This page is not a step-by-step manual how to built video downlink for FPV flight. I did try to make my system as simple as possible but with time it became a "monster" - don't use it as a gauge but rather as a resource that may give you an idea or two. The quickest way to start flying aircraft from First Person View is to purchase ready to fly video downlink system. For example BlackWidowAV.com sells one. If you have good soldering skills and desire to experiment than RC-CAM forum will give you great jump start and help in building your own system. This system used many ideas discussed on RC-CAM forum. Thanks to all the people for their help. All rights are reserved.

	Diversity video downlink FPV flight puts a person on the ground in control of the aircraft as if he/she would be sitting inside the cockpit. Needless to say that reliability of the video connection is crucial. Because aircraft itself can turn in any direction it is hard to find a place on its body that will not be blocked from the ground antenna some time during flight. This blockage happens most often with RC helicopters and it means loss of video connection. To avoid the problem I've decided to deploy multiple transmitters on the body of the helicopter. If one antenna is blocked by metal or carbon fiber parts of the frame the other will continue to transmit the signal. Multiple antennas on the ground are serving the same purpose. The ground receiver will have to decide which video channel to show to the pilot and switch quickly to that channel. This type of video system is called diversity and this is what I am after. It took me few nights to put it together and I was delighted when it started to work!
	System unit My diversity system can have three independent transmitters. System unit routes the power to external 2.4 GHz transmitters and GPS receiver. It also keeps together some small parts: R/C activated switch  from hobby-lobby selects data from TL100 or Intuitive Circuits board to overlay on the video. TinyTrackII takes GPS coordinates from GPS receiver and encodes it in audio tones that can be sent over audio channel. PT-5041N 5->12 voltage converter, modified for 10V output. Allowed voltage range 5.0 to 9.0V - it works great with two Li-Ion cells that normally produce 7.5V when charged and 5.5 V when discharged. 5.0V 3A LDO voltage regulator to power up all 5.0 V components. Lawmate 900 MHz transmitter is build into system box. Aluminum enclosure with carbon-fiber plate reinforced walls. Total weight is 200 g.
	Lawmate 900 MHz transmitter 500 mW, 900MHz video transmitter is a part of the system box, shown here during early stages of construction. It is sold by www.BlackWidowAv.com PT-5041N can be also seen along with ferrite chokes. The series of chokes and capacitors are required to suppress high frequency noise from the voltage converter.
	Felsweb 2.4 GHz video transmitter 500 mW video transmitter. It can be ordered through the internet and requires some soldering to connect power and video signal to the transmitter. Headers to the transmitter and receiver connector are available from Felsweb and very recommended to simplify the soldering job. Great feature of this transmitter is 5V power supply requirement. I attempted to solder wires directly to the inside of tuna can and it was pretty difficult to do - I wouldn't try it again. SMA female connector to the RF output is not difficult to solder in.
	2.4 GHz turnstile antenna Antenna is the most crucial element that can make or break a video downlink system. I think of antenna as interface between electronic circuit and the outer space - it transforms motion of electrons into electromagnetic waves. Turnstile (crossed dipole) antenna is my choice for aerial because it transmits signal uniformly in all directions. Quadrature hybrid is a small PCB device that shifts the pahse of RF signal by 90 degrees and helps a great deal in building turnstile antenna for 2.4 GHz. Use these links for design notes and discussion
	Diversity receiver The receiver is the heart of diversity downlink and the most labor intensive unit to build. I've tried to use as many commercial components as possible. For example LED signal strength indicators are VoltWatch devices available for $11 from tower hobbies. The receiver consists of two Felsweb receivers on 2.4 GHz and one Lawmate 900 MHz receiver. The idea of diversity switching is to select the receiver which display the strongest signal (RSSI) at any given moment. The switching between video and audio channels is implemented using Maxim IC. Two manual switches on front panel allows selection between 2.4 GHz (Ch2) + 2.4GHz(Ch3, Ch2) or 2.4 GHz(Ch2) + 900 MHz  Schematic diagram.
	Receiving antenna Diversity system gives higher reliability by utilizing multiple antenna arrangement. You see two 2.4 GHz antennas connected to two different receivers inside diversity box. The antennas can cover independent or overlapping sectors of space. Their polarization and gain can be different. The diversity switch will always select the best antenna to use. Low gain (high beamwidth) antenna can cover large sector when aircraft is close without need to point the antenna. The diversity system will switch to high gain antenna at far distances provided that high gain antenna points in the direction of the aircraft.
	Telemetry TL100 board from Sky-Spy Co Latest revision of the board is shown on the picture which has temperature compensated pressure sensor, GPS data and temperature display capability. Weights 60 gr.
	CCD cameras Panasonic's GP-CX161 and GP-CX171 cameras are good value, miniature CCDs. Their biggest advantage imho is 5V power supply. 330 (161) or 480 (171) lines of TV resolution, 5 lux sensitivity, weight - 21g, power - 5V, f=2.5 mm lens, 75 degree HFV GP-CX171 has better resolution compared to 161 or any other small CCD's that I've tested by at least 100 lines View test picture from GP-CX171
	Antenna tracking Ground station will track aircraft with antenna and display trajectory over the map. Double quad antenna is mounted on winch servo for panning and regular servo for tilting. Low noise amplifier is used to boost RF signal over long and thin coax line from antenna to diversity receiver. The ground station software is written in Matlab and runs on IBM Laptop (233 Mhz PII). GPS data from aircraft are sent using Tiny-Track II module connected to audio channel of video transmitter. AGWPE driver running on Win 98 laptop listens to the audio input and decodes audio into APRS packet format
	Onboard avionics 6 degrees of freedom inertial navigation system from www.carvec.co.uk is attitude stabilization and flight augmentation device. Controls helicopter attitude propotional to stick defelction Rotor RPM governor and HH rudder gyro are built in. Has camera gymbal stabilization option. Has flight recording (aka BlackBox) capability.
	Field case Aluminum gun case hosts all the electronics 2 x 6V 12Ah Lead Acid battery supplies power to all the systems In addition to diversity receiver and laptop there are: LCD monitor Video goggles VGA to NTSC converter PIP inserter Sony's Digital-8 TRV-103 camcorder for flight recordings
	Mobile ground station - it's warm in there! The tracking antenna will automatically adjust for car heading and location. I can fly a model while the car is moving.