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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 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

Software Improvements

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 (see 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 been arrested.(see article here)

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.

DF Error Due to Element Spacing
Figure 1: Antenna spacing of 0.125 to 0.3 wavelengths works well for 4 element mobile array

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.

DF Error due to Antenna Gain Variation
Figure 2: Antenna gain variation cause bearing errors

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)

Low

High

Spacing (in)

Spacing (cm)

100

120

23.7

60.2

120

144

19.8

50.2

150

180

15.8

40.2

180

220

13.1

33.2

220

250

11.1

28.1

250

300

9.5

24.1

300

360

7.9

20.1

360

430

6.6

16.8

430

500

5.6

14.2

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