Like the radios that we listen to, there are two arrangements, AM and FM.

Almost all 2 channel radios are AM (Amplitude Modulation). The control information is transmitted by varying the signal's amplitude. They may occasionally be subject to interference.

Almost all multi-channel (4 or more) radios are now FM (Frequency Modulation). These are more expensive but are less prone to interference.

PPM is the standard transmission/receiving arrangement. PCM (Pulse Code Modulation) is an additional feature of some FM radios where the control modulation is in the form of a digital word rather than just a pulse, so that the receiver only reacts to genuine messages from the transmitter and not other rubbish in the airwaves. ZPCM is a slightly different modulation form used by JR.

A PCM receiver from one brand of radio may not be compatible with a transmitter from another.

Another method of reducing interference - or "glitches" - is a system called Dual Conversion, which runs the signal received through some extra calculations to verify the signal is genuine and not rogue. At the end of this page is a discussion on Dual Conversion - and other things that cause interference.

Many Australian pilots have their planes set up in Mode 1 - ie steering and throttle on right stick, elevator (up/down) on left stick. Mode 2 is with both steering and elevator on the right stick.

Some radios are factory set as Mode 1 or Mode 2. For these - it is necessary to stipulate your requirements to the shop.

Be consistent with your mentor/locals or they won't be able to test your plane, train you or land it when you need them to.

Sometimes works - sometimes not. Many fliers prefer to stick with the same brand so as to avoid problems.

The control surfaces are simply the two elevons at the back of the wing. ELEVONs are a mixture of ELEVator and ailerON. Both up gives up, both down gives down. To turn, one goes up and one goes down. To do this, mechanical or electronic mixing is required. Mechanical mixing can be achieved by having the "left/right" servo (which is attached to the elevons) being slid along rails or rocked or pivoted by a fixed "up/down" servo.

The problem with this is that it requires a number of linkages (thus giving more room for slop in the controls) and the servos to be behind each other in the centre of the craft - weakening it structurally and requiring the second servo to be placed further back, and in consequence extra ballast is required in the nose. The better arrangement (and this is a requirement of the Zagi SI kit, since the servos are in a fixed position) is the use of electronic mixing, either by an electronic mixer placed between the receiver and servos, or, if you have it, the use of the electronic mixing function in your computer radio.

Electronic mixers come in two types - those with the ability to adjust the amount of up/down compared to left/right, and the cheaper ones that don't. You are likely to be disappointed if you go for the non-adjustable model as flying wings require a considerable amount of left/right but not too much up/down.

Computer radios with an inbuilt mixing facility usually also have the ability to adjust the travel/sensitivity of the respective functions, allowing both good up/down stability but plenty of left/right manoeuvrability. If you also put in some exponential into the left/right function, you can reduce some of the sensitivity of your turning in the middle ranges of the stick whilst still having the craft being able to roll real fast with full stick movement.

Sumarising; your basic 2 channel or multi channel radio is fine provided a quality electronic mixed is included, or if you have computer mixing on your transmitter then a separate on board electronic mixer is not required and you can tune the plane to fly the way YOU want it to.

If the minimum cost was going to be (say) A$300 for a very basic unit, or (say) A$500 for the bells and whistles that you are likely to need further on, don't mull over the A$500, simply mull over the A$200 difference that you are paying. Consider if that is money well spent for the extra functionality, plane stability, adaptability to various future planes.

You were going to have to spend the A$300 anyway!.

If the A$500 radio includes enough model memories, it will also defer the time before you will have to buy again (and, because of its mixing features, possibly save some broken planes).

Also, if I had a choice between two radios - a computer radio with multiple model memories and non computer radio with more channels, I would be going with the computer radio.

NECK STRAP - this is an optional extra and not essential - some people like them - some not.

I don't like them because I have seen too many mistakes made by switches or sticks being hit when the strap is being attached or removed - or when the transmitter is swinging from the neck whilst the plane is being attended to. I have electric planes and can't afford that to happen. To ease my weary arms (and legs) I spent the same money and bought a folding chair!

