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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. Those who have made the shift generally love 2.4. Most of the teething problems have been sorted out on the established brands so you just have to be a bit more cautious with new releases.

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.

For greater technical depth, please take a look at my handheld 2.4GHz scanner page or pages where I've researched range and diversity (lockout logger for Assan and Spektrum-based tests).

TECHNOLOGY DESCRIPTION & ADVANTAGES

1. '2.4GHz' is a band which extends from 2400 to 2483.5 MHz in most (but not all) 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' and be 'polite'. They are designed to reject interferance and have many extra resilience features. These are great strengths over the older approach which assumed single use only. As a user, you can't shoot anyone down; channels used 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 only important at long distances 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 allows overlapping/spillover from other channels better. This was a problem with the old channels so adjascent channel interference is significantly reduced, and has much less of an effect should it happen.

4. Receivers are linked ('bound') to Transmitters with unique keys (a big number). These are used 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 make 2.4 radios much more resilient to glitches. They should all go into 'hold' mode initially if signals are ever corrupt, and sometimes a failsafe setting about 0.5s later if a good signal is still not being received.

5. 2.4 radios generally transmit commands to the receiver in bursts (up to ~20% duty cycle). On some radios transmissions include redundant data (ie: the same info sent many times). Together with encoding mentioned above means that multiple radios can share the same frequency. Clearly the more signals that overlap, the longer it takes for your Tx to 'hear' a robust signal so response can become vague or jerky in extreme situations. The risk of crash through loss of control is vastly reduced.

6. The high 2.4 frequency band is 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. However, 2.4 receivers that have 2 antennae will offer greater relience to simpler ones and as on the older FM type receivers. Antenna should be placed at 90' to each other to make most of this feature.

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. However, swamping of channels is likely to be rare.

2. Transmitters/receivers need to be quite clever, eg: checking signal integrity and taking appropriate action. Receivers then need to be able stay in sync with what can be dynamic behaviour by the Tx (FHSS). This is new and more complex than the older FM-based radios. Complexity means teething problems which we have from every RC manufacturer. So the passage of time has helped iron out some of the bugs.

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 available yet.

4. With 2.4 starting to 'take off' and being perhaps more complex than expected, alliances have started (eg: JR/Spectrum/Kyosho, Graupner/XPS). More are likely for those not in the market yet and some may 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, EZC, Art-Tech, etc). Each will use proprietary code. DIY radios also already exist.

6. The plug-in Tx modules are an easy way to enter the market but operate with a slight lag (latency) 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 not much bigger than tiny antenna could have this effect. Materials like metals (eg: motors) and carbon fibre can impair reception. 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'. A benefit of the shorter aerial is that it's actually quite hard to block the signal for long.

8. Signal strength is greatest when the Tx and Rx antennae are parallel (and significantly worse when at 90'). Never point the Tx aerial directly at the model (weakest signal). Should you ever go out of range, hold the Tx up high and rotate it back and forth through 90' to hopefully align the Tx aerial with the Rx to improve signal strength (in the moments that they are parallel).

RISKS & RC OPTIONS

1. The challenge with obstacles on the plane masking or mirroring signals will normally be overcome with multiple antennae. Multiple receivers are an added benefit. Make sure Rx antennae are outside carbon fusalages and perform range tests from all sides of the plane where possible. 2.4 is better than FM but needs 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) or they transmit continuously (PFV/video). They are often 5 to 25MHz which is much broader than the typical 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 (FHSS) will probably be better at these locations.

3. Add-on voltage smoothing capacitors or modules seem more common than before. This is because 2.4 receivers are generally 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) beef up you flight packs.

RC RADIOS

For RC use, the IC's that handle the radio stuff seem all to be off-the-shelf components, usually intended for wireless mice and keyboard type applications. These are interfaced with standard or bespoke microprocessors with a manufacturer's own programming. It is common for both receivers and transmitters to have two-way capability (transceivers).

There appears to be a mix of 10, 100 and 200mW transmitters, perhaps more. Stronger signals are good but they will generate stronger reflections so need better diversity. Some countries limit the allowed power. 1mW = 0db, 10mW = 10db and 100mW = 20db. Power doubles every 3db and range doubles every 6db.

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; the radios seem to have more than adequate resilience for this.

A simple single-aerial 2.4 Tx/Rx should be more robust than a single narrow band setup. However, it is common for there to be extra redundancy which suppliers try to provide 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: One receiver usually with two antennae diversity. This means only one channel at a time but hopping channels regularly to avoid being stuck on one that becomes over-used. This is considered a good approach for handling interference. Their non-unique ID's and over-heating problems have been concerns and price is a problem.

Spektrum/JR: Hops between two channels and there are usually one or two receivers, sometimes with dual antennae diversity. High end systems can go up to four receivers for greater resilience. 1MHz spacing and 80 channels used. ModelMatch feature is fantastic. Lengthy resets when voltages dropped were problems on early receivers but now solved.

XPS/Graupner iFS: Early manufacturer claims were out of sync with modellers observations which 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. Based on the MaxStream Xbee Pro radio.

Multiplex: Said initialy they were concerned about shared use of the band, possible legislation changes, low power levels allowed. Have now entered the market.

Assan (also branded Jamara): Fixed single channel, two on new versions. Often has two Rx antennae but only one is active so not true antenna diversity. Seemed to be the first to offer the choice of 3 and 30cm length antennae allowing better placement. Believed to be based on the Nordic nRF2401 chip. 1MHz spacing.

Jeti: Frequency hopping over 5 channels and claiming the abilty to change those channels should any become congested. Antennae diversity in Rx.

Corona: Single receiver, dual antennae (longer than normal to aid diversity), frequency hopping claimed. Different versions released.

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...

More information on 2.4GHz can be found here:
* Handheld 2.4GHz scanner
* 2.4GHz lockout logger
* 2.4GHz diversity tests

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