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During the summer of 2003 I
decided to try to improve the quality of my on-board video transmissions
by switching to FM modulation. The previous version of my
transmitter used AM modulation. In addition, my
previous receiver was
not tuned by a frequency synthesizer, so I was constantly fiddling with
the tuning knob on it to try to get the best picture and sound quality
possible. The receiver was also very prone to overload when the
rocket was close by and yet developed a little "snow" in the picture when it was far away and not
aligned with the antenna. At any rate, I was able to solve all those
problems with a new transmitter and receiver pair from
G1MFG.
Giles Read at
G1MFG in the UK sells 23cm
(1.2GHz) FM ATV transmitters and receivers that are built and tested. They
are not a kit. He also has 2.4GHz versions. His US web site is at
www.tvham.com and his
UK web site is at
www.g1mfg.com. These units are very good quality and very
reasonably priced. I purchased the 23cm transmitter for $90 and the
23cm "gold" receiver for $110. Both of these operate 1.24-1.36GHz.
I also purchased his new 1W 23cm power amplifier for use with the
transmitter. The transmitter measured 95mW raw output. This
power amp should boost that to nearly 1W. The power amp was an
additional $89. Both
the transmitter and receiver are tuned with a frequency synthesizer that
is switch selectable in 0.5MHz steps from 1240MHz to 1367.5MHz.
However, the upper end is beyond the 23cm HAM band so don't use it there!
Besides video, both units also provide two channels for stereo sound. The
sound subcarriers are at 6.0 and 6.5MHz. Both units operate from 12
to 18V DC. The photos
below show the units from G1MFG as well as how I mounted them into my
existing rocket video module.
Details about the rest of the video module such as the camera, GPS,
overlay card, and so on, are on the page that describes the
previous
version of the transmitter and receiver.
Click here to see that.
The first flight with this new system was on
September 26, 2003 at the XPRS-2 launch at Black Rock dry lake bed in
Nevada. The video for
that flight can be seen here. At this point, I believe the
limitation in this system is now the resolution and image quality from the
CCD camera that I am using and not from the transmitter/receiver.
A second flight was made on November 1, 2003 at the Tripoli Idaho Swan
Falls launch site. That video
is available right here. More flights will be posted on the
video page as they occur.
Click on any photo below to see a larger version of it. |
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This is the 23cm FM ATV
transmitter card from G1MFG. It has three RCA jacks. One for video
and two for sound. There is also a circular plug power connector. The transmit frequency is set by 8 DIP switches.
The RF module is a self contained unit that has an SMA connector output.
I had to drill four holes in the PCA to add 4-40 mounting screws. |
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This is the "gold" 23cm
receiver from G1MFG. It has all the same type connectors as the
transmitter. I had to carefully drill mounting holes on this PCA too. |
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This is the 1W 23cm power
amplifier from G1MFG. The input is via the SMA plug on the 4" flying
lead and the output is via an SMA jack. It is a two-stage design
using discrete bipolar transistors and requires 12 to 14V DC to operate.
It can be tuned to provide up to about 1W with a 14V supply. My unit
measured 0.6W with a 12V supply at 1.255MHz. That's still plenty
enough for my application. (At least this one came with LOTS of
mounting holes! I wish the others did.) |
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I mounted the power
amplifier inside a small box built out of copper clad board that was in
turn part of a larger copper clad mounting board. I wanted to shield
the power amp from the RF from the antenna since it would be so close to
the antenna. A separate power connector was also added for easy connection to
the existing power cables in the camera module. |
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This view shows the same
power amp module inside the small copper clad box, but it also shows how
the transmitter PCA is mounted to the backside of the larger copper clad
card. The flying lead from the power amp also connects over the top
side to the output of the transmitter. |
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Here the top cover for the
power amplifier has been added. It's another piece of copper clad
that is taped in place with copper tape. I didn't want to solder
this in place since I might need to open it up someday to gain access to
the power amp. This view also shows the flying lead from the power
amp connecting to the output of the transmitter. |
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Here's a better view of the
transmitter mounted to the copper clad with the power amp on the back
side. All of the RCA signal connectors and the two power connectors
are on the left side and oriented upward. A short piece of aluminum
channel is bolted to the bottom of the copper clad card to serve as a
mounting bracket in the video camera module. |
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Here the transmitter card
is mounted into my existing video camera module. (After removing the old
transmitter.) Details about the video camera module can be
found on the page that shows it along with the old TV transmitter (the one
that was removed here.)
Click here to
see details about the video camera module.
The RCA signal connectors
and the power connectors can also be seen mated with the appropriate
cables. However, only one sound channel is used here. |
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Here is a little bit wider
angle view of the back side of the whole video camera module with the new
transmitter installed. The power regulator cards are at the top and
have the battery connectors coming off them to the top left. The
back side of the (blue) power switch as well as the three status LEDs
(red, yellow, green) can also be seen. |
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Another view, more
direct-on, into the back side of the camera/transmitter module. The
black thing near the top is the back side of the CCD camera. The
aluminum channel bracket at the bottom of the transmitter card is bolted
to the bottom bulkhead with three small 4-40 machine screws.
The size of the transmitter
card was just right for mounting crosswise in the 5.5" coupler tubing and
also allows plenty of room for RCA cable connectors. It worked out
quite nicely. |
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Here we have rotated around
a little further to the left and can see more details about how the
transmitter card mounts and connects. The CCD camera is the black
device in the upper left corner. Below it, in the aluminum box, is
the GPS unit and the on-screen overlay card. |
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View from the upper right
corner looking down on the unit. Key subassemblies are labeled.
Click on the photo to see a larger image.