SERVO REVERSING - this is common on all modern two and multi-channel radios. It means you can you can flick a small switch on your transmitter to make up - up and down - down. Before servo reversing, you would need to build your plane to match the throw of the servo, and you could buy left and right hand servos.

BATTERY LEVEL INDICATOR - on 2 channel radios this will usually be a series of 3 or 4 LED lamps. On a high level radio it may be a dial with a pointer, and on a computer radio it is likely to be a digital readout. As batteries discharge, they generally maintain much of their voltage level and then drop, generally sharply, at the end. If you transmitter battery is at the stage where it is dropping, you may have VERY little battery left. Don't chance it - bring your plane down immediately.

BATTERY LEVEL WARNING - this is usually only on computer radios and is an audible warning that the voltage is at danger level. Immediately you hear this, return your plane safely to the ground.

Some are pretty basic, but it doesn't take much extra money to buy considerably increased functionality.

DUAL RATES - on both computer/non radios - switches are provided to vary the sensitivity of steering and elevator, so you can have a stable plane but access to greater sensitivity for aerobatic manoeuvres. Computer radios usually allow greater flexibility of settings for each. 

BUDDY BOX FACILITY - many (but not all!) multi-channel radios include a socket to connect a buddy-box lead to link two transmitters. This allows a trainer to hold in a switch to hand control of the aircraft to the trainee - and to take it back in an instant if required. Transmitters must almost always be the same brand, and may only work between similar models. More expensive computer radios include the ability to hand over specific functions (eg steering only, or steering and elevator but not throttle).

END POINT ADJUSTMENT - generally on computer radios only - allows limitation of travel for different channels.

EXPONENTIAL - on computer radios only - allows different rates of travel depending on where the stick is. Eg, on the outer (left/right) the stick can be sensitive for more manoeuvrability, but nearer the centre is less sensitive, so the plane is more stable.

V TAIL MIXING - on computer radios only - up is up, down is down, one up and one down is left, one down and one up is right. (Could also be achieved with a plug in mixer in the plane.)

ELEVON or DELTA mixing - on computer radios only - up is up, down is down, one up and one down is left, one down and one up is right. Required for flying wings and deltas - (could also be achieved with a plug in mixer in the plane, but end point adjustment generally also required.)

FLAPERON MIXING - on computer radios only - the ability to program the ailerons to also act as flaps (down ailerons) handy for slowing plane on landing.

SPOILERON MIXING - on computer radios only - the ability to program the ailerons to also act as brakes (up ailerons) handy for reducing lift when trying to get out of a thermal, landing on the slope, or trying to get a spot landing in competition. This also creates "washout", providing considerable stability for the landing process.

CROW BRAKING - on the higher level computer radios only - the programming of the two ailerons to go up while the two flaps go down - braking the plane and providing good control in the landing.

DIFFERENTIAL - on computer radios only - ability to program in greater up than down in the ailerons (separate channels required for each aileron), giving a smoother turn because the up aileron also creates drag in the wing closest to the centre of the turn.

ADDITIONAL MIXING - on computer radios only - allows compensation mixing, eg mixing down elevator with throttle if insufficient down-thrust has been built in, or elevator compensation with flaps or brakes/spoilers if the nose rises/drops when the brakes are applied.

TIMER - on computer radios only - inbuilt stopwatch, some can be set for count-up and count down - including audible signals.

GRAPHICS SCREEN - more expensive computer radios have a comprehensive graphics screen which shows you more detail about your programming, including graphs of the exponential curves you have applied, variable mixing illustrations and relative servo positions. Less basic models have only a couple of lines of basic information.

FAILSAFE - the combination of an advance transmitter and an advanced receiver is required for this - the receiver may be programmed so that in the event of a loss of signal the servos are set to predetermined settings that the pilot has programmed. For example, motor off, gently turn, to stop the un-guided plane heading off at a great rate never to be seen again .

ADDITIONAL FUNCTIONALITY - some computer radios have even more functions and pre-sets - such as "snap roll" , "landing" and a whole lot that I am yet to get my head around after four years of using it!

