What do these numbers have in common?

27, 35, 459, 868, 2400


No, this isn’t part of an IQ test; these are R/C aircraft bands we can use legally in the UK, so yes, I want to write a little bit about R/C radio, RF modules and dual-band operation.

My first radio was a simple Horizon two channel set on 27MHz and I’m talking decades ago. It was made in Kempston, Beds, I believe. It was a set that had multiple problems and went back a few times. Radio was expensive in those days and a two channel set was all I could afford. After those early radio problems I had a series of other 27 and 35MHz sets: Macgregor, Skyleader, Robbe, Futaba and Multiplex and recently a modern FrSky or two. I had bought a module for the 35MHz Multiplex 3030 set and this gave me my first good experiences on 2.4GHz. Modules are what I am writing about in this piece.

I don’t think any of us oldies have looked back since the introduction of 2.4GHz; no more shoot downs, pegboards and peg-waiting in order to fly. Trouble is, while 2.4 is great, it is not perfect. There are a couple of reasons for this, it is pretty much line of sight and does not like going through or round obstacles in its path. Anyone with a telemetry set will tell you that someone simply walking in front of you can reduce the received signal; let alone a drop below the level of the hill top or behind the little hump on the Beacon’s top.

The second reason is carbon fibre, especially in fuselages. Last year I put my Aresti 2M in on the front of the slope; I attributed the crash to the all-carbon nature of the model and a failsafe. High frequency radio waves do not penetrate carbon very well, since it can act like a Faraday Cage around a receiver, hence the so called ‘2.4 safe’ fuselages. These are fuzzes where the nose of a model is deliberately left void of carbon to ‘allow’ radio waves to reach the receiver. Carbon fibre flying surfaces can also blank incoming signals and cause problems/failsafes.

Signal Strength

Even if you have a radio that gives you RSSI this is not necessarily a fair indicator of control (i.e. it is not an empirical measure of dBm). RSSI on model sets is generally just a number from 0-100, the higher number you receive the better… well, yes and no. Generally anything above 50 is good, FrSky defaults are 45 and 42 and remember this is a logarithmic scale. If the noise floor is high then the RSSI may appear high as well but not necessarily because of your transmitter signal. That RSSI value is could be telling you the strength of all signals resonating in your receiver antenna including background stuff on the same band. While this is not a problem for us in the open countryside flying, in a built up area surrounded by houses like Hockcliffe, I imagine, and all that Wifi can be dodgy. Everything these days seems to use 2.4GHz. My Wifi uses it as does my printer; I have a radio doorbell and an electric garage door opener. I know my brother has three Wifi routers going at the same time; then there’s all that Bluetooth in the ćther as well. Just switch on your computer and see all your neighbours’ Wifi signals. This is the background noise that your tiny receiver has to sift through to get its personal instructions.

Just one hop 865.295MHz in 863-869 ISM band, note the noise floor.                              

A better measure of performance is LQ or Link Quality. Not all radios have this, I know Jeti radio does and so does one of my modules, the EzUHF; the TBS Crossfire system has it as well. This is a better measure of received signal, effectively taking into account lost packets of data. The difference between RSSI and LQ is akin what I used to use when I was a radio amateur years ago. ”I am receiving you 5 and 9” i.e. Readability (like LQ) on a scale of 1 to 5 and Signal Strength (like RSSI) on a scale of 1 to 9. Enough, I digress.

So a rhetorical ‘why change from 2.4GHz’. The simple answer is that you don’t have to change if you’re flying close by, just a couple of hundred metres away in the countryside. If on the other hand someone flies carbon models, large scale sailplanes, F5J, Jet Turbines, GPS Triangle, Cross Country at huge distances and heights or, dare I say it, FPV or quad racing through trees then a case can be made for another frequency band.

