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Please mention you saw it in Monitoring Times!
Radio direction finding (RDF) is both fascinating and useful. It provides the capability of locating sources of interference or unknown signals, beacons from downed aircraft, intentional repeater jammers, hidden transmitters and other radio signal sources.
Most RDF exercises are done at VHF, typically in the 100-200 MHz range, since that’s arguably the busiest swath of communications spectrum. Hams occupy the popular two-meter (144-148 MHz) band, and handy-talkies (HTs) by the zillions are propagated among both licensed and unlicensed users of the VHF spectrum.
On a broader scale, RDF can be conducted virtually anywhere in the spectrum, although propagation characteristics change with frequency, and different techniques are required.
In the civilian sector, most RDF projects are conducted by the hams as they pursue their “fox hunts,” competitive meets in which participants look for a hidden transmitter.
But in more serious sectors, RDF techniques are used both by federal agencies and military organizations hunting for specific individuals or groups using radios or cellular telephones and, on a smaller scale, by law-enforcement investigators tracking suspects, stolen cars (Lo-Jack system), or even robbed cash parcels equipped with hidden radio-beacon transmitters.
While an extensive RDF system with integrated remote terminals can cost $1 million or more, those of us with thinner wallets can get into the tracking game with much less. Let’s take a look at two popular RDF kits available to the experimenter.
Ramsey Electronics is an established company with a wide selection of electronic kits for hams and experimenters alike. Their DF1 “Foxhound” is advertised to work “with any radio, any frequency,” but realistically, with the dimensions given for the antenna array, its range is roughly 100-200 MHz.
Lower frequencies (longer wavelengths) require wider element spacing, and higher frequencies (shorter wavelengths) require shorter spacing, and the radio must supply a signal with a steady carrier like AM or FM, not SSB or CW bursts.
The kit comprises a circuit board, all electronic components, and the four telescoping whips; the optional case is available at extra cost. The builder must supply PVC pipe, couplings, glue and adequate workshop tools, including a drill and hacksaw as well as the expected soldering utensils, to execute the project.
Component quality is quite good; the circuit board is professionally laid out, tinned for soldering and screen-printed for component placement. I would have preferred a BNC jack over the RCA phono plug for antenna connection, and 1/8” (3.5 mm) audio jacks rather than the 3/32” (2.5 mm) provided, since the former choices are more standardized in the communications industry.
The assembly manual is generally quite good as far as it goes, but there are numerous errors of both commission and omission which are expected to be corrected in future editions. Because of the confusion which I encountered during construction, I estimate that I have approximately 8 hours in what have should taken, according to the advertising, about 2.5 hours for an experienced kit builder.
But to Ramsey’s credit, a courteous and helpful customer service department is maintained for just such contingencies, although it is open only during weekdays, not during evenings or weekends when many kit builders are likely to be assembling their projects and encountering problems.
In the simplest terms, the DF1 is a phase detector which measures the time lag (phase difference) between two antennas produced by a signal’s arriving wave front. If both antennas are equally distant from the target signal, the wave front strikes the two antennas simultaneously, but if one antenna is closer to the signal, the wave front arrives first on that one, producing a phase differential in the detection circuitry.
Even the displacement of an antenna by a fraction of an inch is detectable, assuring tight accuracy when taking bearings of the target signal. A pair of LEDs provides visual indication of whether the antenna bearing is to the right or left of center. A meter gives additional null indication, registering more precise orientation of the hand-held array.
In use, the DF1 is plugged into the antenna and speaker jacks of the companion radio, most likely a hand-held scanner or HT. A separate jack is provided for an optional headset or speaker since the radio’s internal speaker is disabled by the audio interconnect.
Once the target signal is being received, the audio output of the radio is adjusted in combination with the gain control of the DF1 until a good left/right null indication is provided.
It is crucial for the operator to have a good map and a compass, otherwise only a visual indication can give a bearing; this might be OK for final close-in on a signal, but it certainly isn’t adequate for a distant start!
An initial compass bearing should be penciled on a map, followed by a second bearing taken at another location; under perfect conditions, the two lines cross on the location of the target signal.
Distant signals are harder to pinpoint than nearby signals, and the wider the angle between the bearings, and the more bearings taken (discarding wildly-divergent bearings), the better the final accuracy.
