the antenna is, but with higher gain also comes more directionality, which can be a problem when used for FPV.

I like to think of it like a torch with an adjustable beam. A high gain antenna is like the spot light which produces a very tight beam but only lights up a very specific spot on the wall, but can shine a long way. A lower gain antenna is the opposite and is more like a flood light which lights up the whole room, but doesn't light up something a long distance away. If this was the beam of the antenna then you can imagine how hard it would be to try to stay flying in this narrow beam and you wouldn't be able to fly behind it, but if you could then you can fly a long way.

Polarisation is the direction in which the radio waves travel with respect to the Earth's surface. There's no need to know in depth radio theory, although it can make interesting bedtime reading, but knowing a little about this is really useful for FPV. There are 2 types of antennas that we use for FPV, linear polarized and circular polarized.

The type of antenna that usually comes with your FPV transmitter (rubber ducky) is a linear polarized antenna. With these types of antenna it's really important that you keep the orientation of the transmitter and receiver antenna the same otherwise there is a significant loss of signal.

The circular polarized antenna is a more advanced design of antenna and works in all orientations, which is great for FPV use as the signal will not drop when you bank and turn or fly upside down. There are 2 types of circular polarisation, left hand and right hand and the most commonly used is right hand polarisation, so make sure you purchase one of these. The other advantage with circular polarisation is that it overcomes the issue of multipath fading effect.

Here is a nice video that explains gain and polarisation really well.

Antennas 101 - Polarization, Diversity & Gain Patterns from Joop Media on Vimeo.

Now that we know a bit more about the various frequencies, antenna gain and polarisation, lets take a look at what we need to put together a basic FPV system. The components we need are: some sort of camera, a video transmitter, a video receiver, a pair of antennas and a video display device. For these basic components I would advise purchasing good quality equipment right from the start, because even as you add more complexity to your FPV system you will be able to stick with these. In my next article I will recommend specific components, but in this article I just want to give a basic overview of what to look out for.

FPV Camera
Most people use a CCTV camera for FPV, mostly because they are lightweight, inexpensive and have good light handling abilities and because they output an analogue signal there is no video lag. You can of course use other cameras such as the GoPro as the main FPV camera, but the wide angle lens makes it very difficult for proximity flying and there is a slight lag in the video output. On the subject of lenses, it's good to pick a lens that gives you a natural field of view and somewhere around 3-6mm seems to be the most popular with many pilots and most FPV cameras come with a 3.6mm lens. Not all cameras are created equal, so a few key features to look out for are WDR (Wide Dynamic Range) and Pixim cameras offer really great performance. With the analogue video transmitters that we use for FPV, the maximum resolution that they can transmit is 480 TV lines, so even if a camera is described as being 700 TV lines high definition, what we receive will still only be standard definition.

FPV Transmitter & Receiver (often written as Vtx & Vrx)
A few years ago the only solution was using adapted CCTV surveillance devices for FPV use, but nowadays there are dedicated FPV retailers that stock FPV equipment that has been tried and tested for our use, so it is no longer as much of a guessing game when it comes to finding a good package. Don't be tempted to try and get  a cheap set from one of the many wholesale sites like dealextreme as it can be really hit and miss as to whether you get a good set or not. Transmitters that come with a built in voltage regulator are great, because they allow you to connect them directly to the main flight battery without the need of a power filter. For the receiver, look for the advertised sensitivity as this can make a massive difference to your range. As dBm is a logarithmic scale, a change of 3dBm equates to a doubling in range, so a receiver with a sensitivity of -85dBm will have half the range of a receiver with a sensitivity of one with -88dBm.

Note: Never power the transmitter without the antenna attached as you will burn it out and keep an eye on the temperature of the transmitter when doing any ground tests as they can get quite hot without the cooling air that it would get in the air.

Video Goggles, Screen or Laptop
Personally, I feel that to get a truly immersive experience then video goggles are the only option. For your initial FPV setup, this will probably be the single biggest investment so it's important to get a good set as this really makes a big difference to the enjoyment of the whole FPV experience. Most FPV goggles at the moment come with a resolution of 640x480 and I wouldn't recommend going for the lower resolution goggles that are available. Some have built in video receivers, which are great if you just want to go for a quick flight, but I've found that the built-in receivers aren't as sensitive and you have to check that they are compatible with your video transmitter (more on this later). Nice features to have are adjustable IPD (inter pupil distance) the ability to wear glasses with them and good light blocking to make it possible to fly on a sunny day.

A cheaper option is to use a video screen that accepts a composite input or even a PC monitor using a composite to VGA adapter. The important feature for screens, apart from running off 12 volts is that they don't do the 'blue/black screen of death'. This feature is common on many screens and instead of displaying static when the video sync signal is lost, they go to a blue or black standby screen which can take several seconds to recover from. Again, try to find a screen that has a resolution of at least 640x480 to make it a more enjoyable experience.

The third option is to try and use your Laptop as an FPV monitor. This has the added benefit of also being to act as a ground recorder to record your flights. There are however 2 big issues that I have with using a Laptop, the first being that you have to find a suitable converter such as easycap and secondly you have to rely on your computer not suddenly crashing. If you can manage to get easycap to work on your computer (lots of problems with driver compatibility) then a good program that allows recording and full screen live video is virtualDUB. You need a pretty powerful computer for this though. Don't forget to turn off any screensaver or auto shut-down and turn your wi-fi off or it could affect your video if flying on 2.4GHz.

This is an area that can be a total minefield for newbies and if you don't pick a good combination for the transmitter and receiver then your range is going to be very limited. First you have to make sure that you choose an antenna that is designed for your chosen frequency and you can't use a 5.8GHz antenna on a 1.2GHz system or vice-versa. One thing that absolutely doesn't work is high gain omni antennas, so avoid these like the plague. The current trend is to use circular polarised antennas and these have proven to be a bit of a revelation, especially at 5.8GHz. A 3 lobe Skew Planar Wheel (SPW) antenna on the transmitter coupled with either a 4 lobe Clover Leaf (CL) or a higher gain Helical antenna on the receiver works amazingly well.

I mentioned earlier on about compatibility issues between different brands of equipment and just want to expand on this a bit more. Some brands of FPV equipment use airwave modules and others use skyRF modules, which although work on the same frequency band actually use different frequencies within the band. If you don't purchase the receiver and transmitter as a package, make sure that the frequencies match, even if they're the same brand. The other thing that may cause problems is the antenna connectors and you need to make sure you get the correct connector for your Vtx/Vrx as it is sometimes possible to mate the wrong types and the centre pin won't actually be mating.

In my next article I'm going to be doing a 'What to buy' article where I will look at the pros and cons of various components, make recommendations of what to purchase and provide a list of quality FPV retailers.

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