WAMASC Technical Information
Recommendations and issues with 2.4GHz Radio Control Systems
The new 2.4 GHz system that is now the new kid on the block at most flying sites around the world is taking over the normal narrow band VHF system of PPM & PCM technology at an alarming rate. Nevertheless it would be prudent to say that all manufacturers of this new system have had problems from the word go.
The main contender on the block, Spektrum were very quick to look into these problems and came up with suggestions in a very professional approach as this was a new field for all players. The main problem was the power supply to the RXs (receivers) and this is common throughout the industry that without adequate supply to the receiver’s micro processor the Rx would shut down and would have to wait for it to reboot just like your home computer.
The reboot times were very high with Spektrum, as much as 10 seconds in some cases and can you imagine waiting for the model to re connect in that time, NOT GOOD. This issue has now been fixed with their “Fast Connect” system.
The loss of the processor was mainly due to the power supply dropping down to the threshold working voltage of the processor when a load was applied to the power supply (battery pack) when servos operated, pulling the voltage down.
Spektrum came up with a super capacitor to be placed in a spare servo outlet to give a reservoir effect to the supply to try and stop the low voltage spikes. The use of 4 cell NiMh or NiCad packs which had been supplied with the equipment were found not suitable and were replaced by a 5 cell pack to give more head room voltage to the processor.
At this point I would like to point out some other issues with power supplies feeding the receivers irrespective of manufacturer.
Some of the problems associated with the cells that make up the power supply to the receivers are the main problem in as much the ability to supply the voltage to the receiver and drive the servos without voltage sag.
The 2.4 GHz system is very sensitive to voltage changes mostly voltage that is dropping across the receiver and that 4 cell (4.8 volts at whatever mAh capacity) are not suitable to operate these systems.
The use of 5 cells is the better option as a supply and is now commonly being supplied with the RC sets.
N.B. If you do decide to use the 5 cell packs that came with your kit or you have changed to 5 cells make sure your old servos are up to the task. Some older servos are only rated at 4.8 volts and not 6 volts as the newer servos are. This could cause a failure in the servos electronics and loss of model.
The ability to supply voltage and current at the correct rate all the time under all conditions is reliant on the internal resistance or impedance of the cells. The lower the impedance the more stable the supply of power becomes.
If I may take this time to give you an example of internal resistance:
Take two lantern batteries with a rating of 6 volts each and place them end to end to give you a terminal voltage of 12volts. Then place a 12 volt car head light across the battery it will glow to some level of brightness but nothing like it should be.
If the voltage is measured at the battery terminals you will be lucky if you have 5 or 6 volts.
The internal resistance (impedance) of each battery is in series with the load (Car Head Light) and is dropping the voltage produced across the terminals. The car battery on the other hand has a very low internal resistance so very little voltage drop is created internally in the battery giving you the power that is required to operate the equipment on the vehicle.
This then is one of the reasons you should purchase good quality batteries and please don’t look for the highest mAh rating as this comes at a price (high impedance).
Regulators and BECs
The use of Regulators and BECs (Battery Eliminating Circuits) should under no circumstances be used unless designed for the powering of the receiver on 2.4 GHz systems.
The use of small regulators of up to 5 Amps and are some times called three legged devices with other associated components to make up a regulator are to be avoided at all times. These devices can run hot under load at their internal junctions and cause voltage drops across them and also go into oscillation causing all kinds of interference on the power line feeding the equipment.
There are some very good units on the market such as the Power Box System ™ but at a price which the large model flyers use and would recommend to you. They also produce a Power Box Royal Spektrum™ which contains an inbuilt Spektrum 2.4GHz receiver at a cost of 729.00 Euros but I have no experience with any of these units; but take note there are units that I have found in my research that come with the following disclaimer:
Products are tested for use on the UK 35MHz aircraft frequency use on other frequencies should not be a problem, however use on frequencies other than 35MHz is at owners own risk .
I think this speaks for itself this product was a dual battery system with regulation.
BEC - Battery Eliminating Circuit
Used on electric powered model that have a regulated supply coming off the speed controller to supply the receiver and servos with a voltage of around 5.5 volts the main battery pack may consist of LiPo cells having voltages above 11.1volts.
Again these BECs have regulators that are not up to the job and can cause problems locking up the receiver causing problems again. Therefore it’s recommended that an independent supply feeds the RX and servos and leave the main batteries to run the power train motor.
Other issues
As stated other manufacturers have had their own problems and one of the other main contenders Futaba which run the FASST system which is a very good solid radio link have trouble with over temperature cut out of the receiver and some Tx (transmitter issues) losing some of the GUID code when subjected warm conditions.
With the RX issues it was proven and repeatable that the processor went down and stopped working when the CPU temperature reach 59 degs C, yes that’s hot but it’s not the ambient outside temperature we are referring to but the rise of the components that are internally in the receiver.
The outside temperature together with the inside temperature of the model plus the working temperature of the receiver elevates the component working temperature to the point that it reaches this temperature of 59 degs C and the unit stops working.
The other Manufacturer jumped on this issue and ran advertisements showing how their receiver case would lose its shape with applied heat and the receiver would still function correctly.
It’s therefore recommended that the receiver be mounted in a way that it can get ventilation to pass around the unit to keep it cool, some operators have gone to the lengths of installing a small fan to blow cooling air over the unit.
The TX should be kept under the shade till it is time to fly so keeping the unit cool, I personally believe that this issue will be addressed in the near future but at this time the issue is still causing concern on some of the forums overseas.
