|
PixController RF
Copyright © PixController Inc., All rights reserved.
Antennas
Simple Antennas for the PixController RF Controller
Board
One of the most important components of the PixController RF
board is the antenna. This is a brief guide to some simple and effective
antennas, how to use them, and what to expect from them.
Background
We’re all familiar with the AM radio. You tune it to the
frequency of your favorite station; it receives the radio waves
transmitted by that station, and, through the magic of electronics,
converts them into the sound of the ballgame coming through your
speakers.
The PixController RF is essentially an AM radio. But it is
tuned to a special frequency used by the X-10 wireless motion sensors.
On this frequency, instead of music and voices, the X-10 transmits
digital data which the PixController RF can decode and use to trigger a
camera.
While the AM station transmits thousands of watts of power to
send its signal out over an entire region, the X-10 is limited to a
small fraction of a watt of broadcast power. So the range is very
limited, and the PixController RF antenna is a critical component.
It can be extremely difficult to make an extremely good
antenna. Fortunately, it’s pretty easy to make a pretty good antenna. So
we’ll cover two simple and inexpensive antennas that will work pretty
well for the PixController RF.
Quarter-Wave Monopole
First of all, don’t be intimidated by the name. It’s really
nothing more than a single straight antenna of the correct length. The
antenna on your boom box or cordless phone is probably one of these.
The “quarter-wave” designation on this antenna refers to the
wavelength of the radio signal. Antennas of this type work most
effectively when their length is a specific fractions of the wavelength
of the signal, such as half-wave or quarter-wave. So we want our antenna
at a precise length to pull in as much signal as possible.
Wavelength is inversely proportional to frequency. In other
words, as frequency goes up, wavelength goes down. The relationship
between the two is based on the speed of transmission, which in air is
the speed of light – 186,000 miles per second (or 300 million meters per
second). For the X-10 transmission frequency, the wavelength in air
works out to be about 38 inches. This needs to be adjusted a little,
since the speed of transmission in a copper antenna will be slightly
less. So we’ll call the wavelength in our antenna just over 36 inches.
A full-wave antenna of 36 inches or even a half-wave antenna
of 18 inches would be quite effective, but wouldn’t be very convenient
to have on a compact camera unit. But a quarter-wave antenna, at just
over 9 inches, is effective and its size is much more suitable for the
application.
How can we make a quarter-wave monopole? The simplest is just
a straight, 9-inch piece of wire. A good choice is 22-gauge solid wire,
because it will keep its shape and orientation pretty nicely. Insulated
wire is fine. (see Figure 1) There are a variety of whips, “rubber
ducks,” and telescoping antennas that can be used as well, but these
mainly give better mechanical properties (ruggedness, weather
resistance, etc) but don’t really perform better at receiving an RF
signal. They can also be helpful in getting the antenna away from other
wiring in the unit.
Figure
SEQ Figure \* ARABIC 1: Simple quarter-wave monopole antenna
So a simple wire is a good choice if it is to be internal to
the unit. Connect the wire directly to the antenna port (J15) on the
PixController RF board. You can drill a hole in the top of the case and
use an antenna skin to accommodate the length if necessary. Keep the
wire as straight as possible. The ground pin of J15 can be left open
(see Figure 2)
Figure
SEQ Figure \* ARABIC 2: Quarter-wave monopole connected to
PixController RF board J15
For an external whip antenna, the connection between the
antenna and the J15 must be shielded coaxial cable, with the shielding
connected to the J15 ground. A lot of whip antennas include a coaxial
pigtail. For other antennas, an SMA, SMB, or BNC bulkhead connector can
be connected to J15 using coaxial cable.
Note that monopole antennas have some directional
characteristics, which is to say they are better at receiving signals in
some directions than others. Best reception is from a direction
perpendicular to the antenna. So if you drew an imaginary line from the
X-10 sensor to the antenna, you’d like the antenna to “cross the T” of
the imaginary line at a right angle. The potential reception range will
decrease to a greater or lesser degree as you move away from this ideal
geometry. Of course we don’t live in a perfect world, so this condition
can’t always be met (especially with multiple sensors in different
directions) but the quarter-wave monopole is reasonably forgiving.
Therefore, the typical layout would have the antenna mounted
vertically. This will give best reception in all compass directions out
from the antenna, with somewhat worse reception above and below the
unit.
The PixController RF kit includes an 18” antenna wire with a
connector suitable for J15. If you choose to use the simple quarter-wave
monopole antenna, just cut this wire to 9 inches, attach it to the
PixController RF board, and you’re ready to go.
This antenna setup should give reliable reception at 50 feet
or more, depending on orientation and obstructions. We’ve seen reception
as far as 150 feet under ideal conditions. This is affected to some
extent by other wiring and ground locations near the antenna. Having the
antenna perpendicular to the PixControllerRF board also seems to help
maximize range. Sometimes, due to considerations such as these, it’s
worthwhile to consider using a coil antenna.
Coil Antenna
Compared to the quarter-wave monople, the coil will be more
compact but less effective. It is a good alternative if space is at a
premium and the distance to the X-10 sensor is not overly far.
Coil design is a little more fuzzy than monopole. The length
of wire, coil diameter, and coil spacing all interact in this type of
antenna, and the best antenna is often revealed through experimentation.
One design that we’ve found to work pretty well uses an 18”
wire, which is included with the PixController RF kit. Simply wrap the
wire around a plastic ball-point pen (see Figure 3). Pull out the pen,
and there’s your coil (see Figure 4).
Figure
SEQ Figure \* ARABIC 3: Wrapping a coil Antenna
Figure
SEQ Figure \* ARABIC 4: Finished coil antenna
With this setup, you should get reliable reception at 25-50
feet, again depending on orientation and obstructions.
Conclusion
We think these simple antennas will cover a lot of your
needs. Of course your uses of the PixController RF will expose it to a
lot of conditions of layout, foliage, etc. that we never imagined in our
testing. We encourage experimentation, and we hope you’ll share your
experiences on the
PixController forum.
|
Phone: 724-733-0970 
