Spread Spectrum: Are You Ready for Full Range? - 2006/12

Spread Spectrum:
Are You Ready for Full Range?
by Steve Kaluf and Dan Williams
AR7000 receiver(s) with short pigtail connecting the two discrete receivers. The
remote receiver should be at least 2 inches from the main receiver with the antenna
oriented perpendicular to the main receiver’s antenna.
Models such as this Hangar 9 Katana 90 will soon be able to take advantage of spread spectrum radio technology.
ROUGHLY 18 MONTHS ago a new
company introduced spread spectrum
technology to the radio-control-modeling
world. This first system was a surface unit
for use in cars/trucks.
Almost exactly a year ago that
company debuted a six-channel aircraft
radio also utilizing spread spectrum
technology. This new system was limited
to park-size/mini-helicopter use because
of range and antenna considerations.
That unit has literally taken the parksize/
mini-helicopter world by storm.
Modelers who are using these systems
have enjoyed no longer having to worry
about frequency control and interference
from outside sources.
However, we’ve all been left wanting,
wondering when we would finally be able
to fly our larger, glow/gas aircraft with a
similar system.
Well, members, the wait is over!
Spektrum and Horizon Hobby—which
brought you the Spektrum car system and
then the DX6 park flyer system—released
the Spektrum DX7 Full Range system at
the iHobby Expo in October. Now
everyone can finally fly utilizing the
benefits of spread spectrum technology.
This is an exciting time for RC, and it
is truly the beginning of a new era for us.
Let’s look at what it took for Spektrum to
introduce this new system and get it ready
to operate our larger aircraft. (I hope you
have been following along for the last 18
months or so and have read the
information we have published in MA and
on the AMA Web site.)
For this update I will turn this article
over to AMA Electronic Technology
Committee member Dan Williams. He
wrote the article “Digital Spread
Spectrum: The Story So Far,” which was
published in the July 2005 MA. I’ll drop
back in following Dan’s words to tell you
a little more about what to expect with this
new system.
The main problem that exists in a larger
model when attempting to use a spread
spectrum system such as the DX6 is the
propensity for the large metallic and
electrically conductive components to
block, or directionally shield, the
receiver’s antennas from the signal.
The small size of the (DX6’s) AR6000
receiver and its antennas can be
completely masked by a large gasoline
engine, muffler, or carbon-fiber structure.
At 2.4 GHz the transmitted signal’s ability
to “go around” the engine is greatly
diminished.
For that same reason, wireless
components for PC networking have the
same problem going around I-beams or
furnace ductwork in your house. Longwave
radios, such as AM broadcast, work
well around buildings and under bridges.
FM VHF broadcast radios can suffer from
shadowing of buildings and structures.
Your satellite radio really hates anything
blocking its view of the sky and the
satellite. The same problem applies to the
2.4 GHz digital spread spectrum (DSS)
systems.
The AR6000 DuaLink technology uses
two transmitted frequencies that send the
information to the two receivers in the
receiver enclosure. That’s why there are
two antennas protruding from the receiver.
This works well to take care of any
blockage of one or the other receiver from
its signal on a small model. Small motors
and carbon fiber used on small models
don’t block both of the receiver antennas
at any one time, so there’s always a solid
link to the aircraft.
Another problem with using signals at
2.4 GHz is that the antenna orientation can
become critical. By employing a separate
antenna for each receiver, and orienting
them 90° to each other, they see around
the various objects in the aircraft
differently.
December 2006 65
12sig3.QXD 10/25/06 10:15 AM Page 6566 MODEL AVIATION
How the new Spektrum DX7 looks on a spectrum analyzer. The sweep rate is slowed
to show the two frequencies being broadcast. The waveform on the right is still
building. Because of the speed at which two frequencies are sent, it is difficult to
capture both full waveforms at the same time.
AR7000 installed in a helicopter. Main receiver is mounted in the normal location.
Remote receiver has Velcro on its back and is stuck to Velcro straps holding main
receiver in place. Note perpendicular orientation of the two antennas.
Also, as the aircraft goes through
different orientations when performing
aerobatics, there’s always one antenna that
is going to see the transmitter signal for a
solid lock. However, as the model size
grows, so does the chance of large objects
in the model blocking the signal to the tiny
receiver box and antennas.
As model distances are increased,
because of the greater ability to see large
models at farther distances, the critical
nature of antenna direction becomes
worse. The chance that antenna orientation
between transmitter and receiver causes a
signal drop becomes much greater. Big
models can fly longer distances. The
farther away the receiver is, the less signal
it can receive.
Those are fundamental limitations on
the size of model the DX6 and AR6000
receiver can fly reliably. Regardless of the
supposed success stories using this
receiver with big models, there’s a
possibility that the sheer size of the
receiver vs. the model can cause a problem
with the RF link.
The DX7 and AR7000 receiver solve
all of that. Besides the intelligence added
to the DX7 transmitter, which Steve will
describe, the AR7000 receiver is a total
redesign of the AR6000. It too uses
DuaLink technology, but it is now known
as “Full Range” DuaLink technology. The
receiver has undergone development to
eliminate the problems inherent with
combining the AR6000 with a large
model.
