This is a bit of a work in progress and will be updated occasionally.
RC – Radio Control
Used for all sorts of applications but mostly thought of for model control; planes boats, cars etc. This is about model control and references New Zealand frequencies but the theory can be applied to other uses and frequencies..
Until a few years ago most “real” RC was FM and operated on lower frequencies like 35, 40 and 72 MHz. Referred to as “long wire” systems because the receivers had a wire antenna about a meter long. These systems operated on one frequency at a time generally using plug-in crystals on the transmitter and receiver to set the frequency.
Just as synthesized receivers and transmitters were becoming cheap enough, the model world began to switch over to 2.4GHz systems. Synthesized means that the frequency is set by a PLL (phase locked loop) controller and usually a small micro-controller, and can be changed without changing crystals.
2.4GHz is synthesized and automatically selects or “hops” around within a block of frequencies at about 2500 MHz referred to as the 2.4GHz ISM band (Industrial, Scientific and Medical). This is a free-for-all band that is used by computer wifi, bluetooth, cordless phones and many other things. It’s a very busy band. The reason it works is that the equipment is smart enough to change frequency on the fly and avoid conflicts with others using the band. This works well up to the point that all or nearly all of the band is in use, at which point it becomes unreliable.
The long-wire systems transmit a low power signal that goes for kilometers. This means that two models on the same frequency must be a long way apart to avoid interference. Long-wire models generally can’t be flown that distance because at the low frequencies, the antenna is very inefficient, the receivers are not very sensitive and the noise floor is high. Noise floor just means there is a lot of background noise that competes with the signal. Long-wire signals are better at getting through and around trees, people and small buildings.
2.4GHz systems also transmit a low power signal but smaller antennas are more efficient at high frequencies, the receivers are more sensitive and the noise floor is much lower. The only real problem is all the other equipment using the same band. 2.4GHz signals are blocked by trees and people, and need a good line of sight between the transmitter and receiver.
When a typical long-wire system gets to the limit of its range due to distance or interference, control becomes twitchy and if you notice, you you can turn around and come back. With 2.4GHz systems it’s all handled by a small computer in the receiver which copes well with poor signals up to the point that it just quits and does nothing or applies fail-safe settings.
Under ideal conditions and with a good installation in the plane a long-wire system will operate out past 1.5KM. Range and reliability are reduced by transmitter problems on the ground, interference or noise and installation problems in the plane. Interference coming from other sources can also be a problem.
These generally work or they don’t. Maintain it well, keep it clean and dry and look after the battery. Dust and dirt will eventually get inside the case and is most likely to cause problems with the stick pots. Moisture (condensation) will cause corrosion and may also affect the pots. and contacts.
Don’t run the transmitter for extended periods with the antenna down. It’s tuned to operate most efficiently with the antenna fully extended. A short antenna will make the transmitter run hot and use more power. Keep the antenna in good condition.
Installation in the Plane
This is the area of greatest gain or loss but it’s not that difficult. Keep wiring short and tidy. Keep the receiver antenna away from wiring and metal or carbon push rods and keep it as straight as possible.
Running the antenna the length of the fuselage next to wiring or push rods can seriously reduce the range. You will end up with the equivalent of a 4 inch antenna hanging out the back. Those helical or rubber-ducky receiver antennas are a trade off; smaller size for reduced performance.
Secure the receiver but make sure it is vibration and shock protected. The crystals (many receivers have 2 or 3) are sensitive to vibration and don’t like being dropped. Wrapping the receiver in soft foam is a good idea. It’s also a good idea to make sure that the receiver is not between the battery and the front of the plane. When you crash, the battery is going forwards, very quickly.
Interference and Noise
Other electrics in the plane generate interference which is usually of higher frequencies and noise which is usually of lower frequencies. If the frequency of the interference/noise is one that the receiver is sensitive to, then the smallest signal will interfere with the receiver and it is almost impossible to filter out. With FPV the camera and transmitter generate high frequencies which can interfere with the RC receiver. A ESC and servos mostly generate lower frequency noise but these are very powerful noise sources and are mostly conducted via the wiring. Fortunately they are the least harmful to radio range and the easiest to reduce. You can never completely eliminate interference or noise.
Digital servos and those with mosfet switching create more noise than the older analog servos. Some servos are sensitive to noise conducted in on the PPM control wire; HXT900 seem to be quite bad for this when used with a digital servo in the same system.
For normal flying where most models never get more than 300-400 meters away a tidy installation will generally be OK. If you are flying FPV or large gliders and push the range a bit everything you do to reduce noise and protect the receiver will increase the safety margins.
UBEC : A lot of people use a UBec or a really expensive BEC if they run more than a few small servos. A UBec is a switchmode voltage regulator and will produce electrical noise. Most of the hobby units are short on input and output filtering so they can be kept small, light and cheap. Be very careful if using a UBec and pushing the range limits; it could reduce receiver sensitivity.
