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Adjacent antenna elements should be between 1/8 and 1/4 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 3/16 wavelength at the nominal operating frequency. This spacing can then be used over a full octave bandwidth.
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.
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 5 degrees and an eight element array has a one sigma or typical accuracy of about 2 degrees. For mobile operation, 5 degrees accuracy is more than adequate, but for fixed site triangulation, 2 degrees is much better.
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.
On the original 4000 and 5000 series, 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 the current 5900, 6000 and 6100 series, 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 R7000 in the NBFM(2) mode (15 KHz bandwidth). Typical values measured in the 150 MHz range are:
| Series | Sensitivity (dBm) |
| 5900 | -126 |
| 6000 | -130 |
| 6100 | -130 |
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. The method is protected by US patent.
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.
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.
The preamps used are rated at 100 milliwatts maximum input and 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 100 milliwatts, relocate the transmit antenna. If you still MUST use a transmitter in the close proximity to the DF antenna, you should consider using the input protection devices made by AntennaAuthority (www.AntennaAuthorityInc.com).
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. Alternatively, you can simply change the whips. For example, when using the system at 2 meters, you can change the whips from 500 mm (19 in.) to say 167 mm (6.33 in.) if the DF appears to be overloaded. On the 5900/6000/6100 systems, the rf summers contain matched 20 dB attenuators at each of the antenna inputs which can be enabled from the display unit.
The direction finders work with any narrow band FM receiver. Series 6000/6100 are calibrated for the ICOM R8500, the AOR AR5000, and the AOR AR8600 MkII receivers. The BearingTrack and AutoTrack software that comes with these DF models can configure the direction finders to use either of these receivers and the software can also be used to control the frequency setting of these receivers. Other receivers may be used; however, the user will need to calibrate the direction finder after installation and manually set the receiver frequency.
AutoTrack and BearingTrack can set and read the frequency of the receivers mentioned above. Additionally, BearingTrack has some scanning capability allowing a user to scan a band of frequencies. If more advanced receiver control is desired we recommend Spectrum Manager or Spectrum SentryNet by Signal Intelligence.
Yes, but be sure to disable the transmitter by, for example, disconnecting the microphone.
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.
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. Series 5900/6000/6100 provides a selectable sweep rate, and by setting it to 1200 or 2400 Hz, the deviation is increased to the point where a wide band fm signal may be tracked. Accuracy and sensitivity, however, will be lower.
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.
Normal voice modulation used on NBFM signals may cause a few degree variation in the display. 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.
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 a such a signal. See also next paragraph.
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. The series 5900/6000/6100 direction finders contain selectable averaging that permits multiple bearing readings to be averaged differently to compensate for varying signal conditions.
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.
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.
The DDF6092 antenna can be used (with some loss of sensitivity) in the aircraft band, but only with a NBFM receiver. If you wish to listen to the AM modulation, you must use a separate AM receiver.