The AR7000 continues to use a type of
dual-diversity receiver design. A dualdiversity
receiver uses signals from two
separate receivers to process the
information, and dual-diversity receivers
use dual antennas.
The idea in a dual-diversity receiver is
to have the two antennas located some
distance apart. In doing that, any signal
blockage or other problem in receiving the
signal with one antenna won’t be a
problem with the second antenna, which is
located separately.
The big difference in this system is that
two RF channels are sending information
simultaneously to two receivers. Add to
that all the digital signal processing done
from both receivers, and one has a robust
signal link to the dynamic environment of
a model.
The AR7000 is actually composed of
two receivers with an umbilical cord
between them. Now they can be located in
different parts of the aircraft to take full
advantage of the dual-diversity design.
By placing the receivers in different
positions in the aircraft, any blockage
from, say, an engine probably won’t affect
the other. Signal integrity is maintained
and there is no signal loss.
Another advantage to the new AR7000
design is in the antennas themselves. In
the AR6000 park/mini flyer receiver, each
antenna is more or less a single “whip”
style, or monopole antenna. The new
respect to the transmitter antenna will be
taken care of. No matter what attitude the
model is in, an antenna will be oriented
correctly for a good signal lock.
Now Steve can take on the rest of the
radio-system details.
Thanks for that update on the technology
and what was involved to get spread
spectrum to the point we can use it in all
of our aircraft, Dan.
As I mentioned in the introduction, this
is an exciting time for RC modelers.
Although we certainly do not see this new
AR7000 utilizes two “dipole” wire
antennas instead of whips.
A dipole antenna has added gain (more
signal received) than a wire whip antenna,
plus the added “capture area” of the
dipole. Capture area is physically the
amount of wire exposed to the RF signal
from the transmitter. The longer the
antenna, the more signal that is received.
Finally, by separating the two receivers
for placement and following the
recommendation to orient the antennas
perpendicularly (known as “orthogonal” in
the RF world), all model orientation with
Photos by Steve Kaluf
12sig3.QXD 10/25/06 10:16 AM Page 66technology causing our 72 MHz systems
to disappear, it will certainly ease the
congestion at many flying sites and make
flightlines at large events more enjoyable
for those using spread spectrum systems
and 72 MHz systems. The 72 MHz band
will open up a bit as more spread
spectrum systems become available.
The new Spektrum RC system is, once
again, based on an existing JR transmitter;
this time the new JR 7202. So everything
except the RF link (which is significant) is
JR. I’ve been fortunate to have been given
access to this new system in early August
2006 and have enjoyed flying with it and
learning about it ever since.
As does the DX6, the DX7 uses what
Spektrum calls DSM (Digital Spread
Spectrum Modulation). However, the DX7
and its receiver (AR7000) use what
Spektrum calls DSM2: a secondgeneration
form of this modulation.
The AR7000 receiver is also actually
two receivers. However, unlike the DX6’s
AR6000, the AR7000 is actually an
internal and external receiver. The internal
is located on the main printed circuit
board of the AR7000, and the external
receiver is attached to the board with a 6-
inch extension lead.
As Dan mentioned, each receiver has a
short dipole antenna. That allows the two
receivers to see a different RF
environment, which improves path
diversity, allowing the AR7000 to see the
transmitter’s signal in all conditions.
Spektrum recommends that the
receivers be mounted at least 2 inches
from each other and preferably with the
antennas oriented perpendicular to each
other. This technique maintains as much
path diversity as possible. Spektrum does
point out that the receiver placement has
been proven not to be critical.
Along with this article are pictures of
the AR7000 installed in a helicopter so
you can see how I arranged the two
receivers. Installation is simple, and I, too,
found receiver orientation to be
noncritical.
All flight testing and ground range
testing I have conducted has shown
excellent range for this system. In fact, I
got tired of walking while doing the
ground range tests. The external receiver
is tiny and thin, so it’s easy for which to
find a home.
This system takes advantage of spread
spectrum technology with another safetyenhancing
feature called “Model Match,”
which is patented technology. It identifies
each model stored in the transmitter’s
memory with a code that is unique to that
aircraft memory position. That code is
also transmitted to its receiver. This
means if you attempt to operate Model 1
and the transmitter is set to the wrong
model (such as Model 3), the receiver will
not operate.
The DX7 is also compatible with the
DX6’s AR6000 receiver. However, it is
important when using the DX7 with the
AR6000 receiver to limit aircraft types to
park size and mini/micro helicopters only.
We hope you have enjoyed this look at
the latest technology available to the RC
modeler. The next couple years are going
to be interesting and fun as more
manufacturers jump on the spread
spectrum bandwagon. I suspect each will
have a slightly different way of doing
things.
With the cooperation of each
manufacturer, we hope to be able to bring
you a technical update on each new
system as it is released. AMA thanks
Horizon Hobby and Spektrum for beingcooperative with us in the last two years
as they have been working on this
technology. This has allowed us to learn
about it and hopefully educate you, our
members, properly. MA