-= R\C MODEL RADIOS =- TYPES: AM - Amplitude Modulated FM - Frequency Modulated PPM FM - Frequency Modulated PCM AM Is an older mode and uses an encoding similar to PPM. The bandwidth is as much as three times that of the more modern FM modes of PPM and PCM. The AM signal consists of a carrier and two sidebands which the digital pulses are imposed on. FM Utilizes a signal carrier which is shifted up and down by the imposed digital information. FM is about 1/3 as narrow as AM. This is a desired advantage since the entire signal is focused into a narrow beam of RF energy. This also results in a greater range and less chance of unwanted interference affecting the signal reaching the plane. When comparing AM and FM types of radios you may think of a tiny light bulb attached to the top of a D cell as the AM_mode. For FM_mode use the same small type bulb placed in front of a reflector as you would in a flash light. Now... at midday walk to one end of a football field and at the other end have someone hold the two lights aimed at you. Which is brighter? Obviously the flashlight, because the light is being focused by the reflector into a concentrated narrower beam as are the FM_modes of today's PPM and PCM radios. PPM Pulse Position Modulation utilizes a series of pulses spaced apart within a 60 millisecond frame. Each pulse represents one of the inputs from your transmitter. There is a beginning timing pulse, and following it are a series of pulses for throttle, rudder, elevator, ailerons, landing gear, and so on. As you move the stick on your transmitter, the proportional spacing between the pulse controlled by the stick grows longer or shorter, depending upon the direction you move the stick. It is the proportional distance change that the receiver detects and causes the control wire voltage being fed to the corresponding servo to increase or decrease. The servo sees the control wire voltage change and moves either one direction or the other to match its internal shaft orientated potentiometer with the change in the control wire voltage. Hence the result is you see the servo move one of your airplane's parts in proportion to the movement you make with the transmitter stick. PCM Pulse Code Modulation is entirely different from the pulse position systems. There is one bad error which can occur with PPM... static ! Every time electricity is turned on or off by a switch, a spark occurs. The spark creates a super wide band bit of interference... or a erratic pulse in the case of PPM. If you're flying at a high speed inverted ten feet off the deck this could spell disaster. Most static spikes are very short in duration and much weaker than the signal coming from your transmitter going to your planes receiver. THANK GOODNESS FOR THAT ! The PCM system incorporates a Hexadecimal numeric coding. The coding includes a checksum algorithm which simply adds the value of each data group sent and tags it onto the group. When the data group reaches the receiver the value of the data is added up and compared to the value of the checksum assigned to that group. If the checksum value and the group value are equal the group is accepted... all others are rejected. The very nature of most radio interference is so erratic... not all of the data groups are affected... allowing the good groups to prevail. With the bad ones tossed aside, a successful flight is still within the reach of most pilots. BOX The BOX or PAD you stand on when flying has an important role few pilots fully know or understand. The pads are carefully spaced a certain distance apart for RF NULLING. Radio Frequencies travel in waves, like the waves in the ocean. The actual frequency determines the distance the waves are apart from one another. If you were at sea in a little row boat with five foot waves trying to wave hello to a friend of yours in another row boat you might find the only time your friend would be visible is when you were both on the crests or tops of two waves. The same applies to radio waves. Instead of wanting to be able to see your buddies' waves, the pads at the air field are positioned precisely so that all pilots are in the valley of each others' radio waves. So when speaking in terms of radio waves... as far as the model planes are concerned the other pilots' transmitters are invisible to one another and no mixing of frequencies can occur causing unwanted "hits" ( a slang term used by many R\C operators). ANT If you leave your antenna only partially extended... only a small portion of the radio wave can be emitted. This causes a dramatic reduction in range ! Hence the reason for not extending it during a "range check". A good tip to remember: The hottest spot from your transmitter is any 45 deg. angle from the antenna toward your airplane's receiving antenna. SO IF YOU SEE YOUR PLANE ACTING LIKE IT IS GETTING OUT OF RANGE... DON'T POINT THE ANTENNA RIGHT AT THE PLANE... HOLD YOUR TRANSMITTER SO THE PLANE IS OFF AT ABOUT A 45 DEGREE ANGLE IN ANY DIRECTION FROM THE ANTENNA ON YOUR TRANSMITTER.
by Peter Magee