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2.4 GHz

So what's this 2.4 stuff all about? Well it's generally a much better radio system for model aircraft especially if you go with one the popular suppliers. If you are just starting out, 2.4 GHz is the obvious place to start. The more conservative modellers who don't have frequency/radio problems will probably wait a while for things to settle a bit more.

This is an 'enthusiastic amateurs' analysis of the 2.4GHz radio frequency being used for model aircraft radios. 2.4ghz is used by many other wireless technologies so I am interested in understanding those implications too. This analysis is meant to be more detailed than the wooly marketing bumph and therefore includes some technical stuff but not too much. I don't know everything and things may change so use this info to identify what to look for rather than being the only version of the truth.

TECHNOLOGY DESCRIPTION & ADVANTAGES

1. '2.4GHz' is a band which extends from 2400 to 2483.5 MHz in most countries. This is much higher than the older 27, 35, 40, 60, 72MHz (etc) frequencies. This means much less interference from other sources such as metal to metal noise, ignitions, etc.

2. The 2.4 band is an environment shared with other radio equipment. The band has been set up knowing this so all radios are required to have 'collision avoidance'. This is a great strength over the older approach which assumed single use only. You can't shoot anyone down; frequencies are controlled by the radios themselves.

3. The older RC frequencies were narrow-band signals, ie: very 'spikey' often with 10kHz spacing. Their signal needed to be spot on the chosen frequency for good reception. By contrast, the 2.4ghz spacing is very broad with the smallest channel typically 1Mhz, using the term 'spread spectrum'. This makes precise tuning less important and any overlapping narrow-band signal or random noise should be a tiny blip in a broad-band signal resulting in much less interference. This concept actually allows overlapping/spillover from other channels. This was a problem with the old channels so adjascent channel interference is significantly reduced.

4. Receivers are linked ('bound') to Transmitters with unique keys (a big number). The broad-band approach uses these to encode signals which is how frequencies can be shared. Receivers are looking for signals that are both on the correct channel and have the correct codes. The rest are ignored. These all make 2.4 radios much more resilient to glitches.

5. 2.4 radios generally transmit commands to the receiver in bursts (10% duty cycle perhaps). Together with the above encoding means that multiple radios can share the same frequency. Clearly the more signals that overlap, the longer it takes for your Tx to get a 'slot' and your signal may become more vague or jerky. But you should not crash! This is another major improvement over narrow band.

6. The high 2.4 frequency band is considered a short wavelength and so only needs short antennae to transmit and receive signals. Great for scale and fast models (drag) and less susceptable to damage. 2.4 receivers that have 2 antennae will offer superior reception to the one on the older FM type receivers.

SOME DISADVANTAGES

1. Sharing a frequency band with others without strong regulation poses some new challenges. Bandwidth is quite large but still has limits so congestion (eg: local WIFI) and swamping of channels (eg: wireless video) are concerns.

2. Transmitters need to be quite clever, eg: checking for other tranmissions before transmitting themselves to minimise overlap. Receivers then need to be able stay in sync with what can be dynamic behaviour by the Tx. This is new and more complex than the older FM-based radios. Complexity means teething problems which we have from every RC manufacturer.

3. The 'radio' part of an RC system has become more program-based and there are many options available to manufacturers (eg: DSSS/FHSS). None are perfect so they all want to implement them in different ways. Common standards for RC do not exist. Early attempts may not be right and some RC equipment is already not backward-compatible from the same supplier. Interoperability between manufacturers is therefore even less practical. Once you choose your make (and sometimes model) you will be locked in. Sadly this also means cheap receiver clones are unlikely to be common.

4. With 2.4 starting to 'take off' and being perhaps more complex than expected, alliances have started (eg: JR/Spectrum, Graupner/XPS). More are likely for those not in the market yet and some will change.

5. Programming is a skill many people possess and some highly configurable underlying radio platforms are available off-the-shelf very cheap (eg: suitable 2-way radio chips retail for a few pounds). So instead of seeking licence arrangements, there will be more new entrants to the market selling matching modules and receivers (eg: Assan, iMax, Corona, etc). Each will use proprietary code. DIY radios already exist.

