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Doppler Systems LLC

Radio Direction Finders and Direction Finding Systems

Doppler Radio Direction Finders and Direction Finding Equipment


Our direction finders consists of two parts – a base unit which does the signal processing and an antenna unit which controls the multi-element antenna array. The base unit issues commands to the antenna (for example the sweep rate) via an RS485 data link. The same serial link is used to synchronize the rotation of the antenna to the signal processing in the base. The system is designed to support many possible arrangements of antennas, processors, receivers and network configurations. The most basic fixed site arrangement is shown below.

Block Diagram of Doppler Direction Finder

In this configuration, a single processor unit is connected to a single antenna and a single receiver. Bearing data is displayed on the PC which is also used to control the DF and the receiver. The DF processor provides a host USB port for connection to the receiver allowing it to be remotely controlled. Audio from the receiver is processed to compute the bearing angle. It is also filtered to remove the commutation tone and drive the loudspeaker shown. The received signal strength (RSSI) is read through the USB port and output along with the bearing data to the PC.

Each fixed site DF antenna has a bandwidth of one octave (115-250, 250-500 and 500-1000 MHz). Antenna switching is accomplished within the antennas themselves, thereby allowing a single control cable and a single coax cable to be run to the stacked antenna array as shown here.

Block Diagram of Three Band Direction Finder

Multiple channels can be simultaneously monitored and bearings computed using the same DF antenna. This is accomplished by connecting multiple processors together in a daisy chain arrangement. The Master DF controls and synchronizes the antenna and the Slave DF processors. Two channels are shown below but the scheme may be extended to thirty two.

Direction Finding on Two Frequencies with one Antenna

In mobile operation, the fixed site antenna is replaced with a magnetic mounted RF summing unit connected to the roof top magnetic mount antennas.   Using the MPT User Interface software a simple homing direction finder can be constructed by simply adding a narrowband FM receiver.

Simple Mobile Radio Direction Finder

When running an application such as TargetTrack, a laptop PC will be connected to the MPT using the Ethernet port and the GPS will be connected to the MPT USB port either directly or through a USB-to-serial converter. A compass or yaw rate sensor may also be connected if required.

Doppler Mobile Radio Direction Finder for Geolocation