FPV, or rather long range FPV, has largely been the reason for interest in frequencies other than 2.4GHz for model flying. Some of those practicing long range FPV weren’t happy with the 1.5 km range offered by 2.4 sets and wanted to get further. Going down in frequency helps increase range and penetration through vegetation. Initially they turned to 433MHz, a frequency available to modellers in many parts of the world; at a power of 1 or 2 watts FPVers could reach out many, many kilometres. In the UK we still have the 459MHz frequency available to us (a low 100mW) though only one R/C firm offers us this choice as far as I know, EzUHF. I have such a module. It puts out 100mW on 459MHz and will travel at least 4km. Like 2.4 this is also a frequency hopping system. (Anyone remember the Reftec 459 radio in the 70s btw?)


The latest trend to come is to operate in the 868/915MHz ISM band. I have the FrSky R9M module that enables me to use 868MHz, the UK part of the band. While only 25mW output is allowed with telemetry, 500mW can be used without telemetry in one part of the band. The TBS Crossfire module uses this frequency as does the new very expensive Jeti DS-24 dual band radio ($2100). This should go a long way even at 25mW. The Jeti MC-24 is a purpose built dual band radio but adding a Crossfire or FrSky 868/915MHz R9 module to a Taranis or Horus gives us the benefit of a dual band system at less cost. (N.B. 915MHz part of the ISM band is for US and other countries.)

 (I think the new 5G mobile system will have a similar multi-band thing going on. The proposed 3.4GHz and 26GHz bands in urban areas would not work very well so they will parallel a 700MHz band as well to obviate the lack of penetration.)

Those of you who use a Spektrum transmitter will be well aware that one or more satellite receivers can be added easily and bound together to receive the best 2.4GHz signal possible; I know many power fliers do this and use a Power Box. Having a dual band system means that you can have master and slave receivers in the same way but on entirely different frequency bands. This is a new development… dual-band redundancy. In my case a FrSky Horus on OpenTx 2.2.1 using the internal FrSky IXJT 2.4GHz module and G-RX8 Variometer equipped Rx with telemetry as the master and an 868MHz R9M external module and R9 receiver for back up. The two receivers are connected by SBUS. Switch one transmitter signal off and the other takes over seamlessly and vice versa. It works, at the time of writing I am just setting it up on my Horus X12S and revamped Kinetic. This system also works with a regular Taranis and the new X10 series I believe. The beauty of the module system many radios have standardised on is that one can also obtain modules for other Radio systems thus enabling one to use their existing 2.4GHz receivers. This is the beauty of digital.

Here, in my image at the top I have a Multiplex 2.4 module enabling me to use all my old Multiplex receivers without telemetry. I believe Spectrum, Orange, JR Graupner, Futaba FlySky and FrSky XJT modules are available as well for the FrSky range as well as a Multiprotocol version that should fly anything. A second 2.4GHz XJT module can extend a Horus/Taranis to 16 PWM servo outputs, enough for anyone, I should think. The moral don’t throw away your old 2.4 receivers you may still be able to use them on lesser models.

So what of the future? We may see more dual-band radio. The 868/915 MHz ISM band like 2.4GHz is available to all and sundry, so it is still shared, and likely to become even more populated as more and more devices use short range radio communication right down to RFID tags embedded in frozen legs of lamb in the supermarket!Such a pity we never got a nice chunk of sub-1GHz for ourselves alone. With the 700MHz band being cleared for mobile telephone and data use by the Whole World for 5G pressure on sub-gig frequencies is increasing.


RSSI - Received Signal Strength Indication

Noise Floor - Background level/noise of frequency

LQ - Link Quality, the accuracy of the signal if you like

dBm - a log measure of power 100 mW is +20 dBm, 25mW is +14dBm

(e.g. receiver sensitivity -112dBm = 6.3095734448e-12 mW !)

Hockliffe - Site of club field


This article was originally written by me, Graham Woods, for publication in the Ivinghoe Soaring Association Newsletter in March 2018.