WHAT YOU NEED TO KNOW ABOUT RECEIVERS
by Ed Anderson
aeajr on the forums
Revised June 2015

Preface

Today, most radio systems are based on 2.4 GHz. Much of this article
is still valid but make note of where I am talking about 2.4 GHz systems vs.
27 or 72 MHz systems. Frequency control is very important in the 27 MHz and
72 MHz worlds but is virtually unknown when using 2.4 GHz systems. So read
in the context of what frequency system is being used.

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You control the plane by moving controls on the radio, but it is the
receiver that "hears" the radio and directs those commands to the proper
servos to move them according to your wishes. So, what do you need to know
about receivers when preparing and flying your plane?

By convention all radio systems use a transmitter and a receiver. However in common
use, in the RC airplane hobby we typically refer to the thansmitter as the radio. While this
is technially incorrect, everyone knows what we mean, so I will speak of the radio and the
receiver. For those of you who are radio systems wizards, I hope you will forgive me
for this convenience.


FREQUENCY AND CHANNEL

Receivers are specific to a given frequency. For example, in North America,
NA, our planes can be flown on 27 MHz, 72 MHz, and now 2.4 GHz. Your
receiver has to match the frequency of your radio in order to be able to hear it.

In North America 2.4 GHz radios are now the most common but 72 MHz is
also considered a valid frequency for flying RC aircraft. 72 MHz is
split into 50 sub frequencies, or channels so that we
can have more than one person flying a plane at any given time.

In NA, 27 MHz is typically only seen in low end RTF planes and is shared
with low end cars and boats. 27 MHz is limited to 6 channels. Also note that
27 MHz is the frequency band used by Citizen Band radios. If you tend to fly
near a highway you may encounter interference from CB radios. While CB
radios are not as common today as they were years ago they are still out there.

For all practical purposes, 2.4 GHz is now the standard for RC systems. The
main attraction to 2.4 GHz is there is no need for frequency control, which we will
discuss later. Also, this system operates well above the frequency level of most of
the "noise" that is generated by other components in the airplane so the 2.4 GHz
systems are less likely to pick this up as interference. However because of the
very short wavelength they are more prone to having the signal blocked. More on
this later.

With non 2.4 GHz systems your receiver needs a crystal that matches the
channel of your radio. In RTF packages, this is already done, so you don't
need to worry about it. However if you are buying your own receivers, you
must match them to the frequency and channel of your radio when you buy
them. Your supplier can help you with the details. One suggestion is that
you not mix crystal brands. They may work but this introduces a risk that
you are better off avoiding. If you get a Hitec receiver, get a Hitec
crystal.


AM and FM and FM SHIFT (72 MHz)

Just like your car radio, RC radios can use AM or FM to transmit their
instructions to the plane. AM is an older technology but it is still in
use, primarily in low end 2 and 3 channel radios. However most new radios
are FM. Both work!

In North America, 72 MHz systems are grouped by those using positive shift and
those that use negative shift. Typically we speak of JR and Airtronics as
positive shift. Hitec and Futaba are negative shift. In some cases these
brands can be made to change shift through a function called shift select or
reverse shift that can be set at the radio.

Shift refers to how the radio codes instructions for the receiver. One is
not better than the other, they are just different. This is only important
when you are buying a new receiver as you need to be sure that your FM
receiver and your FM radio are using the same shift.

Crystals are not specific to shift, but they may be specific to AM vs. FM.
Be sure you get the right type of crystal for your receiver.


FM/PPM and FM/PCM (72 MHz)

PPM and PCM further define how the radio codes commands to the receiver. We
normally speak of PPM and PCM in the context of FM radio/receiver
combinations. If you are buying an AM receiver/radio, or a 2.4 GHz system you don't
need to take this into consideration.

FM receivers can be either PPM or PCM. When people say FM, they typically
mean FM/PPM. If they say PCM, they mean FM/PCM.

As long as the shift is right, you can mix brands of FM/PPM radios and
FM/PPM receivers. On the other hand, FM/PCM receivers are highly brand
specific. If you have a Futaba radio capable of PCM transmission and you
wish to use a PCM receiver, you must have a Futaba PCM receiver that is
compatible with that model radio. No mixing brands in PCM.

As far as I know, all FM radios can transmit in FM/PPM. Some can transmit
in FM/PCM also. I don't know of any that are FM/PCM only, but there may be
one out there. If PCM is listed, it is normally an extra feature, not a
requirement you use PCM.

