Page 153 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

132 Bu il d Y o ur O w n Q u a d c o p t e r

original strength at a 128 meter distance. That is a considerable reduction. Transmitter
strength or power is measured in dBm units that are defined as follows:

power dBm = 10 log (power in milliwatts)
Therefore, 1 milliwatt, or 1 mW, is equivalent to 0 dBm. Strictly speaking, the impedance
used in the dBm measurement should be 300 Ω resistive, but if it is not, the measurement is
still largely valid.
The Spektrum DX-8 transmitter that is used in my Elev-8 system has a maximum contact
output (almost touching the antenna) of -10 dBm. That is not a lot of absolute power, but it
is sufficient to its task. At one meter, the actual measured power is -25 dBm for the DX-8.
Every doubling of the distance per the spreading rule of thumb means a linear reduction of
10 dBm, which is why using dBm units is so handy. If you use the example from above, the
128 meter distance would mean that the transmitter power at the receiver antenna would be
-10 + (-70), or an absolute -80 dBm. This is a very small power level but remarkably well
within the Spektrum AR8000 receiver’s capability.
The other major culprit in signal loss is the line-of-sight restraint. The 2.4-GHz signals
operate in the RF frequency region where signals travel in a straight line or line of sight. If you
cannot see your aircraft, you can be pretty sure that it will not be able to receive your signal.
A simple maneuver, such as flying your aircraft to the side of your house, will likely cause
loss of signal due to losing line of sight. However, although it is true that the signal could
reflect off a nearby object, such as a neighbor’s house, I would not bet my quadcopter on that
happening. Obviously, flying your quadcopter so far away that you can no longer see it
would be bad on several levels, as discussed previously. You would also be flying it too high
and breaking some civil regulations to boot.

AM and FM Modulation and Noise
AM is probably the one modulation type that is most susceptible to noise because almost
any nearby electrical source can generate spurious radio waves. You are probably familiar
with the somewhat noisy AM radio in your car. It will often pick up noise from your car
engine as well as from nearby cars. The same condition may happen if you are operating
your model outside your home. For instance, a nearby gas lawn mower could generate noise
across a broad spectrum, especially if the spark plug is unshielded. AM receivers have no
means for detecting and counteracting interfering signals that may cause the devices they
are controlling to go out of control. AM is often used in extremely low-cost R/C toys, which
is fine, since they are usually quite small and will not harm people or things if they suddenly
go out of control. However, AM-based systems are to be avoided in the Elev-8 system or any
other quadcopter application. Using an AM system is to invite disastrous consequences.
FM is much more resistant to noise, as you might realize if you think about the car radio
example again. FM radio stations always sound clearer and mostly noise free when compared
with AM stations. Part of this clarity has to do with the broader spectrum allocated to an FM
station as well as to the nature of the modulation process. FM R/C systems use what is
known as a narrow band (NB) channel in which the carrier frequency is slightly changed in
response to the data-amplitude change. The narrower the frequency change or deviation,
the more susceptible the FM channel is to interference. You should understand that the
interference results from the receiver losing its lock, or phasing relationship, with the carrier
wave due to noise, not to the introduction of pops or clicks in the signal. Interference is most
often due to multiple reflections of the signal from buildings and terrain.
Another common source for FM interference is the presence of other NB FM transmitters
operating on the same frequency. It is commonly referred to as cross-channel interference.