Since the DF1 is operated on a 9 volt battery and current drain is a consequential 30-35 mA, the unit should be left on only long enough to take bearings. For more extensive periods, a 9-12 volt jack is available on the panel to connect an external DC source such as a battery belt pack or car battery.
For such external power applications, the internal battery must be disconnected since it remains in parallel with the external power jack. A circuit-breaking jack would have been a much better choice here to avoid frying the internal battery.
My trusty Uniden Bearcat BC3000XLT hand-held scanner was connected to the Ramsey DF1. Since these are all low-impedance lines, any loose or intermittent contacts produced enormous fluctuations in readings during bearing-taking. Cables with proper plugs soldered at both ends are strongly recommended over adaptors.
Using a Hewlett Packard signal generator out in the open as a signal source, I tested the DF1 from 25-850 MHz. The lower the frequency, the more accurate and stable the direction finder.
Clear through VHF high band (170 MHz or so), bearings were quite good, but at higher frequencies, the fixed separation of the antennas allowed multiple readings since the pattern assumed the familiar cloverleaf pattern. It is also important to shorten the lengths of the whips as frequencies go higher to avoid multi-lobing and high takeoff angles from excessive length in terms of electrical wavelength.
A means of substituting antenna arrays of different spacing would certainly allow reliable frequency range extension of the DF1.
Signal strength and gain control adjustments were also critical; when properly adjusted, the audio tone produced by the switching circuit would smoothly disappear and the meter deflection would null as the array was pointed at the signal, but as it was rotated, distorted tones would come and go and the meter would fluctuate.
On assessment, the DF1 makes a good direction finding accessory provided the operator was fully aware of and familiar with its idiosyncrasies.
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Most readers have encountered the Doppler effect, a gradual raising or lowering of a pitch from a siren, car horn, jet aircraft, train whistle, or other audio source as it rapidly approaches or recedes from a listening point.
The same effect may be noted from stars (the “red shift”) and radio signals as well, although the speeds are much faster.
Doppler direction finders work on the principle of rotating antennas – in this case, electronically-switched antennas, thus called “pseudo-Doppler” since there is no actual physical movement.
If a circular array of antennas can be switched rapidly and consecutively, their relative positions to an arriving radio wave can be compared. Those on the side moving toward the signal will deliver an upward frequency shift (as with an approaching car horn), while those switching away from the signal will record a downward shift (as heard when a sound source passes by and recedes into the distance).
Explained simply (too simply – this is a sophisticated product!), a resolver circuit provides a readout of this comparison on a circular compass rose of 16 LEDs, alerting the observer to the directional bearing of the signal. This 22.5 degree spacing provides adequate homing bearings for most applications. A very detailed description of the DDF1 circuitry is presented in the manual.
To save time, instead of ordering the kit, I ordered the factory-assembled DDF1, comprising the control unit, four whips, flexible magnet strips, and the manual (the same one included with the kit version).
All the user needs to do is stick on the magnetic antenna pads to mount the roof array, plug in the DB9 interconnect cable, attach a source of 12 volt DC power (100 mA average drain) and the audio cable, and dial in the desired frequency on the user-supplied radio! Virtually any frequency in the VHF/UHF land mobile spectrum may be selected.
A comfortable audio level from the DDF1 speaker is set and the automatic antenna scan is started, resulting in a 500 Hz tone overriding the signal. Audio is readjusted to extinguish the overload lights so that only one LED in the circular compass array should remain lighted. Others will flicker erratically, but this jitter may be dampened and a stabilized reading taken with a control for that purpose
A phasing switch assures that the pattern shifts in the correct direction with respect to the movement of the vehicle, and the calibrate control is adjusted to align with the forward direction of the vehicle. You’re ready to catch that fox!
With the four-antenna array stuck to the roof of my Jeep Liberty, and the cable running from there to the control unit and my scanner, I was ready for action. The manual warns against road speeds with the antennas in place, yet the manual recommends testing the system by driving past a known transmitter.
It’s a respectable warning; the antennas are held in place by extremely weak rubber magnets, and the slightest motion will tip them over. Even when they are fully collapsed to 4” or so, a slight breeze tips them. