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Maximum legal power . . . easy to build . . . fantastic range . . . cheap . . .
2.4GHz Audio/Video Link
Invested in a home theatre system? Maybe you have cable
or satellite TV? What do you do when you want to watch the
program on another telly in another room?
by Ross Tester
I
t’s fairly unusual these days to find
a home with only one TV set. But
it’s certainly not unusual to find a
home with one audio/video setup –
say a VCR, DVD perhaps. How do you
get audio and video signals from the
source to the remote TV set?
And what if you have cable/satellite
TV? It’s very unusual to find homes
with more than one cable or satellite
receiver.
Running cables is one way. But it’s
often not easy – sometimes nearly
impossible to do a neat (hidden) job.
And it’s so passé these days, since
there is a much simpler way to do
it: you transmit the audio and video
signals via dedicated transmitters and
receivers.
Sounds expensive? Not at all. Especially if you use these pre-built 2.4GHz
modules from Oatley Electronics.
They are called “kits” but all you have
to do is connect suitable 12V DC power
supplies (and they have those, very
cheap too!). Of course, you’re going
to need a source of audio/video (and
suitable RCA connection leads) for the
80 Silicon Chip
transmitter, and leads to connect the
audio and video outputs of the receiver
to your suitably-equipped TV set. (No
RF output is available; your TV needs
to have direct video and audio inputs.
Fortunately, most modern sets do have
such inputs).
To preclude any interference from,
say, a neighbour using similar modules/frequencies (or even other 2.4GHz
devices which are now legion), both
the transmitter and receiver modules
have four channels, set on the PC
boards via a four-way switch. As
long as both are set the same, they
should talk to each other. And if there
is interference, simply select another
channel. Four LEDs on each PC board
identify the channel in use.
Is that all?
Well, yes . . . and no!
Yes, it can be as simple as that.
Both transmitter and receiver have
miniature coax antenna leads connected. But if you bare 32mm of coax
braid from each, you’ll have a fully
functioning 2.4GHz antenna. Align
both antennas in the same direction
– and you’ll have a range of about 10
to maybe 50 metres or so.
Because the operating frequency
is so high, you may find that there
are some dead spots caused mainly
by metal objects in the way (eg, the
reinforcing rods between two floors
in a concrete home or unit).
And that’s where the “no” comes in.
As it stands, the transmitter output is
just on the maximum allowed for these
devices – 10mW. But if you connect
an external “gain” antenna the range
can be significantly increased.
Adding an appropriate gain antenna
does not increase the output power of
the transmitter. Instead, it concentrates
the power in one direction, meaning
that the range in that direction is
greater.
Wifi antenna
And where do we get an appropriate
gain antenna?
Regular SILICON CHIP readers may
recall our article on WiFi back in
November, 2002. You may also recall
www.siliconchip.com.au
The transmitter (left) and receiver (right) mounted in their cases. The four LEDs which show channels are clearly visible
in the receiver pic but are bent straight up in the transmitter. The channel-setting switch is alongside the LEDs. Note our
comments about the input sockets being oriented differently: the colour coding is clearly visible here.
that there were several WiFi antennas
in that article, designed to operate on
a frequency of 2.4GHz.
2.4GHz? Mmmm! Sound familiar?
Oatley Electronics have taken that
basic antenna design (based on a PC
board) and refined it by adding a
“reflector”.
As its name implies, a “reflector”
reflects energy that would otherwise
be transmitted behind the antenna to
the front of the antenna, effectively
increasing the amount of energy in
that direction.
In other words, it’s a “gain” antenna,
as we were discussing a moment ago.
The reflector is nothing too spectacular, nor expensive. In fact, it’s
downright cheap – something you
can find in just about every kitchen:
aluminium foil.
The Oatley Antenna Kit consists of
a PC board etched with the appropriate pattern and a suitable weatherproof plastic case. (They leave you to
scrounge the small piece of aluminium
foil you are going to need).