Some will say that PCM is better and more reliable. I can neither confirm
or dispute this point as I have not done testing. I use both and have found both reliable.
I will point you to a couple of articles that discusses PCM, how it works and their opinion
of the advantages.

Futaba FAQ on Advantages of FM/PCM over FM/PPM
http://www.futabarc.com/faq/product-faq.html#q102

Article on PCM vs. PPM
http://www.aerodesign.de/peter/2000/PCM/PCM_PPM_eng.html#Anker143602

PCM receivers tend to be more expensive, larger and heavier. From what I
gather FM/PPM is what the overwhelming majority of flyers use. FM/PCM seems
to be most popular in the high performance world, giant scale and
competition planes. Choose whichever you like as either will fly your
plane.


RANGE

For practical purposes, range is determined by the receiver, not the radio.
It is a function of sensitivity of the receiver and its ability to pick out
the radio signal and filter out noise. Many brands state the rated range of
their receivers and some do not. I suggest you stick with brands that state
their rated range or at lest advise of their intented purpose. Otherwise you
could end up flying beyond the range of your receiver.

How much range is enough? That depends on the application. You can
NEVER have too much range, but you can have too little. If the plane
gets out of range it will crash or fly away. More range is always better.

Here are my suggestions for minimums:

Indoors

Indoor planes are usually very weight sensitive, every gram counts.
To get extremely light weigh, sometimes range has to be sacrificed but that
is OK indoors as long as you know what it is. I suggest 200' minimum and
more is better but you may be fine with less. Many indoor flying spaces are
less than 100 feet along any span and you are not going to accidentally fly
past the walls.


Outdoor

Micro planes, micro helis and small electric planes under 36" wing spans can
often get by with ultra light receivers with ranges of as little as 500
feet. This is adequate if you have a small model or fly in a small field of
under 500 feet in span. Many of these small models can be hard to see at
ranges of more than 300 feet, approximately the length of a football field.
I prefer more range, but many people do fine with 500 foot receivers.

Today there are plenty of micro receivers, sometimes called parkflyer
receivers, with 1000' rated range that are under 1/3 ounce, about 9 grams.
I have a large field that is 1600 feet long so it is easy for me to get a plane
out beyond 500 feet without realizing it. While it can become hard to see them
at that range, I don't want to lose it because I ran out of receiver range.

For 2M and larger gliders and anything with wing spans over 5 feet I recommend
a full range receiver which will typically have a range of 1 KM or more and
may be good beyond a mile.

If your receiver is rated for "line of sight" that means that as long as you
can see the model, you should be able to control it. These receivers will
be your longest range receivers. They often carry the "full range" designation.


SIGNAL PROCESSING - Single and Dual Conversion, DSP and more (72 MHz)

In addition to range, 72 MHz FM receivers will usually specify if they are single
conversion, dual conversion, or that they use some other method of signal
processing. I will leave it to the engineers to go into depth here.
However, as a general rule, dual conversion is better than single but there
are excellent single conversion receivers that have digital signal
processing and other ways of making sure they pick up the right signal.

I have no hesitation to use single conversion receivers with 2600 foot, (
1KM or .6 mile) rated ranges in my models that will be flown less than 1500
feet out. Most of my electric planes can't be easily flown further than
that and since I am operating at less than 70% the raged range I feel comfortable
that good quality single conversion receivers should be fine. This includes
my 2M sailplanes.

For my larger sailplanes I use only dual conversion receivers. Here I am flying
planes, that may be over 1/2 mile out and 1000 feet or more in altitude. I need
every bit of signal processing I can get to insure I get clean control. I can't afford
even a single glitch. If my plane is on 72 MHz I want a dual conversion system.

You make decisions based on your type of flying. This is what I do.

Some receiver brands offer single conversion, dual conversion and perhaps
other types of receivers. Be sure you get the right kind of crystal based
on the receiver. For example, Hitec dual conversion receivers and single
conversion receivers take different types of crystals. I don't know what
makes them different but you can not interchange them. They won't work.


CHANNELS (all frequencies)

We spoke of channels above in terms of frequency. We also use the word
channels to describe how many servos/devices you can control. So a 4
channel radio can control up to 4 devices. It is OK to have
more channels in the receiver than your radio has as some slots are used for
things other than channel control. For example, if we have a 4 channel
radio and are flying a 4 channel plane your slots might be used like this:

1 per control channel = 4
1 receiver battery
1 for plane locator or battery monitor

In this case you might want a 6 channel receiver to give you 6 slots. Or you
can use one or more Y cables to share slots. However I prefer to have a
receiver with extra slots rather than use Y cables. I feel it will give me
greater reliability. Rather than putting money into Y cables I would rather
put the money into the receiver.