6. The after-market plug-in modules are an easy way to enter the market but operate with a slight lag because the orignal Tx has to produce a standard PPM signal which the module then has to convert into it's proprietary signal. Said to be like PCM systems which have a similar overhead although not many people really notice it.

7. The shorter 2.4 wavelength makes signals easier to block and reflect. An object the size of the tiny antenna could have this effect. Materials like metals (eg: motors) and carbon fibre may therefore impair reception although some manufacturers claim to have proved that carbon encased receivers work perfectly. Most antennae tend to be directional but the size and design possibly exacerbates this in 2.4. It is therefore common for 2.4-based radios (including the WIFI types) to benefit from multiple aerials, a concept referred to as 'diversity'. RC receivers often have two antennae, usually orientated at 90' and preferably physically apart which then gives excellent directional coverage. One antenna may be on the board and not visible.

OTHER 'CONTESTANTS'

In most countries, the 2.4ghz band requires no licence to operate and has lighter than normal regulation. Applications such as RC have to be approved in each country and radios need certification but significant flexibility remains. Given its many advantages, the band has become popular for many wireless applications, eg:

1. WIFI/WLAN (802.11): 5MHz channel spacing starting from 2412MHz. The protocol requires 25MHz bandwidth so one WIFI Access Point would overlap 30% (25/80MHz) of the available band and absorb capacity needed for our RC radios. However, they have collision avoidance, their antennae are usually orientated for horizontal coverage and range is usually quite limited. So on one hand the risk is small but on the other power output can be greater than RC equipment. So on balance, the existance of WIFI in the immediate vicinity of a flying field is likely to be 'unhelpful' and best avoided. It is easy to scan for WIFI channels using a laptop with WIFI and the free Netstumbler or other tools.

2. ZigBee (802.15.4): Also 5MHz channel spacing starting from 2405MHz. Channels do not overlap and as usual radios check if channels are free before transmitting. The 5MHz spacing means up to 6% (5/80MHz) of the spectrum could be occupied by one ZigBee network. ZigBee transmiters are usually weaker but can have similar power to RC and in fact form the basis for some RC radios.

3. Bluetooth: 1MHz spacing, 80 channels with very rapid channel hopping. Also usually very weak but again some high power versions exist.

4. Cordless phones, video/security cameras, microwave ovens, etc.

5. There are a growing number of cheap 2.4-based 'toy' models that come with their own Tx and embedded radio. The Kyosho Minium is said to have 79 channels, the Yuneec ProCopter 80 so they could be using 1MHz channel spacing and hopefully therefore have a 'light footprint' (and likely to be low power).

RISKS & RC OPTIONS

1. The challenge with obstacles on the plane masking or mirroring signals will normally be overcome with multiple antennae, and multiple receivers should definitely be better than any narrow band solution. There are mixed views on whether airframe parts from materials like carbon fibre are a problem but again diversity will help. So 2.4 may need more careful Rx placement to avoid blind spots.

2. Other 2.4 wireless sources pose a risk of absorbing airtime/capacity but only if their output is high (eg: ZigBee or WLAN). They are often 5MHz or more which is much broader than the 1MHz RC channels. So non-RC 2.4 traffic should be minimised at flying sites. You might like to assess the amount of 2.4 traffic if close to residential or industrial developments. Channel-hopping RC kit will probably be better at these locations.

3. Add-on voltage smoothing capacitors or modules seem more common than before suggesting that some 2.4 receivers are more susceptable to voltage dips (typically caused by sudden servo load). So load test your Rx batteries. For models that may experience sudden load changes (eg: large, heavy and 3D models) buy the extra devices where available.

RC RADIOS

For RC use, the core radio modules are a mix of bespoke chips (eg: Futaba) or off-the-shelf modules interfaced with microprocessors with their own programming. It is common for off-the-shelf radios to have two-way capability (transceivers). Bespoke chips should allow greater flexibility/sophistication (when they mature).