Theoretically, you could add a gain
antenna to both the transmitter and
receiver. In practice, though, you won’t
require it unless you’re thinking about
V-E-R-Y long range. With one antenna
on the receiver, wall-to-wall signal was
achieved at more than 100m.
As well as increasing the range
significantly, adding a gain antenna to
the transmitter would do two things:
first, it would make the system very
much more directional – perhaps too
directional. You would probably need
very good aim of both antennas.
Second, and possibly more important, adding a gain antenna to the
transmitter could make it illegal.
We haven’t checked on this but
suspect increasing the power in one
direction could be in breach of the
rules under which 2.4GHz audio/
video transmitters can be used without a licence.
The modules are FCC (US) approved for the purpose, operating
into a dipole or monopole antenna.
Australian and NZ rules tend to mimic
the US ones.
Building it/them
As we mentioned before, apart from
the receiver antenna (which we will
look at shortly) there is very little you
have to do except mount the modules
in suitable boxes. We used a couple of
DSE Cat H-2512 cases – a little large as
far as the transmitter was concerned
Just to reinforce
the point that they
are different, this
is the front panel
for the receiver.
Hang on, or is it the
transmitter . . .
www.siliconchip.com.au
but about right size for the receiver.
Front panels in each were drilled to
accommodate the on/off switch, DC
power socket and three RCA sockets
(stereo audio and video). Note that the
receiver and transmitter are different
and the three RCA sockets are also in
different order.
That could be a trap for young players but fortunately the RCA sockets
comply with international colour coding now used on practically everything
A/V: red and white sockets are right
and left audio respectively, yellow
socket is video.
3-way A/V connecting leads are
readily available with the same colour
coding. And you should find the same
colours used on the back of your VCR,
DVD, cable/satellite/HDTV box, etc.
The LEDs we mentioned earlier
are not even brought out to a panel,
because they are basically “set and
forget”. If you really wanted to, they
could be taken off the PC board and
mounted on the panel with leads back
to the PC board but we hardly think
it’s necessary.
Mounting the PC boards is a little
different to normal because they are
pre-assembled modules and their
mounting holes do not line up with
any mounting holes in the cases.
All we did was push the switch and
sockets through their respective holes
in the front panel, then sit the PC board
in the case and drop hot melt glue over
the four corners of the board.
The board will naturally sit up off
the case bottom because of the other
January 2004 81
were horizontally polarised. Maybe
another dollop of hot melt glue could
hold the antenna exactly where you
want it.
Remember that signal radiates basically at right angles to the antenna wire
so for best results, in worst cases you
may need to orient the cases themselves at the same angle to each other
but this probably won’t be necessary.
The gain antenna
Reproduced from
the “WiFi” feature
in the November
2002 SILICON CHIP,
the diagram
above shows the
dimensions of the
2.4GHz antenna
which Oatley
have made into
the PC board
version at right.
Note the method of
anchoring the coax
to the board.
mounting pillars underneath it, so a
good dollop of hot melt glue will be
needed to bridge the gaps.
When dry AND after you’ve organised the antennas, put the rear panel
in place and screw the case together.
That’s it!
The wire antenna
If you only need a small (say 10m)
range – which, incidentally, should be
more than adequate for most domestic use – you can use the coax cable
feedlines already connected to the PC
boards as your antennas. (You should
use one of these for the transmitting
antenna anyway).
All you have to do in this case is
carefully cut the outer insulation and
coax braid off the cable so exactly
32mm of inner wire is showing. You
don’t have to remove the inner insulation – it won’t affect transmission or
reception one iota!
You could lay both cables flat inside
the case (say along the rear panel) and
this would ensure that both antennas
At its simplest, the WiFi antenna
could be glued to the top of the receiver
case so that it stood perpendicular to
the case (ie, one of the longest board
edges glued to the case). Hot melt glue
makes life easy.
The centre wire and braid of the
antenna coax lead simply solder to
the two centre pads of the antenna
PC board. It doesn’t matter which
one goes to which. Technically, best
results will be achieved if the coax is
soldered straight onto the copper side
of the PC board.