That brings me to the installation of these receivers and the mounting of their antennas in an environment that the transmitted signal can reach the receiver antenna without some of the signal being lost with having to pass through material that is not able to let the signal pass through freely (attenuation).
Installation of receivers and their antennas
These are some of my own recommendations.
These receivers are quite small, so are their associated antenna system and require more care and thought put to their installation than the normal narrow band VHF setups we have all been used too.
First of all the receiver takes much more current than the normal PPM receiver that consumes around 10mA at rest no servos moving these 2.4GHz units take up to around 100mA at rest and @ 6 volts that is 600mw of power before it does a thing just sitting there.
Now you can see why these units get warm so my recommendations are:
When mounting the receiver don’t use cable ties to strap it hard down to the mounting point or wrap it fully in foam.
Use Velcro (hook and eyes) strips with self adhesive backing for mounting the unit with the nice soft material (eyes) on the back of the receiver.
N.B. Although the receivers are constructed using small surface mounted components they are still susceptible to damage by vibration and the Velcro Strips gives it just the right protection.
Wiring of the receiver
All the cabling that exits the receiver including battery harness and servo leads should exit straight away from the antenna’s end of the receiver and not pass within 50 mm of the antennas. Never I REPEAT NEVER parallel to the tip of the antenna proper, this will detune the antenna and reduce the efficiency of the receiver to function correctly. This applies to all other metal and carbon fibre materials that come close to the antennas. This also includes covering material that as aluminum particulates in its makeup and other metallic types of covering film.
For the best results the antenna wire should stick straight out and not be laid across other materials and stuck down with tape to these materials, remember what I pointed out previously about things being parallel to that little antenna detuning the antenna and reducing the efficiency of the receiver?
The VHF 36MHz transmitters can have a maximum power out put up to 1 Watt the norm being 750mW the 2.4 GHz transmitters have a maximum power out put of only 100mW so can you see the power that is getting to the receiver in good conditions is very small compared to the 36MHz receivers.
The placement of the 2.4GHz antennas is very critical and remember the 36 MHz antennas were mounted mostly on the out side of the model.
One other point in comparison of 36 MHz and 2.4 GHz the 36MHz signal can pass through walls very easily 2.4 GHz cannot without losing most of the signal strength.
NB. Some of the major manufacturers like Spektrum are now producing receivers with extended antenna leads so the antenna proper (the tip at the end around 31mm) can be mounted in the outside world away from attenuating materials. The Spektrum AR6350 Carbon Fuz and the AR9300 Carbon Fuz are two of such receivers.
Other items to look out for when installing are such things as water ballast tanks in gliders as water stops the 2.4GHz signal passing through it and will leave a dead spot that the signal can’t penetrate.
If the antenna can see the sky then your installation is looking good as 2.4GHz is very similar to light, if it can see the light you are on the right track to a good and secure radio link.
Let me point out once more all material other than the material PTFE (which has a very, very low attenuation at 2.4GHz) will attenuate the incoming signal to some extent so the more the antenna is out in the clear the better the link.
Power supply leads and switches are a must for the operation of our gear but make sure you get the best available (Gold flashed pins and socket) no compromise and heavy duty to the extreme (remember the voltage drop?).
Likewise the on/off switches should be heavy duty and a good positive feel when moved into the ON position. “I have tested switches with more that 100mV drop across the switch mechanism with only 1 amp flowing through it.
Think about a dual feed to the RX with a battery lead at each end of the RX bus rail this gives a lower resistance for the power supply in the event of one of the battery plug and sockets failing. The extra power lead plug can be plugged into any free servo socket on the receiver.
The servo leads that feed control surfaces such as ailerons and flaps are normally flapping loose in the fuselage after the model is assembled for flight. DONT LET THIS HAPPEN. The servo leads could pass in front of the Antennas causing problems.
The batteries that power the system
I have mentioned the use of NiMh and NiCads and the internal resistance (impedance) these cells contain internally that can cause voltage drops, well there are alternative batteries on the market and these are in the form of the LiFePO4 and are commonly called A123 cells which produce 3.3 volts per cell and come in similar format to AA & C sizes although both are a little longer.
These cells do not self discharge or have a memory effect and can produce 30Amps continuous in the AA format 1100mAh size and also be fast charged at 5Amps. For the larger models the C size is 2300mAh.
Range checks
You should always carry out the range check as explained in your manual that came with your RC set.
I would recommend the walk around 360 degs test before the maiden flight and after any movement of the internal components of the model.
When you reach the point where the link starts to break up in the range testing mode on you transmitter move forward till the link is just maintained. Then walk around the model or get a helper to turn the model around 360 degs. Make note of any dead spots in the link and try to rectify these dead spots by moving the antennas or internal components.
If this fails you may consider trying out one of the out of model antenna systems used on Carbon Fibre models and check again.
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That’s it for now and what gives me the authority to make these recommendations you may ask?
Well put it like this I made my first RC set , single channel carrier wave system around 1959, then a single channel tone system followed by multi channel tone systems and then the Galloping Ghost system that was the for runner of today Proportional units we use today.
I then contracted myself out in designing and developing some of the first 4 channel AM Proportional units in the UK which were in the form of DIY construction kits.
I was also involved with some of the first dual conversion receivers in the 70s when the so called FM units first came out.
I am a retired qualified Electrical Engineer with certificates in both industrial and radio communication electronics and have held an amateur radio license from the sixties to this day.
I have also carried out lecture work in communications for students wishing to obtain their amateur radio qualifications to the rating of full call.
I think I can then say I have done a little in the subject although these new 2.4 GHz systems are new to all.
My very best to all and safe landings.
Malc Nicklin AUS 56289
07 March 2010
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