The 2.4GHz spectrum has been in use for wireless communication for a long time and it was natural that RC radio links also trickle down to use that spectrum due to its high resistance to interference and additional bandwidth for multiple pilots to fly at once.  The FrSky R9M 900Mhz system and the Team Black Sheep Crossfire 900Mhz system has been around for a while now. The FrSky R9M 900Mhz system has been a centre of controversy ever since its release because Team Black Sheep has classified the R9M system as a “clone” due to its similar structure, working and specifics. But keeping aside the politics, let us see which one is the better one for you. In this two-part series, we will first see what the difference between 2.4GHz systems and 900MHz are and then in the second series, we will look into which 900MHz system is suited for you.

What are the advantages and drawbacks of 2.4G radio systems?

Like mentioned within the introduction, 2.4G radio systems provide tons of bandwidth which is important when trying to send across more data but it's not the main reason why people have chosen this frequency. The most reasons are that 2.4G is taken into account as an open license-free frequency in most parts of the planet and it's less interference to nearby devices, which could even be other pilots.

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

The main disadvantages of two 2.4G are that it lacks good range, that's unless you've got a transmitter with very high output power. In most cases, you aren’t getting to get quite the 1 mile of range you are supposed to with stock transmitter modules in your radio (~100mW).

This isn’t much of a problem in proximity flying or in races except for freestyle pilots or long-range flights, this may be something that holds them back from reaching the complete potential of their flight rigs.

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

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

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

The 900 MHz system (868/915MHz counting on which country you live) is additionally hospitable getting used during a low power grid without requiring any license to work it. But the most advantage is that, since it's a lower frequency, it can penetrate more through objects and have an extended range than the upper-frequency 2.4G systems, because lower the frequency - longer the range and penetration at an equivalent output power 

The drawback would be the apparent reason for being limited to the number of individuals using the 900MHz system within the same area directly. But even with this, you'll have dozens of individuals with none issue which is why it's also employed by racers since they will do long-range and proximity racing also.

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 got to be made longer also. The length of the antenna of a 900MHz system is nearly 3 times the length of a 2.4G system. this will make it tricky to mount it on a drone with limited space, like a whoop.

Another advantage of the 900MHz system is that the manufacturer usually provides quite one level of power output, so just in case you would like to fly accessible, you'll put it on 25mW and if you would like to travel long-range, you'll put it on 1W or 2W counting on the modules.

The FrSky R9M 900MHz system can do from 25mW to 1W of power. And at 1W of power, you'll expect almost 10KM (or even more!) in light of sight flights, which is amazing! The TBS Crossfire full version can do up to a boggling 2W off power(which is illegal in most places!)

Not just that but you get tiny receivers also, which also good just in case you've got no space in your quad for the larger receivers. it's (R9MM) even smaller than an XM+ receiver!

This concludes part 1 of the two series blog. Stay tuned for the second part :)