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RADIO SYSTEM BASICS
By Ed Anderson
Updated March 2015


We will discuss the parts of the radio system and their function. While the transmitter is the star of the show, without the other parts of the system, it can't do anything for you. So, I will briefly cover the other components of the radio system first.

I can't place photos in the article. Below are the photos. They were uploaded in the correct order.

Some of the photos in this article have been taken of my own equipment either on the bench or mounted in my planes to help you see how it all goes together. I would also like to take a moment to thank Hitec RCD USA, Inc., for providing access to their image library to help us provide additional pictures that will help illustrate the article.


PARTS OF A RADIO SYSTEM
(photos referenced below)

First we have examples of two radios. One is a dual stick, typical of radios with four or more channels. The other is a single stick typical of two to three channel radios.

Next is a photo of three servos. The top is a Futaba standard size servo, the middle is a Hitec micro servo and the bottom is a GWS sub micro servo. I selected these three to illustrate the range of servo sizes. In this group, each servo is about half the size and weight of the one above it.

Next we see an electronic speed control, an ESC, which would be used to control an electric motor in an e-glider. Pure sailplanes and fuel powered planes would not have an ESC.

In the next photo we see the interior of one of my planes, a Great Planes Spirit, showing two servos, the receiver, a switch and the battery as they are mounted in the plane. Note that the battery is wrapped in Velcro, which is glued into the plane to hold the battery in place. Likewise the servos are mounted with screws and the receiver is secured with double-sided tape on the bottom.


The white control arms on the servos will be connected to rods that operate the rudder and elevator. You can see one of the rods, which is yellow, on the top and to the left of the servos. The receiver antenna runs down the interior of the plane and exits at the rear so that it is fully extended for best reception. This is a typical set-up for a beginner plane. Add a radio, connect and adjust your surfaces, balance the plane and you are ready to fly.

Here is another installation. This one is in my Zagi slope glider. This is a flying wing. You will see in the photo that the components are embedded into the foam of the wing. At the top is the green battery, then you see two standard size servos, one to either side. Finally in the center you see the receiver.

The wires have not been finally connected in this photo. The 72 MHz antenna runs through a tube that is embedded in the right side of the wing and exits at the wing tip where the antenna wire hangs lose. In this kind of plane, the electronics are sealed in with the reinforcing tape and covering.

This photo shows the wing finished. Notice the control rods that connect from the servos to the elevon control surfaces. Note the switch, which sticks out through the covering for access. The receiver and the battery are completely buried.


Let's take a look at each component and its function.

The Receiver

The receiver captures the signal from the transmitter, decodes it and uses that information to control the other parts of the radio system. The important thing to know for now is that the receiver needs to be matched to the transmitter for them to work together, though in some cases they don't have to be the same brand. They do have to be on the same frequency or channel. You can get more detail in the articles on receivers, a different chapter of this e-book.

In the photo below you will see two holes in the center of a typical 72 MHz receiver. Receivers on 72 MHz require a crystal to set the sub frequency within the 72 MHz band, similar to having channels on your TV set. This is where the crystal is installed that will designate the channel for the receiver. On 2.4 GHz radio systems there are no crystals.

Note that in RC flying, if we are using a 2.4 GHz radio system we talk about the channels on the receiver. If we are using a 27 or 72 MHz radio system we use the word channel in two ways, which can be confusing. We use channel to describe the sub frequency of the radio/receiver combination. We also use channel to describe how many devices the transmitter/receiver can control. They are completely different and the use of the same word for both in unfortunate. We will discuss more about channels later.

You also see in the photo the pins on the right where the various components plug into the receiver. Here you plug in your, servos, ESC or some other accessory pieces. The receiver shown is a six channel receiver so it can manage up to six devices under the direction of a transmitter if the transmitter is able to transmit on 6 channels. We can still use this receiver if your transmitter that has more or less than 6 channels

Frequency Bands

Today new transmitter/receiver systems are almost exclusively on 2.4 GHz. The radio system takes care of insuring there is no conflict with other RC radio systems. If you are using older equipment, 27 MHz or 72 MHz, the transmitter and receiver must be on the same frequency band and on the same channel within that band. The pilot must follow "frequency control procedures" to insure that no other flyer is on the same channel or the radio systems will come in conflict and the planes will crash.


