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Doppler Systems LLC

Radio Direction Finders and Direction Finding Systems

Frequently Asked Questions

What is the optimal spacing for the mobile antennas?

Adjacent antenna elements must be less than 1/2 wavelength apart. Deviation and hence sensitivity increase with spacing up to the point where the phase difference between the signals from adjacent elements begins to approach 180 degrees. A phase of 180 degrees does not, however, correspond to 1/2 wavelength because the mutual impedance between the antennas causes the phase to increase over the free space (uncoupled) condition. We recommend that the spacing between the elements be 1/4 wavelength at the nominal operating frequency. This spacing can then be used over a full octave bandwidth. That is:

Frequency 0.707 x Nom. Nominal 1.414 x Nom.
Spacing (wavelength) 0.177 wave 0.250 wave 0.354 wave

How large a ground plane is required for a mobile monopole array?

As a minimum, 1/4 wavelength should be provided outboard of the array. Do not try to make an antenna with the four elements located in the corners of a square ground plane. It will not work properly.

What determines the accuracy?

Assuming that proper processing is done, the accuracy is determined by the number of antenna elements. A four element array has a one sigma or typical accuracy of about 2.5 degrees and an eight element array has a one sigma or typical accuracy of about 1 degree. For mobile operation, 2.5 degrees accuracy is more than adequate, but for fixed site triangulation, 1 degree is much better.

How do you measure accuracy?

We measure accuracy under very controlled conditions that are designed to eliminate multipath reflections. The DF antenna is rotated on an index head and the bearing angles are recorded at 1 degree intervals. The error at each angle is then calculated and the mean and standard deviation computed. This process is repeated at other frequencies across the bandwidth of the antenna. The mean of the error corresponds to the calibration angle and the standard deviation is stated as the "one-sigma" error value.

What is the sensitivity?

On our first direction finders, the receiver's squelch was used to determine whether a signal was present or not. It was necessary with these models to adjust the squelch so that the receiver remained squelched under no signal conditions. The sensitivity was then equal to the squelch threshold of the receiver which was approximately the same whether the receiver were connected to the direction finder summer/antenna or to a single antenna element of the same type used in the direction finder. (For mobile operation this would correspond to a single magnetic mounted whip). On current models, the direction finder may be operated with the receiver's squelch opened, and the sensitivity is greater.

We measure sensitivity by driving the inputs to the RF summer through a signal splitting device that simulates the antenna. The input signal level is adjusted until the standard deviation of the bearing is approximately 5 degrees. This input level, after compensating for the loss in the signal splitter, then corresponds to the published sensitivity. The measured values will depend on the receiver used, and in our case, we use an Icom R8500 in the NBFM mode (15 KHz bandwidth). Typical values measured in the 150 MHz range are:

Series Sensitivity (dBm)
5900 -126
6000 -130
6100 -130
7000 -128*
*Summer with overload protection

Why do you use "smooth summing" of the RF signals?

Earlier simulated Doppler direction finders switched between adjacent antenna elements. Our RF summers combine the antenna inputs on a continuous basis. The method provides improved sensitivity because hard switching generates RF noise at the front end of the receiver, and it also produces sidebands which have the effect of causing adjacent channel signals to appear on the receiver's frequency when the direction finder is operating. Our designs also provide a matched input impedance which allows the antenna to be located remote from the summer.

If I build my own antenna, how closely must I match the antenna feed lines?

At frequencies up to 500 MHz, try to cut the feed lines within 6 mm (1/4 in.). If the feed line is relatively short, say 3.5 m (12 ft), this is not difficult. It is much harder to match longer feed lines or to match at higher frequencies.

What is the best way to check out and calibrate a system?

On a mobile installation, drive to an area clear of reflections with a good line of sight to a known transmitter, such as a NOAA weather station, radio repeater site, etc. Point the car at the transmitter and calibrate the bearing angle to zero degrees. Drive the car slowly in a tight circle and verify that the bearing angle clocks smoothly around the display.

Can I transmit close to the DF?

The preamps used are rated at 100 milliwatts maximum input and in our series 7000 direction finders we provide additional protection that increases the rating to about 1/2 watt. Transmitting 10 or more watts from a mobile antenna a few feet away in the same band will probably damage the unit, while transmitting 5 watts inside a car will not. Transmitting a few watts from a nearby antenna on a different band from the direction finder antenna will probably not hurt the unit. Vertical separation helps also, so if you can place the transmit antenna on the trunk lid or boot of a car and the DF antenna on its roof, the coupling will be greatly reduced. When in doubt, it is best to run a test by connecting a mobile antenna output into an RF power meter and checking the power induced from the transmitter. If it is more than the maximum rated power, relocate the transmit antenna.

Do I need an attenuator when I get close to the transmitter?