---------------------------

The following is a VERY interesting discussion on DUAL CONVERSION receivers.

It was taken from the SFRC (Slow Flight Remote Control) email discussion group.

Date: Sat, 07 Jun 2003 10:12:33 -0400

From: "Don Stackhouse @ DJ Aerotech"

Subject: Re: [SFRC] Receivers

Geraldine Roberts asks:

>I understand that certain receivers are better at

>glitch-free performance than others, and I want to

>know which one to get. Someone told me the tendency

>to glitch is a problem of the receiver and not the Tx.

> Is this true? Any advice?

Simple question, but no simple answer. Glitches are the result of the

failure of the transmitter and receiver to communicate with each other. An

entire book could be written on the subject. And, while some types of

receivers can deal with certain problems better than others, none of them

are completely immune, and none of them are totally unreliable either.

The other thing to keep in mind is that (especially with regards to

airplanes) when you add safety in regards to one issue, you usually have to

give up some safety and/or capability in another area. This is

particularly true with regards to receivers for slow fliers. In general,

the receivers that are especially good at rejecting some forms of

interference tend to be heavier than the other choices. More weight means

less performance, which means a greater probability of getting into a

situation your plane can't fly itself out of, and also means more kinetic

energy to be dissipated by splintering structure in a crash.

One of the big buzzwords you hear a lot about these days is "dual

conversion", as opposed to "single conversion". This is a scheme (but not

the only scheme) for combating what's referred to as "third order

intermodulation" or "3-IM". I'm sure some of the electronics gurus on this

forum could explain it better than me, but I'll give it a shot.

Our receivers are typically based on the "superheterodyne" operating

concept. These use a crystal oscillator in the receiver to generate a

frequency that's slightly different from the frequency we're trying to pick

up. The incoming signals from the antenna are mixed with the crystal

oscillator frequency, creating a much lower intermediate frequency

(typically 455 khz) that the receiver is tuned to pick up.

It's like that phenomenon you can hear when tuning a guitar or other

stringed instrument. If you finger two strings to make the same note, with

one string slightly out of tune, then pluck both of them simultaneously,

the sound waves from the two strings will alternately reinforce, then

cancel each other. You will hear a "beating" in the volume of the sound

that corresponds exactly to the difference in the frequencies of the two

strings. That "beating" of the volume is the intermediate frequency.

The frequency range (or "bandwidth") that a tuned circuit (like the one in

a receiver) is sensitive to is a certain percentage of the nominal

frequency it's set for. That percentage if something around 72 MEGAHERTZ is

going to be a much bigger number than that same percentage of only 455

KILOHERTZ. Thus, this scheme of converting the signal to a much lower

intermediate frequency results in a more precise receiver.

Unfortunately, the spacing of the frequencies used for model planes in the

US since 1991 is just right so that if you have two other transmitters

operating near you on just the right combination of frequencies, their

signals could mix with each other and create an intermediate frequency that

almost matches the intermediate frequency that your receiver is creating

internally. The receiver can't tell the two intermediate frequencies apart,

both get through into the controls, and you and your model proceed to have

a very bad day.

For this to happen, you need to have just the right combination of

transmitters operating in close proximity. If you fly by yourself or in a

very small group, your odds of encountering that combination are very low

(note, it takes at least two other transmitters in addition to your own to

cause a 3-IM problem). Also, some frequencies in the R/C airplane band (in

general the ones in the lower and middle portion of the band) have a

greater number of possible transmitter combinations that will cause this

than transmitters operating in other parts of the band. For example, most

of my planes are on fairly high channels, and even at big fly-ins I rarely

have problems with 3-IM. Meanwhile, Joe Hahn typically flies on frequencies

in the middle of the band, and he's had significantly more problems with

it. Because he flies a lot in large groups, Joe now uses dual-conversion

receivers in many of his planes.

"Dual conversion" simply means that in addition to going through the first

stage of conversion to an intermediate frequency, the signal goes through

two of these to yet a second intermediate frequency. 3-IM can get through

the first stage successfully, but not both. The down side is that now we

have a bunch of extra circuitry we've added to the receiver's front end,

some of which is bulky, heavy and relatively expensive.