Range

One important thing you need to know about receivers is that the receiver is usually the part of the radio system that determines the effective working range. I discuss receivers in more depth in another article, but the fact that the receiver determines range should show you that you need to consider what receiver you are using in each application. If your receiver does not have enough range, you could fly beyond its capabilities, resulting in a crash.

I bring this up because some receivers have working ranges of as little as 100 feet while others have ranges in excess of a mile. Therefore you must be sure the receiver you are using is appropriate to your application.


Servos

Servos contain motors that are used to make things move in the plane. They will be connected to the rudder, the elevator, the ailerons, the flaps and other things in the plane that need to be moved to make them operate. Servos come in a wide range of sizes and strengths. We will want to match the servo to the job to be sure it is strong enough, can fit in the space available and be no heavier than necessary as excess weight on a plane is a not good.

A simple 3 channel electric plane will have two servos that manage the pitch and roll functions while the electronic speed control manages the throttle. More complex models may require six or more servos. We won't get into servos to any degree here except to say that the servos plug into the receiver and respond to instructions sent from the transmitter to the receiver. Unlike the receiver, virtually all servos work with all receivers as long as the plugs match; and most do. If you are buying new servos with universal plugs, you can readily mix and match brands with little likelihood of a problem.


Batteries

Your radio system works on electricity, which is supplied from a battery pack that is made up of cells, usually rechargeable cells. Nickel Cadmium, NiCd, Nickel Metal Hydride, NIMH, and Lithium Polymner, LiPo, are the most common rechargeable cells used in our airplanes.

NiCd have the lowest power to weight ratio while Lithium cells have the highest of the ones mentioned here. As a result Lithium and specifically LiPo packs are growing in popularity.

Whatever cell types you use make sure you use a charger that specifically works with that battery type. Charging with the wrong type of charger can damage the cells or lead to a fire. Make sure you have the right type of charger.

There will also be a set of batteries in the transmitter. If your radio system uses rechargeable batteries, the package may include a battery charger that plugs into the wall and charges the radio. There are more sophisticated chargers available, but for most people these simple chargers work just fine for their transmitter batteries.


The Switch

Most electric planes do not have switches. You power them on by connecting the battery. When you are finished with that pack, you disconnect it and remove it for charging. Note that if you leave the battery connected, there will be a constant flow of power to the speed control. This will drain your battery pack. If the pack is NiCd or NiMh, little damage will occur. If the battery is Lithium, this may destroy the battery pack.


Electronic Speed Control - ESC

As covered in an earlier article, the device that controls the motor is called the ESC or electronic speed control. Typically there will be wires that go to the motor, wires to the battery and wires to the receiver so the ESC can distribute power to the receiver, and respond to commands from the transmitter to control the speed of the motor.

multiplex speed control.jpg

Accessories

There are lots of other things that can hook into the receiver and become part of the radio system. Things like altimeters, lost plane locators, battery monitors and the like. However they are not core to the operation of the plane.


The Transmitter

Finally we get to the star of the show, the transmitter. This is the part of the radio system you hold in your hands. It has those lovely sticks and switches and things that allow you do what you need to do to fly a plane that is hundreds or thousands of feet away while you are standing there on the ground.

Transmitters come in a wide variety of prices and capabilities. Of all the investments you will make in getting started in RC flying, your radio transmitter is one of the most important. Which radio you select will determine how easily you can set-up your plane, how easily you can adjust and trim it and how complex a plane you can fly.

I divide transmitters, typically called radios, into two groups, standard and computer radios. Standard radios retain the settings for the currently flown plane. If you want to use that radio to fly a different plane you must manually make adjustments to match the new plane. With computer radios, you can store the settings for many planes in what we call model memories. You select the plane you want to fly from a menu and the radio resets everything to match that model. There are many other features that computer radios bring to the hobby, but for the moment this one feature adequately separates computer from standard radios.

Like all areas of electronics, new models come out often, with each having more features and usually delivering those features at lower prices than the previous generation. Features that were only available in high end systems just a few years ago, like elevon and V tail mixing, are now standard features on some of the lowest cost radio systems on the market.


Budget

For most people budget will be the determining factor in what radio system they buy. Let me say here that you can get a quality four channel radio for under $50 that will fly any of the typical starter electric planes. However, as you will see, there are many more capable radios that are worth your consideration. They offer convenience and higher degrees of control and can handle more complex planes. Depending on where you plan to go in this hobby, a larger up front investment may be money well spent.


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