More FlightDopp Information

Major Features
User Requirements
Diagrams and Pictorials
General Description
Minimum System
Description of the DF unit
Description of ANTENNA boards
The Control Port
The Audio Port
Antenna Installation Details
More Info and Inquiries

Major Features :
  • VHF/UHF receiver + Doppler ADF, ( 4 antennas ) primarily for UAV / RPV operation
  • 10 MHz tuning span, design range 100 to 1000 MHz ( customer specified )
  • AM and/or FM detection with analog audio output to monitor the signal
  • 100% remote control operation via RS232 from a user-provided host computer
    Receiver frequency, 1 KHz resolution
    AM audio level, 16 steps, 3 db/step + on/off
    FM audio level, 16 steps, 3 db/step + on/off
    Squelch level, 256 steps
  • DB15M connector for I/O + 4 SMA connectors for antennas
  • Generates RS232 Agrello format DF messages, relative bearings
  • Includes MAIN DF Unit and four ANTENNA boards
  • Software upgrades via ICSP or chip replacement ( 40 pin DIP FLASH micro )

    User Requirements :
  • The user must provide a host computer with an RS232 port, to operate the FlightDopp DF
  • The host computer software must be provided by the user ( it will be specific to the UAV/RPV requirements )
  • The user must fit/install the four ANTENNA boards into the UAV/RPV, following guidelines described on this page
  • The user must provide DC 11-18 volts, 105 mA power
    Specifications :
    Reciever Freq : TBD ( customer spec ) design range 100 to 1000 MHz, span 10 MHz, 1 KHz resolution
    Reciever Sensitivity : TBD, -110 dBm ( measured ) for DF reports @ 450 MHz
    Reciever Selectivity : TBD, standard 15 KHz, others available
    Reciever Modes : AM / FM modulation, independantly / remotely adjustable levels
    Antenna : Four vertical dipoles, detachable elements
    Ant Scan : Scan rate 700 Hz standard, rotation clockwise ( viewed from above antenna )
    Audio Output : 1/2 watt 8 ohms, LM380 output amp ( for ramp checks or downlink audio )
    Control Input : 4800 baud RS232 from user-provided host computer, 8N1, no parity, no handshake
    Message Output : RS232 Agrello DF message, 4800 baud, 8N1. ( 10 messages / second )
    Command Vocabulary : Recieve frequency + any optional commands ( 28 available )
    DC Power : 11-18 VDC, 105 mA
    Weight : 9 ounces, with antennas
    Size : 3.1 W x 3.6 L x 1.4 H (inches) less connectors, mating plugs and mounting flange
    Voice Filter : Switched-capacitor filter, ( SCF ) 4 sections, bandwidth 0.5 Hz
    Micro : Microchip PIC 18F442 micro, 40 Mhz CPU clock, 3K byte assembly code
    Software Upgrades : Via ICSP or chip replacement ( uses a socket-mounted IC )


    Diagrams and Pictorials :

    MAIN DF Unit [block diagram] ( JPG 48 Kb)
    MAIN DF Unit [mounting pictorial] ( JPG 31 Kb)
    ANTENNA board [block diagram] ( JPG 19 Kb)
    ANTENNA board [mounting pictorial] ( JPG 28 Kb)
    MAIN DF DB15M connector [pinouts] ( JPG 30 Kb)

    General Description :

    The FlightDopp DF is a small / lightweight Doppler-type ADF intended for use in UAVs / RPVs. It includes an integral single-band VHF/UHF reciever and a Doppler DF. Operation is 100% by remote control, via RS232. The user must provide a host computer ( and software ) to operate the FlightDopp DF, and to employ the resulting DF data, as desired. Detected audio ( AM and / or FM ) is available ( if desired ) for relay to a ground control point, via a user-provided control link. AM and FM audio levels ( and squelch level ) are remotely adjustable. Additional software features ( if desired ) can be installed prior to delivery, or after delivery via ICSP or by chip replacement.