But if you are going to the trouble
of using the WiFi antenna kit, you
might as well go the whole hog and
add a reflector and mount the antenna
inside a suitable case. The Oatley kit
includes such a case but no mounting
hardware.
First, file or cut a small notch in
each of the four ribs on the ends of
the case, so the PC board can “click”
into place.
While the distance between the
reflector and the dipole tracks on
the PC board are theoretically critical, we haven’t found that to be the
case in practice. (It may be more so if
The reflector (left) is simply a piece of
aluminium foil glued to the bottom of
the case. The PC board antenna slots
into place above it after the side guides
have been notched to accommodate it.
82 Silicon Chip
www.siliconchip.com.au
WHERE FROM,
HOW MUCH?
This project is available exclusively
from Oatley Electronics. There are
several components to the project,
depending on what you want:
Transmitter & Receiver (K199)
– Two PC boards, built and
tested, inc. antenna coax $59.00
These two photos give a good idea of
how the gain antenna case is mounted
on the receiver case. Not seen here is
the small hole drilled in the top of the
case through which the antenna feed
coax passes.
you use one of these antennas on the
transmitter).
Next, cut a piece of aluminium foil
to fit inside the box. It doesn’t have to
be an exact fit. Secure this to the bottom of the box with adhesive or thin
double-sided tape.
Now connect the antenna lead coax
to the WiFi antenna board as detailed
above. Run the coax as straight as
possible towards the edge of the board
with the two holes in it and secure the
coax with a small cable tie.
Then click the board into place. The
antenna is now finished and ready for
use but ideally the whole box should
be mounted on the receiver case.
Our photos give an idea of the way
we did it: a short length (100mm-ish)
of 20mm OD PVC conduit was cut
and glued vertically to the centre of
the receiver case lid. A small hole was
drilled in the case lid for the antenna
lead coax (OK, caught us – this has to
be done before soldering the lead to
the WiFi antenna board!).
The antenna box was then secured
to the PVC post with a cable tie. This
allows the whole antenna box to turn
(to the limit of the coax) so as to orient
the antenna to its optimum.
Now it’s ready for use. Connect the
video and audio source to the transmitter, connect the receiver to the TV
set video and audio inputs and apply
power. All else being equal, it should
work perfectly, first time. If not, try
adjusting the angle of the receiver antenna. It is much more directional than
the simple wire antenna. If still no go,
open the case and check that a LED is
on in both transmitter and receiver –
and that the same LED is on.
If not, switch either receiver or
www.siliconchip.com.au
2.4GHz Gain Antenna (K198)
– The antenna PC board AND
weathproof case:
$7.00
transmitter to get both on the same
channel.
Once again, if there is obvious interference, try switching channels. You
may also experience reception problems if the transmitter and receiver
are used within, say, 10m or so of each
other with gain antennas. This could
be overload.
Then again, you wouldn’t be using
a gain antenna at such a short range,
would you!
SC
12V DC 400mA plug-packs
–
$5.00 each
Cases as used in project
(DSE H2512) –
$9.75 each
Contact Oatley Electronics on:
Phone – (02) 9584 3563
Fax
– (02) 9584 3561
email: sales<at>oatleye.com
website: www. oatleye.com
Mail:
PO Box 89, Oatley NSW
2223.
Silicon Chip Binders
H Heavy board covers with mottled dark green vinyl covering
H Each binder holds and protects up to 12 issues (or more!)
H SILICON CHIP logo printed in gold on spine & cover
REAL
VALUE
AT
$12.95
PLUS P
&
P
Just fill in & mail
the handy order
form in this issue; or fax (02)
9979 6503; or
ring (02) 9979
5644 & quote
your credit card
number. You can
also order on line
at www.siliconchip.com.au
Price: $A12.95 plus
$A5 p&p each (Australia only; not available elsewhere).
Buy five and get
them postage free.
January 2004 83
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