Radio Direction Finders and Direction Finding Systems
Doppler Radio Direction Finders and Direction Finding Equipment
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.
New Version of MPT User Interface
December 02, 2016
We added a feature to the MPT User Interface program to allow a user to save the direction finder settings to a file. Once you have the direction finder settings the way you like them you can save them. If someone changes the settings you can always restore them to their saved values. The new version is available for download on our downloads page.
Doppler Systems helps find culprit jamming aircraft transmissions
December 01, 2016
Recently someone made hoax calls to pilots, causing a passenger flight to
abort landing as it approached Melbourne Airport in Australia. In total there
were 15 unauthorized transmissions on aircraft frequencies in the Melbourne area
article here). The Australian Communications and Media Authority
deployed a Doppler Systems radio direction finder to assist the Australian
Federal Police in finding the culprit. Thankfully he was found and has
Antenna Placement in a Mobile Installation
September 30, 2016
Frequently we are asked "What antenna spacing should I use for my mobile
antennas?" This is a good question because many people think that antenna
spacing would be critical for good performance and people are surprised when our
answer is "Somewhere between an 1/8 and 1/4 wavelength." In this
post I'm going to present to you the results from the detailed model of our
antennas to show the effect on error of antenna spacing and also antenna gain
and phase errors. I'm also going to make some recommendations not only on
spacing but on placement of the antennas on the vehicle.
Figure 1 below shows the effect on bearing error due to antenna spacing.
Results for both the 4 element and 8 element antenna are shown. Notice
that the curve is fairly flat from 0.125 wavelength to 0.35 wavelength and then
it increases rapidly. This is really good news if you are going to be
operating over a relatively large portion of the band since you don't want to
have to jump out and change the spacing when you change frequency.
Another source of error in the DF is mismatch in the gain of the antennas. Figure 2 illustrates this effect.
You may ask why do we care about this since all the antennas are identical?
The answer is that the antenna's ground plane has an effect on the gain of the
antenna. Therefore it is important to have adequate ground plane around
all of the antennas. Typically we suggest at least a 1/4 wavelength of
ground plane around each antenna.
In conclusion use a spacing somewhere between 1/8 and 1/3 of a wavelength but
try to make sure that each antenna has at least 1/4 wavelength of ground plane
all around it. The table below is a good reference
Frequency Range (MHz)
Mounting a DF Antenna to the Side of a Tower
September 06, 2016
Over the years many of our customers have asked about mounting the DF antenna
on the side of a tower or mast. Will it work? The short answer is not very well.
We've posted the results of some extensive
modeling we've done to predict the error associated with side mounting the DF
array on a tower. To achieve a less than 5 degree peak error at 150 MHz the
array must be mounted over 8 feet from the tower. To achieve less than 2 degree
peak error the array must be 75 feet from the tower.
The conclusion is that for best performance the DF antenna array must be top mounted
on the tower or mast and away from any significant RF transmtters. Please
contact us if you have any questions.
Improved Mapping in TargetTrack
September 02, 2016
We released TargetTrack 3.2.0 last week. Several of our users had requested that we improve our bitmap based mapping functions. The previous versions of TargetTrack limited the map file size and the level of zooming. The new version allows for much larger file sizes and considerably more zoom levels. We've also added some more map like zooming and panning tools.
The bitmap mapping facility in TargetTrack allows our users to make their own custom maps using screenshots or scans of physical maps. Use our MapFix utility and you then have the map that you want.
We also have our Great Map function so that you can use online mapping data from several different mapping data providers e.g.OpenStreetMap, Google, Bing.
Download the latest version of TargetTrack from our Downloads page and let us know what you think.
Welcome to our Blog
September 01, 2016
Welcome to the Doppler Systems blog. Our blog will give us the opportunity to keep you up to date on our latest products and software releases as well as offer helpful hints to enable you to use our products more effectively.
Thank you for visiting and contact us if we can be of any assistance