    The FlightDopp DF includes the main DF unit and 4 ANTENNA boards. The DF unit contains the receiver and DF circuits, supervised by a microcomputer and controlled via RS232 from the user-provided host computer. The ANTENNA boards are installed at the base of each DF antenna, ( 4 total ) and provide switching for the antennas that drive the DF unit.

    The FlightDopp DF is primarily designed to provide a "DF sensor" facility for UAV / RPV aircraft, but is also suitable in terrestrial applications, if desired. Each FlightDopp is ( semi-custom ) designed and built to customer specs, including the desired VHF / UHF band, and command features and vocabulary. At this time, ( September 2008 ) four FlightDopps have been built and bench-tested at 450 MHz. Three have been delivered to a customer, and are awaiting flight tests. The fourth unit remains here to facilitate any desired software changes the customer might require. ( ultimately it will be delivered, also )


    Minimum System :

    The figure below shows the general interconnections for a minimum FlightDopp DF system.


    Description of the DF Unit :

    The FlightDopp VHF / UHF receiver is a single-band ( 10 MHz span ) dual-conversion superhetrodyne with 10.7 MHz first I/F and 455 KHz second I/F. The first L/O is a fractional-N PLL with 1 KHz resolution which can be built to operate across any 10 MHz span between 100 and 1000 MHz. The second L/O is crystal controlled, at 10.245 MHz. A 4-pole quartz crystal filter is employed at 10.7 MHz, followed by a 4-pole ceramic filter at 455 KHz. A variety of bandwidths are available, standard BW is 15 KHz.

    The FlightDopp front-end circuit employ two dual-gate MOSFETs and a PIN diode "RF summing" switch, controlled by the DF circuits. The I/F circuit employs a type MC3371 IC, with an FM quadrature detector, a noise squelch circuit, and an RSSI output with over 60 dB dynamic range that is monitored by the microcomputer. AM signal detection is accomplished with the RSSI output, which is "fast enough" to follow the modulation envelope of an AM signal. The audio circuits include a 3-channel digital pot, controlled by the host computer. One channel for FM audio, one for AM audio, and one for an optional "SquealTone" output, which indicates signal strength with a variable-pitch audio tone. A fourth digital pot is used to adjust the squelch level. An LM380 audio power amp drives the DF Unit output, producing as much as 1/2 Watt of audio for an external speaker. ( for ground tests, non UAV applications, or telemetry audio )

    The FlightDopp DF circuits employ a switched-capacitor filter ( SCF ) with 4 sections, one for each antenna. The SCF is synchronized with the antenna scan rate, and serves to reject any voice audio that may be superimposed on the signal. DF audio is obtained from the reciever FM output, via a path that is not controlled by the digital pots. ( DF audio level = constant ) The antenna scan rate is optimized for the bandwidth of the reciever employed, ( standard rate = approx 700 Hz ) and the bandwidth of the SCF itself is about 0.5 Hz.

    If no DF "tone" is detected in the reciever audio, RS232 DF reports are suppressed. Otherwise, RS232 DF messages are transmitted 10 times per second. DF messages comply with the ( popular ) Agrello DF message format, shown below :

    Agrello DF message format :



    Where % = message start character BBB = signal bearing degrees ( 000-359 ) Q = signal quality ( 0-9 ) ( fixed @ 7 in this design ) <cr> = carriage return

    The RS232 message is transmitted at 4800 baud, with an 8N1 byte format. The output port is true "RS232 compliant", using a MAX232 chip.


    Description of ANTENNA Boards :

    The FlightDopp antenna boards are designed for mounting directly at the drivepoint ( mounting point ) of the four DF antennas. The ANTENNA boards are designed to accept a dipole antenna, each antenna element being threaded with a 4-40 thread at the mounting end. An onboard broadband BALUN transformer provides a clean signal transition from the balanced dipole signal to the unbalanced co-axial feedline. A PIN diode provides direct antenna switching at the drivepoint, and is operated by DC switching signals, superimposed on the feedline center conductor.

