Fly Electric!

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 love how solid it is. 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. 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 scanner page, range and diversity tests (Assan / Spektrum) and Spektrum technical FAQs.
The 2.4 receivers I have made can be found here.


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'.

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. Multiple 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/FPV) are concerns.

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.

4. With 2.4 starting to 'take off' and being perhaps more complex than expected, alliances have started (eg: JR/Spectrum/Kyosho, Graupner/XPS).

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 are becoming quite common.

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. 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).

9. These microprocessor-based radios are more sensitive to power supply. So 'brownouts' are an issue (temporary power drops often caused by servo movements). Robust rx batteries/bec's and add-on voltage smoothing capacitors are more common than before.


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 FM setup. However, it is common for there to be extra redundancy with 2.4. The following is a simple decription of how each RC manufacurer approaches this; it's a moving target and by no means a complete list. In particular, failsafe behaviour should also be considered in assessing options.

Futaba: Hop between most channels in the range so very resilient. Less modern IC's so receivers tend to be larger and are expensive. Their non-unique ID's and over-heating problems have been concerns which seem to have receded.

Spektrum/JR: Hops between only two channels. Their "Model Match" feature is fantastic but Failsafe is limited to Ch1 on low end receivers. 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. 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): Nice radio but only uses fixed factory selected channels.

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.

Airtronics: "Safety Link" number is similar to Spektrum's Model Match feature.

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