This is high frequency radio interference coming from others using the same, adjacent or even other quite different frequencies as well as accumulated background RF noise. There’s not much you can do about it except to change frequencies or fly somewhere else.
A radio scanner can be useful for determining that the channel is clear before flying and looking for interference. BUT, the signal you receive on the ground is only a fraction of what the model will be receiving 50+ meters up. Typical scanner antennas are really bad at 35-72 MHz so you’ll hear even less.
Note: a standard 35MHz RC transmitter in normal use, can be picked up by a typical scanner with a reasonably good antenna 3-5 KM away over a built up area.
To Do – For Best Reliability
There are a few simple things you can do to improve reliability. Most are common sense and normal maintenance.
- Keep the receiver antenna is full length, straight, undamaged
- Make sure the RX antenna is clear of wiring, push rods etc.
- Never cut or coil the receiver antenna
- Keep all other wiring short and tidy
- Use twisted servo leads
- Use a cores on ESC and digital servo leads
- Use a good filter capacitor on the receiver
- Use good connectors, replace when they become loose
Twisted Servo Leads are supposed to reduce the radiation and pickup of noise. This is true to a certain extent but is probably of little benefit in a model. It will have little or no effect on high frequencies but may help with high current noise pulses as the servo moves or holds against a load.
Ferrite Cores are basically filters that reduce the amount of high frequency noise that gets past, without affecting the DC voltage or current. These are well worth having if you use powerful or digital servos and in FPV planes where there can be a lot of reasonably high power RF floating about from the video transmitter.
Filter Capacitors are plugged into a spare receiver channel and clean up the 5V DC supply that runs the receiver and servos. These are very worth while. The capacitor must be on very short leads and directly plugged into the receiver. Long leads reduce the benefits.
Range and reliability are reduced by transmitter problems, receiver antenna positioning or damage and external interference.
2.4G systems mostly operate internally at 3.3V or less which means they have some safety headroom on the 5V BEC supply. If the supply momentarily dips too low, the receiver will reset which can take a second or two, during which time you are not in control.
The 2.4G band has very little natural background noise unlike 35-72MHz. In populated areas there is a lot of other activity on 2.4G that all contributes to noise. There’s not really much that can be done about this. Nothing at the transmitter nor in the plane will help. When 2.4G stops working at a site due to band noise, you’ll have to go back to long-wire.
Frequency hopping allows the system to move to a part of the band that is less noisy or available. When the hopping system is operating without interference it is quick and reliable. When there is interference and it looses lock, it can be slow to recover.
This all means that it works well, up to the point that it just can’t cope with the interference and quits.
For Best Reliability
Pretty much the same as for long-wire. Cores and a filter capacitor and you will be as safe as you can be.
The transmitter antenna should be horizontal, not sticking straight out pointed at the plane. The antenna radiates from the sides, not the tip.
Antenna placement is the biggest issue within the plane. For normal short ranges it doesn’t matter too much. Just keep it clear of anything that could block the signal such as carbon, servos and batteries. For longer ranges you should have two receivers in a diversity arrangement. The antennas must be at right angles to each other so that for any orientation of the plane, one antenna is always side-on to the transmitter.
It’s likely that some receivers with 2 short antennas on opposite sides are not rue diversity receivers. For one thing, the antennas are not at right angles to each other. It’s possible that one is the grounded half of a dipole. These type of receivers should have a second satellite receiver.
Don’t cut bend or damage the antennas. The thin wires are often a miniature coaxial cable with a precise length of centre core exposed at the end. The longer antennas are suitable for use with carbon fuselages where the active part of the antenna must be outside.
General Electrical Reliability
If it looks like crap, it’s probably going to be unreliable. Keep wiring short and tidy.
Solder connections and use heat-shrink. Electrical tape is crap and will just make a sticky mess.
When connectors become loose, replace them. Hot glue can be useful for ensuring that connectors don’t come apart, but don’t get the glue on the contacts.
Don’t pull on wires to unplug connectors.
Things I’ve Had go Wrong
A lot really, but I’ve not yet lost or damaged a plane beyond repair.
Servo failure in flight (a number of times).
Receiver failure only at high throttle due to vibration.
Receiver with very short range due to prolonged vibration.
Battery failure in flight
ESC overheat in flight
Electrical noise causing servo jitter
What destroyed my plane – It was a glitch, lock-out, brown-out , servo failure, battery failure, poor quality Chinese product….. B.S. Chances are a crappy installation contributed.
Important Note: good and quality don’t mean expensive. The days of having to pay a lot to get a good product are gone. But that said, there is a lot of cheap crap available. There’s also a lot of expensive crap available. Don’t believe the marketing . Every purchase should be checked and tested before use.
If you agree or not that’s OK. Leave a comment….