    The antenna design employs a modified version of the Joe Moell Wideband Doppler Antenna design, which operates by switching each antenna feedline at both ends, using PIN diodes. This "double-ended" switching method yields a wideband antenna design which allows the length of the antenna lines to be arbitrarily selected. ( although all 4 lines must have similar lengths ) The feedlines are hard-soldered directly to the ANTENNA boards, to eliminate the weight of connectors. The opposite end is terminated into SMA type connectors, to allow disconnect of the DF unit.

    The 4 feedlines consist of RG-175 cable, which must cut to proper length and installed by the customer. ( SMA is already installed, if desired ) The length of the feedlines for the NORTH and WEST antennas must be equal within 1/4 inch, and the length of the feedlines for the EAST and SOUTH antennas must be 1 inch longer. ( the difference is due to 1 inch of "extra" signal path length for the NORTH and WEST antennas, inside the DF unit )


    The Control Port :

    The control port is 100% "RS232 compliant", using a MAX232 chip. RS232 profile for both TX and RX is 4800 baud, 8N1, no parity and no handshake. RS232 lines provided on the DB15M connector are RX, TX and SG. ( see DB15M info in Diagrams and Pictorials for pin numbers ) The command vocabulary ( minimum ) allows remote programming of the desired frequency, and DF reports will be generated automatically whenever a signal is detected on the channel, as long as the signal is present. ( 10 messages per second )

    Additional commands ( up to 28 presently ) are available, to operate the AM, FM and SquealTone audio channels, and squelch level. ( on / off / up / down ) More commands include maintenance commands to stop antenna scanning and select a single antenna. Circuitry is provided to also measure the signal frequency ( discriminator DC voltage ) and RSSI values, which can be reported with a STATUS inquiry command. A list of the present available command features is provided below : ( exact command syntax is not shown )

      AM OFF
    AM ON
    AM UP
    AM DN
    FM OFF
    FM ON
    FM UP
    FM DN
    SQL ON
    SQL UP
    SQL DN
    ANT N
    ANT E
    ANT S
    ANT W
    Turn AM audio channel off
    Turn AM audio channel on
    Increase AM audio level 3dB
    Decrease AM audio level 3dB
    Turn FM audio channel off
    Turn FM audio channel on
    Increase FM audio level 3dB
    Decrease FM audio level 3dB
    Turn RSSI TONE audio channel off
    Turn RSSI TONE audio channel on
    Increase RSSI TONE audio level 3dB
    Decrease RSSI TONE audio level 3dB
    Turn SQUELCH off
    Turn SQUELCH on
    Increase SQUELCH level 1 step
    Decrease SQUELCH level 1 step
    Auto-set SQUELCH level
    Enable antenna scanning
    Select NORTH antenna ( kills DF reports )
    Select EAST antenna ( kills DF reports )
    Select SOUTH antenna ( kills DF reports )
    Select WEST antenna ( kills DF reports )
    Reply with present DF status
    ( RX freq, signal RSSI + FREQ ERROR, pot levels, ANT SCAN status )

    Commands are echoed back to the host ( verbatim ) only if they are recognised and executed. In some cases, a command may fail to echo back if the command cannot be executed, i.e. "AM volume down" while the AM channel is turned off, or if the AM volume is already at its minimum value. The commands are saved in non-volatile EE memory, and restored upon coldboot, to allow recovery if DC power is interrupted. This includes the settings for frequency, audio channel levels, etc.


    The Audio Port :

    The audio port can generate up to 1/2 watt of audio into 8 ohms, being driven by an LM380 type audio power amplifier IC. There is no need for a speaker in a UAV / RPV, but this facility can be useful for ground tests or non-UAV applications, and also provides audio for a downlink TX, if remote monitoring of signal modulation is desired. Three audio channels are provided, each with independant remote control of the audio level, 16 steps of 3 db/step. Each channel can also be independantly turned on or off, with the last level setting "saved" in non-volatile memory ( when channel = off ) and later "restored". ( when channel = on )

    The AM and FM channels contain detected signal audio from the AM and FM detectors, respectively. The SquealTone signal is generated by the microcomputer, and expresses ( acoustically ) the relative strength of the signal, with a variable-pitch audio tone. ( higher tone freq = stronger signal ) The RSSI output of the receiver drives this signal, and the RSSI output spans more than 60 db of dynamic range for signal strength. The corresponding audio tone spans 3 octaves, to express this 60 db RSSI range.