No, unless the emitter being tracked is located close to other transmitters which would overload the RF summer. The RF summer design is broad band and has a high dynamic range, but it is possible to overload it when the direction finder is close to high power broadcast stations. It is necessary to attenuate the signal at the antenna side of the RF summer, not between the summer and the receiver, so four attenuators must be used. On our current models, we provide a built in attenuator feature. The front panel has an ATTEN button that disables the preamplifiers in the RF summer thereby attenuating the signal by about 24 dB.

What models of receivers will work the direction finders?

Any narrow band FM receiver with an external speaker connections can be used.  However, software control of the receiver from TargetTrack, Signal Track, or the MPT User Interface requires one of the following receivers

ICOM R1500
ICOM R2500
ICOM R8500
ICOM R9500
AOR SR2200
AOR AR5000
AOR AR5001D
AOR AR8600

Our DF processor has calibration coefficients built in for these receivers as well as the Motorola MCS 2000 transceiver so in mobile applications no calibration is required.

Does your software provide receiver control?

TargetTrack and SignalTrack control the frequency, squelch, and volume of the supported receivers.  Additionally, TargetTrack provides scanning and search functionality for the AOR SR2200.

Can I use a transceiver with it?

Yes, but be sure to disable the transmitter by, for example, disconnecting the microphone.

What are the connections between the DF and receiver?

The receiver's antenna jack connects to the RF summer output and the external speaker connects to the audio input on the direction finder. Series 5900/6000/6100 contain loudspeakers for monitoring the signal. Series 7000 has an audio output jack for connecting a loud speaker.  Note that some receivers use an isolated external speaker output that contains a DC voltage. In our Series 5900/6000/6100 the audio input is single ended, so you will need to provide transformer or capacitive isolation.  Our series 7000 direction finders are capacitively isolated so no additional hardware is required.

What signals will the DF work with - AM, FM, TV, SSB, Noise, CW, Pulse?

When the unit is used as recommended with a narrow band FM receiver, it will work with any signal that has a carrier that remains within the IF bandwidth of the receiver. This would include unmodulated carriers, narrow band fm modulated signals, and amplitude modulated signals. Television video carriers can also be used, but the system will not track broadband noise or single sideband signals.

Commercial broadcast (wideband) FM signals require that the receiver bandwidth be increased to 150 KHz. Our direction finders provides a selectable sweep rate, and by setting it to the highest level, the deviation is increased to the point where a wide band FM signal may be tracked. Accuracy and sensitivity, however, will be lower.  Using a larger number of averages is also helpful; however, you will need to make longer stationary readings in the mobile application.

The latest firmware version provided with all of our direction finding systems allows signals as short as 80 milliseconds to be tracked. Longer duration signals provide more stable and accurate bearings and we recommend using the system with pulsed transmitters of 100 milliseconds or longer.

Does transmitted modulation effect the unit?

Normal voice modulation used on NBFM signals may cause a few degree variation in the display. Data telemetry transmitters may affect the operation depending on the frequencies and modulation methods used. If there is a specific transmitter that you want to use with the direction finder, please contact the factory.

How can I tell if there is multipath?

You can generally tell that you are in a bad multipath environment when the received tone sounds harsh. The bearing may or may not be incorrect under multipath conditions, but it is best to ignore the bearings when you hear such a signal. See also next paragraph.

How can I avoid reflections/multipath?

The best way to avoid multipath is to stay high in elevation away from surrounding reflective surfaces. Hill tops, parking garage roofs and elevated freeways are good locations. Multipath errors tend to average out spatially, so it is best to keep moving. This is another reason why it is best to stay on an elevated freeway as long as possible. Our direction finders contain selectable averaging that permits multiple bearing readings to be averaged differently to compensate for varying signal conditions.

Can I locate the DF at my repeater site?

Probably not. The preamplifiers used in the RF summer have a high dynamic range but they are broad band, and it is likely that a nearby transmitter will cause overload. Before attempting to install a fixed site direction finder, evaluate the site first with a temporary or mobile setup. Try to find a receiving location that is relatively quiet at RF frequencies.

Can I use the DF with an AM receiver?

The direction finders will work with AM receivers such as those used in the aircraft band, but the antenna needs special consideration. In mobile service in the aircraft band using a NBFM receiver, the antenna elements should be cut for resonance at the operating frequency. The system will then work at frequencies above and below resonance with only a small loss in sensitivity. This is because the relative phase between elements is not affected by the resonance condition. However, the relative amplitude (i.e., the antenna gain pattern) is very much affected by the resonance and in fact there will be a 180 degree reversal in the direction of the major lobe as the frequency is increased from below resonance to above resonance. To obtain unambiguous bearings using an AM detector, the mobile antenna elements should be resonant at a frequency that is always below the operating frequency range. If you plan to use the system in the aircraft band (108 - 136 MHz) , we suggest you order elements resonating at about 100 MHz.