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Doppler Systems LLC

Radio Direction Finders and Direction Finding Systems

Doppler Radio Direction Finders and Direction Finding Equipment

Mobile Direction Finder Configurations

August 27, 2018

We sell our mobile direction finders in two different configurations: our component configuration and our packaged configuration.  The direction finding performance is identical for both configurations so why do we offer two configurations?  The answer boils down to customer needs and preferences. 

The minimal component configuration consists of a direction finding processor, an antenna summing unit, and a set of antennas.  To this complement of equipment the customer adds a narrow band FM receiver and a laptop computer.  Additionally, if the customer wants to use our TargetTrack software a our DDF7055 GPS receiver can be added.  The customer has a choice of antennas that cover 100 - 1000 MHz in three bands.  The customer can choose one or more of the bands depending on his needs.  Customers who are looking to permanently install the direction finder in a vehicle usually opt for the component solution as well as customers that prefer to design their own custom packaging arrangement.

The packaged configuration comes packaged in two Pelican cases.  One of the cases houses a chassis containing the DF processor, a receiver, a laptop power supply, and the necessary power and signal cabling.  The other Pelican case contains the antenna summing unit, the antennas, the GPS receiver, and all the necessary external cables.  The packaged system offers a nearly turnkey experience and is preferred by customers who may need to deploy the direction finder on short notice on any of a number of different vehicles.

The table below summarizes the differences in the component and packaged configurations.  For futher information call us at 480-488-9755 or e-mail us.





DDF7001 MPT DF Processor Assy




DDF7080 MPT Summer Assy




DDF706x Antenna Assy







Required to use TargetTrack software

Narrow band FM receiver




Mobile Power Distributor




Laptop Power Supply









Customer Packaging

Transit Case

Packaged System comes in two Pelican cases

Windows laptop computer



MPT User Interface and TargetTrack software provided

1Must be purchased from Doppler Systems
2 Required for DF operation. May be purchased from Doppler Systems or furnished by the customer

In-band Interference from a Nearby Transmitter

July 03, 2018

In general it is not good practice to locate a transmitter close to a direction finding antenna.  But how close is too close?  We've updated our application note on this subject.  In the application note we calculate the separation required to insure that there is no damage to the direction finder.  We also calculate the level of desense caused by nearby tranmitters and the separation required to provide optimum direction finding performance.  If you are contemplating mounting your direction finder near a transmission source and have additional questions please contact us and we would be glad to help you.

Doppler Delivers 15 Fixed Site and 7 Mobile Radio Direction Finders

February 19, 2018

Doppler Systems recently completed a build of 15 fixed site systems and 7 mobile systems for one of our customers. We thought you'd like to see some photos we took during the manufacturing and test of these system. All of the final assembly and test is performed in our Carefree, Arizona facility.


Importing Placemarks in TargetTrack

January 26, 2018

We just released version 3.2.11 of TargetTrack. When using the Great Maps facility in TargetTrack this version allows the user to import placemarks from a .gpx file. This is a standard export format for Garmin software.  To download the latest version of TargetTrack head on over to our downloads page.  Feel free to e-mail us or call us with any questions.

Direction Finding Simultaneously on Multiple Frequencies Using One Antenna

November 13, 2017

One of the often overlooked features of the Doppler DDF7000 series direction finders is the ability to direction find on several frequencies simultaneously using a single antenna.  This is most often used in a fixed site system.  All that is required is a DF processor and receiver for each frequency to be monitored and a RF splitter.  The input of RF splitter is connected to the RF output of the antenna and the output of the splitter connects to each receiver.  One DF processor acts as the master processor.  It controls the frequency band (115-250, 250-500, 500-1000 MHz) and the antenna rotational parameters.  The other processors are daisy chain connected to the master processor.  Each processor has its own IP address so it can be directly accessed by a computer through a network or via fixed assigned static IP address.  Each receiver is tuned to a separate frequency of interest and direction finding data can be simultaneously collected on all monitored frequencies.  A simple block diagram of a two frequency system is shown here, but up to 32 frequencies are supported.

If you would like more information on how to configure a system to direction find on multiple frequencies simultaneously please contact us by phone 480-488-9755 or by e-mail.