    The squelch circuit is a typical FM reciever noise-seeking squelch, triggered ( = closed ) by ultrasonic noise in the FM detected audio. No "carrier squelch" function is presently available, but would be easy to add. The squelch level can be remotely adjusted in 256 steps, to allow for variations of local noise levels or interference.


    Antenna Installation Details :

    Antenna installation is not highly critical, but some attention to specific details is important for best results.

    The ideal antenna installation would employ square geometry for the 4 antennas ( one at each corner of the square ) with each side of the square no longer than 1/3 wavelength, and 1/4 wavelength reccomended. The axis of the antenna elements should all be parallel, and orthogonal to the geometric plane containing the square. ( the "baseline" square ) The antenna elements should each be 1/4 wavelength long, and threaded at the end ( 1 inch of threads ) with 4-40 threads. ( yields a 1/2 wavelength dipole at each antenna board )

    Each ANTENNA board has a small arrow "printed" onto it, to indicate the direction of relative RF phase for that antenna. This arrow should be oriented identically on all 4 antennas... all 4 must point up or all 4 must point down... it doesn't matter which is used, but all 4 antenna boards MUST be oriented the same way.

    The 4 co-axial cables for the ANTENNA boards must be cut to length and "hard-soldered" to the ANTENNA boards, at one end. The opposite end will have SMA-type RF connectors, to allow disconnect of the DF unit. The length of these 4 cables can be any value, but a relationship BETWEEN the cable lengths must be observed. The cables for the NORTH and WEST antennas must be EQUAL, within 1/4 inch. The cables for the EAST and SOUTH antennas must be 1 INCH GREATER than the length for the NORTH and WEST antennas, within 1/4 inch.

    It would be wise to provide a few "extra" inches for these cables, to provide some slack to simplify the install / removal of the DF unit. ( a convenience )

    The SMA connectors should not be over-tightened when installed... they are anchored directly on a thin PC board, and excessive torque will cause damage to the board. Perhaps 15 to 20 degrees of nut rotation beyond "finger tight" is plenty... just enough to ensure the connectors will not "vibrate loose" during flight.

    Acceptable tolerances for all these "rules" exist, and some significant comprimises can be employed, if necessary. In EVERY case, it is STRONGLY reccomended that the host computer employ some kind of correction table for DF bearing errors, which can compensate for a LOT of problems with less-than-ideal installations. A more detailed explanation of the allowable comprimises ( and their consequences ) are provided in the following document :

    Antenna Details


    More Info and Inquiries :

    At this time, ( September 2008 ) four FlightDopps have been built and bench-tested at 450 MHz. Three have been delivered to a customer, and are awaiting flight tests. Additional information about the FlightDopp can be obtained by directing an inquiry to the development engineer : [Bob Simmons].

    Prices are not yet firmly established, and are ( somewhat ) negotiable, based on each customer's specific needs. Prices will range from $1800 to $2500, with delivery 30 to 60 days ARO. A 30% payment upon order placement will be required, with the balance to be paid upon completion, and prior to shipment of the main DF unit. The ANTENNA boards and cables can be shipped after the initial 30% payment, to facilitate customer installation work, and to accelerate UAV completion. Some additional accessories ( not shown here ) are available to simplify ramp checks and tests... enquire for details.

    It is reccomended that an open dialogue be provided between myself ( Bob Simmons ) and one or two technical people ( customer staff ) directly involved in the installation and integration of the FlightDopp DF, to facilitate a successful installation.