The output of the power
amplifier connects to a right angle SMA connector which in then connects
to a RG316 double shielded cable that goes to the antenna.
SMA connectors and cables
are available at
www.digikey.com. |
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I added a small piece of
1/16" thick G10 fiberglass sheet over the top of the transmitter card to
help protect it from accidental damage during battery installation and
removal in the field. It also prevents the sharp edges on the RCA
connector brackets from digging into the batteries and shorting out.
Holes were cut in the G10 for access to the DIP switches and to the FM
deviation control on the transmitter card. The G10 is bolted to the
same aluminum channel bracket that is at the bottom of the transmitter
assembly. |
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Here the batteries have
been added. They are NiCad units normally used for radio controlled
electric race cars. Each one is 7.2V nominal and they are wired in
series to provide 14.4V nominal for powering all of the on-board
electronics. When the batteries are freshly charged they will be
over 8V each, so I also use a custom designed low drop-out regulator to
provide exactly 12V to the camera, GPS and power amplifier. (G1MFG
specifies no more than 14V for the power amp.) However, the
transmitter unit will accept up to 18V, so it is connected directly to the
batteries rather than to the regulated 12V supply. These batteries
give about 90 minutes of transmit time. |
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Right side view with
batteries installed. |
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Left side view with
batteries installed.
All that's left now is to
add the external cover tubing and the top bulkhead that also mounts to the
nosecone and transmit antenna. Pictures of that are the same as for
the previous transmitter and
can be seen here. |
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This completes the
presentation on the transmitter. In the future, I hope to add some more
information here about the custom designed transmit antenna I am using.
It is an omni directional antenna that fits into the nose cone of the
rocket. Check back for details about it. |
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This is the receiver I
purchased from G1MFG at
www.tvham.com. It
operates from 1240MHz to 1367.5MHz and is tuned by a frequency synthesizer
that is set by DIP switches. Tuning is in steps of 0.5MHz. |
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I mounted the
G1MFG receiver
electronics onto a piece of copper clad board that was sized to slide into
the case I planned to use. All of the connectors and the power switch are
mounted on the front panel. |
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Here is another view of the
copper clad board and the back side of the front panel. In this view
the receiver is on the other side of the copper clad. The copper
clad is secured to the front panel by a right angle aluminum bracket that
the power connector mounts through. This avoided adding any
additional mounting screws to the front panel and gave it a more
professional appearance. A power Schottky diode was also mounted on
the bottom of the copper clad and wired in series with the power connector
to provide protection from accidental reverse polarity on 12V. |
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Here is a front side top
view of the front panel and receiver electronics. It shows the short RG316
cable that connects the receiver antenna input to the (gold colored) SMA
bulkhead connector on the front panel. The connections for video and
sound were "hardwired" to the receiver board rather than using the RCA
connectors. |
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Here is a better view of
the front panel itself. Click on the photo to see a larger image.
The lettering is a single decal that I had made by a local sign shop.
I gave them a DFX file from of my CAD program and they produced a very
nice stick-on decal that exactly fit. (Too bad I didn't get it EXACTLY
straight when I applied it.)
The metal thing labeled
"POWER" is just an LED holder that illuminates when power is on. The
12V power connector itself is a standard 5.5mm OD/ 2.1mm ID circular power
connector with center tip positive. The big red rocker switch is the
on-off switch. I only use one channel for sound, so there is just
one audio jack along side the video jack. The antenna connector is
an SMA type jack. |
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The front panel and
electronics assembly slides into an extruded aluminum case. Here the
top cover plate has been omitted. The case I used is made by Hammond
Manufacturing (part number 1455N1601) and is available from
Allied Electronics
(stock number 806-3620). It is roughly 6.3 inches long, 4 inches
wide and 2 inches deep. It has many internal slots that allow a PCA
to slide into the case for simple mounting. The front and back panel
are secured with four screws at the corners. |
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Another view of the case
without the top cover plate installed. You can see the lots on the
interior of the left side wall that the copper clad board slides into. |
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The finished unit with four
screws added to the corners. Note that there is no provision for tuning
this receiver without opening up the case to gain access to the internal
DIP switches on the receiver card. This works fine for my
application since the transmitter and receiver are simply preset to the
same frequency. I use 1.255GHz. |
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The antenna I use is the
same one as for the previous version of the receiver. It is a 22
element Yagi antenna that is 5.5 feet long.
I added two handles to it so that it could be easily pointed at the rocket
during flight. The handles came from a used tool supply store.
They were originally the side handles for a heavy duty power drill.
They came complete with a threaded stud out the top of them for easy
mounting.
The basic antenna is a
model 23CM22EZ from M2
Antenna Systems Inc. It is specifically
made for the 1.25-1.30 GHz band and offers a gain of 16 dBd.
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This view is looking down
the length of the antenna from the rear end of it. The antenna
connects to the receiver with a three foot long piece of 50-ohm RG-316
cable.
The antenna requires an N connector and my receiver requires an SMA connector. |
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In the future, it would be
nice to change the receiver antenna to a circularly polarized unit so that
alignment with the rocket is not an issue. However, with about 0.6W
of transmit power, right now, that alignment has not proven to be a
problem. At least with 10,000 foot altitude flights. |
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To view the video
transmission at the launch site, I use a small portable TV that runs on
12V from a car battery. The TV and receiver are both powered by the
same battery. This TV is a combo unit that also includes a
VCR. I record the transmission on VHS tape and then later (at home)
I play it back, digitize it and convert it to mpeg for display on the
computer. I also use a sun shield made from black foam core board to
shield the screen from bright sunlight. Otherwise it gets washed out and
you can't see anything! (Sun shield not shown here.) |
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