2.4GHz Radio Link has been used for a very long time for Remote Control toys and equipment such as drones. Reason being that 2.4G has good bandwidth and frequency allocation to accommodate multiple transmitter and receivers in an area without causing interference. 2.4G transmitters provide a decent range, a Flysky i6 transmitter provides few hundred meters and the FrSky transmitter provide less than a mile. The FrSky R9 900MHz System is a long-range system designed to provide almost 10KM of range!

In this article, we will see why we might require such a system and its pros & cons.

 

What are the benefits and disadvantages of 2.4G radio systems?

Like mentioned in the introduction, 2.4G radio systems provide a lot of bandwidth which is necessary to send across more data but it is not the main reason why people have chosen this frequency. The main reasons are that 2.4G is considered as an open license-free frequency in most parts of the world and it has less interference to nearby devices, which could also be other pilots.

Back in the day when 27 and 72 MHz were more common, only a few pilots could fly at one point of time because it would cause interference to the person flying next to you as each channel are separated by a very small frequency spacing due to the lack of bandwidth. This is when people started switching to 2.4G radios as you could have dozens of pilots together without causing interferences to the next person.

The main disadvantages of 2.4G are that it lacks good range, that is unless you have a transmitter with very high output power. In most cases, you aren’t going to get more than 1 mile of range with stock transmitter modules in your radio (~100mW).

This isn’t much of an issue in proximity flying or in races but for freestyle pilots or long-range flights, this will be something that holds them back from reaching the full potential of their flight rigs.

To overcome this issue, radio manufactures started using the ~900MHz system, another license-free (mostly) spectrum that is used.

 

How does the 900MHz system overcome the shortcomings of 2.4G?

You might be wondering why are we going back to a lower frequency system when we have already learnt about its drawbacks like the lack of bandwidth etc. Let us find the answer!

The 900 MHz system (868/915MHz depending on which country you live) is also open to being used in a low power system without requiring any license to operate it. But the main advantage is that, since it is a lower frequency, it can penetrate more through objects and have a longer range than the higher frequency 2.4G systems, because lower the frequency - longer the range and penetration at the same output power.

The drawback would be the obvious reason of being limited to the number of people using the 900MHz system in the same area at once. But even with this, you can have dozens of people without any issue which is why it is also used by racers since they can do long-range and proximity racing as well.

Another major drawback would be the length of the antennas. Since the wavelength of 900MHz systems are higher (Wavelength 1/F, where F is frequency), the antennas need to be made longer as well. The length of the antenna of a 900MHz system is almost 3 times the length of a 2.4G system. This can make it tricky to mount it on a drone with limited space, such as whoop.

Another advantage of the 900MHz system is that the manufacturer usually provides more than one level of power output, so in case you want to fly close by, you can put it on 25mW and if you need to go long range, you can put it on 1W or 2W depending on the modules.

The FrSky R9 900MHz system can do from 25mW to 1W of power. And at 1W of power, you can expect almost 10KM (or even more!)  in light of sight flights, which is amazing!

Not just that but you get tiny receivers as well, which also good in case you have no space in your quad for the bigger receivers. It is  (R9MM) even smaller than an XM+ receiver!

 

In this article, we have briefly seen about the 900MHz system and what it provides over the 2.4G system. We will look into this topic at a deeper level in the upcoming articles where we will talk more in-depth about the FrSky R9 900MHz system.