There are other schemes. The intermediate frequency created by 3-IM is not

exactly the same as the one generated by the receiver's crystal oscillator,

so an extremely precise receiver can tell them apart. Many of the JR

receivers use a system they call "ABC&W" to accomplish this. The Berg 6

receivers used a similar system, and there are others. The JR receivers

have an excellent track record in high-density RF environments such as

contests. Unfortunately, the problem with having a very precise receiver is

that your transmitter must also be transmitting exactly on its assigned

frequency. Some folks had problems with the Berg 6 in particular, and the

problem turned out to be their transmitter (and in some cases these were

high-dollar competition grade computer transmitters!). The receiver was

listening to the correct frequency, but their transmitter was transmitting

on a slightly different frequency, not where it was supposed to be. It's

tough to convince a customer that the problem is his brand-new megabuck

supercomputer transmitter and not this new receiver, especially when his

other (less selective) receivers work fine with it. However, in nearly all

cases that was indeed the problem. Some of the transmitter manufacturers

even ended up publicly admitting that some of their transmitters weren't

doing what they were supposed to and issuing a recall.

Other systems that seem to be getting quite popular lately are various

forms of "digital signal processing". A microprocessor checks each

incoming transmitter pulse train to see if it's within the parameters of a

valid signal. If it doesn't (such as if the signal has been corrupted by

3-IM, or by other forms of interference as well), then the microprocessor

rejects that pulse and holds the existing control settings until the next

valid pulse train arrives. This can actually be smaller, cheaper and

lighter than a dual conversion system, and it filters out all sorts of

other interference

besides just 3-IM. However, the software the microprocessor uses is very

critical, and if not done just right it can cause more problems than it

solves. Some of the early attempts at this strategy had this problem.

However, I've heard good things about some of the new ones. I have a Berg

Microstamp and a Plantraco DSP-4SC I'm planning to test soon, and I'll post

the results here after I've accumulated some flight time with them.

The newest dual-conversion receivers seem to be improving with regards to

size and weight as well. One of the receivers that very rapidly has made an

outstanding reputation for itself is the FMA M-5. I have one of these in

the prototype of our Roadkill Series Boeing B-17F, and I've been extremely

happy with it. It's not terribly bigger or heavier than the GWS GWR-4P

single conversion receivers I use in most of my planes, but it has dual

conversion and full-size receiver range and performance. For small models

where every gram is important I still like the GWS single conversion

receivers, but for something that has as much blood, sweat and tears

invested in it as that big Boeing, I like the extra "confidence factor" I

get from knowing that M-5 is in there doing its job.

Some receivers got off to a rocky start, and some of them solved their

problems and went on to develop a reasonably good reputation. JR came out

with a sub-micro receiver called the 610-M that had some serious

reliability problems. Supposedly they eventually got those fixed and the

new ones are supposed to be reliable, although I've personally tried two of

them, one early and one later one, and...well, let's just say that neither

of them are in any of my airplanes right now. The Hitec Feather receiver

initially had some problems with its electrical design and with the

durability of the pc board (too thin and prone to cracking from what I

heard). I got one of the early ones, with the short antenna, and the only

change I made was to install a full-length antenna. I've had some glitches

with it, but it's still in my Tiger Moth and generally performs well. I

understand that the revised ones they came out with soon after the initial

problems came to light have performed very well.

Then there's things like losses of signal, such as that rare case where

your own transmitter signal bounces off of a nearby metal roof, and at one

particular spot in the sky combines with the signal coming directly from

the transmitter to the plane such that they momentarily cancel each other

out. That's not a receiver problem; you can't expect it to receive a signal

that's not even there! Typically the plane will make some sort of violent

jerk in its flight path, then regain control (assuming terra firma or some

other obstruction doesn't get in the way first). A receiver with digital

signal processing presumably would eliminate the jerk, so you wouldn't even

know the signal loss had occurred (which is why some of them like the Berg

Microstamp actually log how many signal interruptions they filtered out

during a flight and signal the count to you with a built-in LED). However,

signal loss is control loss, and it's a small consolation if instead of

convulsing itself into the ground, your model maintains smooth, stable,

level flight right into the eagerly waiting branches of a nearby tree.