There appears to be a mix of 10 and 100mW transmitters, perhaps more. Stronger signals will generally be good but they will generate stronger reflections so need better diversity. Some countries limit the allowed power.

Channel spacing ranges from 1 to 5MHz. The 1Mhz channel spacing can be used for up to 80 channels before needing to overlap. ZigBee-based radios have 12-16 channels with 5MHz spacing so will have to overlap more. What your radio uses is probably irrelevant; what matters is how well it handles sharing channels with other radios.

A single 2.4 Tx/Rx should be more robust than a single narrow band setup but people are not sure if it is yet. So there seems to be need for some degree of redundancy and suppliers try to provide this in the ways described below. The following is an attempt to find out how each RC manufacurer approaches this; it's not a complete list. In particular, failsafe behaviour should also be considered.

Futaba: Normally one receiver with two antennae. This means only one channel at a time but hopping channels regularly (they claim every 2ms) to avoid being stuck on one that becomes over-used. This is considered a good approach for handling interference. Said to only use 36 to 39 channels so not sure about channel spacing. The Rx is likely to simply stay on one frequency to eventually pick up the signal if they get out of sequence.

Spektrum: Initially two receivers, now three in top end setups with an option for four! Known to use 1MHz spacing and 80 channels, one per receiver 5 or 6 channels apart according to scans by modellers. The Tx scans for clear channels before setting them; never changed in flight. ModelMatch feature sounds nice. Main risk is their channels become noisy after takeoff but at least they have two, twice what we have with narrow band now.

The Spektrum 'Flight Logger' indicates typical challenges (faced by all manufacturers): Antenna fades/signal loss on one antenna (50-100 being common on a flight), frame loss (both lost, they expect under 20 in a flight), hold (1s pause after 45 contiguous frame losses) and supply voltage (common cause of failure for any portable radio).

JR: Spektrum-based.

XPS (Xtreme): Manufacturer claims have been out of sync with modellers observations which has created unease in the market. Uses one receiver but claims additional receivers are possible. Claims to change channels when too busy/noisy which would be very nice. However, (i) modellers with scanners have been unable to make this happen in tests, (ii) XPS have admitted that scenarios triggering changes are few, and (iii) XPS have announced that the next version will change channels more willingly. Based on the MaxStream Xbee/Pro radio so presumably has a 5MHz spacing. Said to have two-way comms capability (Tx/Rx) which is likely but not sure how used. Say they use 12 channels and claim 120 modules can be used simultaneously implying that 10 radios can be used on each channel.

Graupner: Using XPS marketed as iFS.

Multiplex: Say they are concerned about shared use of the band, possible legislation changes, low power levels allowed.

Assan: Single channel, one unused frequency chosen at startup and no hopping or changes. Has two antennae, with some models offering the choice of 3 and 30cm lengths allowing excellent diversity. Sold as a modular system, typically much cheaper than other makes. Appears to be based on the Nordic nRF24L01 chip which supports 1 or 2 MHz spacing. Evidence of active development so design likely to change.

Corona: Very new product at time of writing. Single receiver, dual antennae (longer than normal to aid diversity), frequency hopping claimed.

iMax (SkyRC): Two receivers so two channel diversity; sold with a Tx with their own branding but the 2.4 part is actually a module.

ACT: Available with two 2.4 transmitters and two 2.4 receivers in addition to one old FM (3 Tx/Rx total) so probably quite robust.

Sanwa/Airtronics, Others...

TIPS

Rx antennae are normally very short. Assan and Corona sell receivers with longer aerials than normal. These appear to simply be lengths of coax cabling with the last section exposed (shielding removed). Ie: the active part is the same length as 'short' antennae and the screened cable allows them to be positioned best to mainimise diverity. Requires cable of the correct impendance for the internal radio chip.

EXTERNAL LINKS

For some good external descriptions of the topic:
* RC orientated
* Spread Spectrum - Fairly technical
* Interference Avoidance - good graphs illustrating the various types of 2.4 traffic

DIY scanners/spectrum analysers
* DIY 1
* DIY 2

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