If you have a 3 channel electric plane, you need a minimum of a 3
channel receiver. You don't typically need a separate slot for a receiver
battery as your electronic speed control normally provides the receiver with
battery power from your motor battery. You can use a 3, 4, 5, X channel
receiver, but it must have at least 3 channels.

You can also use a 2 or 3 channel receiver with a 4 or more channel radio,
but you will only have 2 or 3 channels of control available. An example
might be to use a 3 channel receiver for your R/E/T plane but use a 4
channel radio to fly it. That works!


COMPUTER RADIO AND CHANNEL MIXES
True for all radios regardless of frequency

If you are splitting functions using mixes in a computer radio your
receiver may need more channels. For example, if you have a computer
radio, you might be able to use two servos for your ailerons and have each
work from its own channel. Each aileron will be controlled its own channel.
Some radios can put the second aileron on any channel and some require they
be on specific channels. Consult your manual for guidance here.

Here is an example where we use more than one slot for a function because we
have individual servos on each surface. This is the layout of one of my
gliders and is controlled from my Futaba 9C computer radio. I use an 8
channel receiver and 7 servos.

Ailerons - channels 1 & 7
Flaps - channels 5 & 6
Elevator - channel 2
Rudder - channel 4
Tow hook release Channel 8
Battery - uses channel 3 slot
Plane Locator - Shares channel 8 slot with the tow hood release servo
via a Y cable


POWER TO THE RECEIVER

Note that most receivers operate at 4.8 to 6 Volts though some can operate
at higher voltages. In glow, gas and gliders, this is usually supplied by a 4-5 cell NiCD
or NiMh receiver pack but can come from lithium packs with voltage regulators. In
planes using glow or gas power, or in gliders, this is a battery pack that plugs into the
receiver or into a switch that goes into the receiver. There are some new
receivers that can work on a two cell lithium pack of 7.4V. There are some tiny
receivers, made for indoor flight and micro planes that can
operate on one lipo cell at 3.7 V. Always read your
manual, but in general, never directly plug a battery pack of more than 6 volts
into your receiver unless you are sure it uses a different voltage or you will
release the "magic smoke" and the receiver will not work.

Note that your receiver might be able to operate on 7.4V, 2 cell Lipo, but your servos
may fry at that voltage so be careful about what receiver pack you use.
RTFM, read the friendly manual.

If this plane has an electric motor, the receiver will most likely get its
power from the ESC, electronic speed control. Note that even though your
flight battery might be 11.1V or higher, the ESC has a circuit that steps
this down to 5 volts to power the receiver. This circuit, called the BEC,
battery eliminator circuit, eliminates the need for a separate receiver battery.

If you look at the manual for your ESC, it probably indicates that, if you
use more than a certain voltage for your motor pack, you will need to go to
a separate receiver battery. This is because the BEC can only step the
voltage down so far. Or it may say the BEC can handle up to 4 servos on the
receiver up to a 9.6V motor battery, for example, but you are restricted to
3 servos if you go above that. After that it has to be bypassed, you need
a separate receiver pack.

There is an article on the BEC in this e-book. Be sure to read it.


Summary

The receiver is the most critical of all the electronics you will put in
your plane. The most expensive radio with the wildest features is just a
paperweight without a good receiver to carry out its instructions. While
the terms can be confusing at first, you should now be prepared to choose
a receiver with confidence. Remember to always consult your radio manual
for any specific needs of your radio system.

A key point is that it is the receiver and not the radio that really
dictates the range you can expect. I encourage you to be very aware of the
range rating of your receivers so you don't lose a plane by exceeding your
safe range.

Your receiver has to have enough channels to accept commands from your radio
and to accommodate the number of servos/devices you have in the plane.
However the number of channels in the receiver does not have to match the
number in your radio.

2.4 GHz systems and do away with many of the issues and points of consideration
for 27 and 72 MHz.

Receivers and radios are very tightly tied together. If you start with an RTF airplane
package then use the radio that came with that package and consider that radio
dedicated to that package. When you are ready to take the next step THEN you
are ready to look at making that larger investment in a "good radio". There is a chapter
later in the book on selecting your first radio. I recommend you read it before making
your first radio purchase.