Using Doppler Direction Finders with AOR DV1 Receiver

October 26, 2017

Recently one of our customers requested that we include the AOR DV1 as a receiver option.  We are always happy to support as many receivers as we can so we gladly agreed.  Our customer sent us a DV1 and we proceeded to add the firmware to control the frequency, squelch, and volume.  Since each receiver requires a different set of calibration constants we performed our calibration routine and entered the constants.  All appeared to be well.  However, occasionally when the receiver was powered off and then back on the bearing would shift by 180 degrees.  We traced the problem down to the fact that the phase of the DV1 audio is not constant for a given received signal.  Something in the processing of the received signal causes the phase to shift.  This issue does not make the DV1 unusable with our direction finders; however, each time the receiver is turned on the user must verify the calibration.  We have reported this issue to AOR and they have indicated to us that they are working on a fix.  When that fix becomes availalble we will release firmware that will support the DV1.

In the interim if you plan on using an AOR DV1 receiver with our direction finders, please make sure you check the calibration prior to proceeding on your direction finding journey.

Direction Finder Sensitivity

October 05, 2017

In our specifications we call out a sensitivity specification of -123 dBm.  A detailed explanation of how we define sensitivity is given in our technical application note entitled RF Sensitivity of Series 7000 (MPT) DF but I thought I would hit the high points of the application note as a way of providing an overview of what we mean by sensitivity. 

Our direction finding processor allows the user to set a sensitivity threshold.  The higher the setting of this threshold the more likely the device is to calculate a bearing.  So why not set it to the maximum?  Because a high sensitivity threshold increases the probability of a false alarm, basically calculating a bearing from noise.  So the object is to set the sensitivity threshold to a value that gives the highest probability of detecting a bearing with the lowest probability of false alarm.  Fortunately, as described in the application note, we have determined that a value of about 2000 gives optimal performance.  We allow the user to adjust this value in case they are pursuing a signal that they know is transmitting but is in the noise.  Under this condition the sensitivity can be increased to provide bearing readings on a very weak signal.  Some of our customers prefer to increase this setting to a value of 4000 to allow the operator greater latitude in determining if there is a signal on frequency or not.

There are two ways we can use to define sensitivity.  The first definition is the level of received signal that results in bearing measurements with a standard deviation of less than 2 degrees.  A weak signal will have considerable noise associated with the signal.  This noise causes the bearing readings to fluctuate.  Averaging the bearings reduces the noise.  The weaker the signal the larger the standard deviation.  For our direction finder a received signal strength of -123 dBm results in a 2 degree RMS bearing error.

Alternatively, the sensitivity is defined as the signal level that has a 90% probability of producing a bearing angle measurement.  This value depends on the threshold setting of the direction finder and is between -123 dBm and -128 dBm for threshold settings between 1000 and 4000.  In other words if you have the threshold set at 4000 you have a 90% chance of measuring the bearing of a signal with a signal strength of -128 dBm. 

For all the gory details of how we made our measurements and our results take a look at the application note.  If you have any questions feel free to call or e-mail us.


Doppler Product Design and Manufacturing

June 28, 2017
Doppler Assembly Area;
Direction Finders being assembled in Doppler's Carefree Arizona Facility

Occasionally we are asked about how we build and test our products. So, I thought I'd share a little about our design and manufacturing process.

We design all our products locally using in house engineers and some outside consultants.  This includes all the schematic design, printed wiring board design and mechanical drawings of our antennas.  Additionally we develop our own firmware and our software.  During the design process we work with our vendors to insure manufacturablilty.

We typically build and test a few prototypes and test them during the design process.  We make the necessary tweaks to the design until we are satisfied that it meets requirements.

We then produce a first article that we exhastively test it to make sure it performs to specifications.

We use outside vendors to produce all of our piece parts and sub-assemblies and we do final assembly and test at our facility in Carefree, AZ.  All our primary vendors are in the United States and most are in the local Phoenix area.  This includes our machine shops, plating vendor, circuit board vendor, electronic circuit assembly vendor, cable vendor and our powder coat vendor.  We have a long relationship with all these vendors and they have performed admirably throughout the years.