Then there's other RF ("Radio Frequency") noise sources, such as nearby arc

welders, industrial applications (we are "secondary users" according to the

FCC, and it is possible for primary users such as industrial cranes and

such to be operating right on one of our model airplane frequencies, and

with far more wattage), power line and other electrical grid effects. Us

indoor fliers might even have to worry about lightning generated

interference, which is not generally an issue for outdoor fliers, who

generally have other concerns on their mind in that sort of weather (Oh,

Boy, Here comes a thunderstorm, let's all go stand out in the middle of an

open field and hold long, sharp metal rods!!). I once had an A6M2 Zero go

in due to interference during an indoor fly-in with a thunderstorm outside,

and we were all making jokes about it being just another case of "a Zero

being shot down by Lightning". Actually, yes there were some of our P-38's

at that event...

Antennas also can be a factor. The RFFS-100 system used in the really tiny

and ultra-lightweight models is a good system, but not as interference

resistant as most of the heavier systems, at least in my experience. The

planes it flies in usually can't tolerate the bulk, weight and drag of a

full length (39.5") trailing wire antenna, and the reception with the

recommended 18" antenna is not enough for safe flying in some RF

environments. I've found that using one of the little coiled-wire antennas

such as the ones available from Azarr or from our website can give dramatic

improvements with this radio.

The bottom line is that there is no receiver that can be absolutely perfect

all the time, and even the ones that approach that usually cost something

to achieve it, typically weight, size, price or all of the above. If you

fly by yourself most of the time, dual conversion may not be an issue for

you. Also, what works well in one RF environment might not do as well

somewhere else. In the area of small lightweight receivers, I personally

have done very well with the GWS GWR-4P single-conversion receivers, the

FMA M-5 dual conversion, and some others. Other folks in other locations

and flying situations might have different opinions. Probably the best

solution is to check with the other fliers in your area and try to find a

consensus on what seems to be working well for them, in airplanes with the

same size and weight-carrying ability as yours.

The other thing to bear in mind is that by far, in the VAST majority of

crashes that get blamed on "radio failure", the actual cause of the crash

was the "nut" on the end of the control stick!

Don Stackhouse @ DJ Aerotech

djaerotech@erinet.com

http://www.djaerotech.com

------------------------------

Date: Sat, 07 Jun 2003 12:08:44 -0400

From: "John Likakis"

To: sfrc@airage.com

Subject: Re: [SFRC] Receivers

I've accumulated a number of single-conversion receivers over the years,

most of which have given good service. My HiTec Feather, for example, has

flown in all manner of models with nary a burp.

At the other end of the spectrum, I have two miniature single-conversion

receivers that pick up more hits than top-40 radio. One of these is on a

high channel, the other fairly low. Although I agree with Don's assessment

that most crashes that get blamed on radio glitching are actually the fault

of the pilot, these two receivers (both the same brand) are definitely

hazards to flight.

For example, I have a light twin powered by a pair of Don's MPS-1 units.

While flying with one of these receivers installed, I got the model up to

about 300 feet. Suddenly the elevators went to full-nose-down and the rudder

went hard right. The model pitched over and rolled simultaneously, achieving

about an 85-degree dive angle before the controls neutralized. At that

point, they froze. From 300 feet, the model came screaming down at full

throttle while I stood with the stick full back. The receiver finally

started responding at about 20 feet AGL, whereupon the model made one very

tight loop before I could center the stick and resume flight.

Both of those receivers now reside in a drawer. I'll never use them in

anything that can't withstand flying straight down at full throttle into the

ground. The number of models that meet that criterion are pretty few, so

those receivers have been gathering dust for the better part of a year.

I think the moral of the story is that you sometimes have to consider some

of the "off-brand" single-conversion receivers to be disposable. Yeah, you

spent good money for it, but if it's wrecking your models then it belongs in

the trash.

John Likakis

------------------------------

End of SFRC V1 #1226

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