Radio Direction Finders and Direction Finding Systems
Doppler Radio Direction Finders and Direction Finding Equipment
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
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
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
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
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
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
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
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.
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
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
February 16, 2017
Last week I attended the NSREN conference in Orlando. A big thanks to
Paul Coburn for inviting us. During the discussion times a number of good
ideas were suggested for improving our software. Throughout our history
our customers have made suggestions for new features and new applications and
our software is so much better because of those suggestions. So please if
you see something in our software you would like changed or you would like to
see a new feature added or if you need a different kind of application go to our
contact page and give us some feedback.
Our software support is free so if you are having any issues please don't
hesitate to give us a call.