Final assembly and test takes place in our facility and consists of

  • Programming the antenna FPGA
  • Calibrating each antenna board so the antennas are precisely gain matched
  • Assembling the antenna circuit boards into the antenna housings
  • Mounting connectors on the antenna housings
  • Assembling the processor circuit card assemblies into their housings
  • Programming the DF processor FPGA and firmware
  • Then each final assembly is 100% tested using written test procedures

Our results speak for themselves.  In the past 5 years we've only had one unit returned for repair.

Release of Firmare Version 2.34

May 19, 2017

We've just released version 2.34 of the firmware.  This version fixes two recently discovered bugs. 
Bug 1 occasionally caused the DF processor to miss commands sent to it by the software.  This occured when the commands were sent rapidly and the underlying Windows IP socket chose to buffer the first command with the second command.
Bug 2 caused the processor, under some conditions, to go offline when the number of connections exceeded the number of maximum allowed connections (10).

Both of these bugs have been fixed and I've been running our unit for several days under the new firmware.

To download the new firmware go to our upgrades page and download the DDF7001 Firmware

Smooth Summing Results in Excellent DF Sensitvity

March 02, 2017

Some people insist that radio direction finders based on the Doppler principle suffer from low sensitivity.  Recently I saw a claim from one of our competitors recently that their unit was 10 dB more sensistive than psuedo-Doppler based direction finders.  This is simply not true and our data backs it up (see RF Sensitivity of Series 7000 (MPT) DF).  Although some direction finders do suffer from low sensitivity it is not due to the psuedo-Doppler principle per se but due to the way pseudo-Doppler is implemented.  In this post I'll attempt to explain the way we implement pseudo-Doppler and how we achieve sensitivity that approaches the sensitivity of the receiver used in the system.

Rotating DF Antenns

First let me make it clear that Doppler Systems did not invent or discover the Doppler principle.  The Doppler effect was first described by the physicist Christian Doppler in 1842.  Doppler discovered frequencies increase as two objects move toward each other and decrease as they move away from each other.  In the radio frequency spectrum the motion of one object relative to another can be achieved using a rotating antenna as shown in the figure at the right. As the antenna moves toward the signal source the received frequency increases and as it moves away from signal source it decreases. As a result the received signal is FM modulated at the frequency of antenna rotation.  Applying the modulated RF signal to the input of a narrow band FM receiver produces a tone at the audio output of the receiver at the antenna rotation frequency (sometimes called the commutation or sweep frequency). This tone is superimposed on the normal audio output and the phase of the tone relative to the clock reference used to sweep or rotate the antenna is the bearing angle. The direction finder processes this audio signal to calculate and display the bearing angle. 

So what we need is a rotating antenna.  The problem is that in order to achieve adequate FM deviation with a reasonalby compact antenna we need to rotate the antenna somewhere between 15,000 and 120,000 rpm which is nearly impossible so instead we use a circular array of four or eight antennas and use electronics to make the array of antennas look like a single rotating antenna.  Thus the use of the term pseudo-Doppler or synthetic Doppler.  The simplest way to achieve this rotation is to simply turn one antenna on and then turn the next antenna on while turning the previous one off.  A typical switching pattern is shown in Figure 1.  Although this technique is the simplest it is not the best because the switching of the antennas takes place when the gain of antenna is at its peak.  This generates considerable switching noise that drowns out weak signals.

Hard Switched Antennas
Figure 1: Hard Switched Antennas Generate Noise Limiting Sensitivity

A better approach is our patented smooth summing technique.  In smooth summing we gradually increase the gain of one antenna while decreasing the gain of the adjacent antenna.  This approach has two main benefits

  • A uniform rotating gain pattern is produced providing better accuracty
  • Antenna switching occurs when the antenna gain is zero so no switching noise is produced resulting in excellent sensitivity

Figure 2 illustrates the antenna gain pattern on each of the four antennas. A similar pattern is used for the eight element antenna.   We at Doppler systems invented this technique and have used it since the early 1990's in all of our radio direction finders.  We've found it to perform extremely well giving us excellent sensitivity.  As a result our customers can DF signals that are just above the noise floor of their receiver.

Hard Switched Antennas
Figure 2: Smooth Summing Provides Sensitivity Approaching the Receiver Sensitivity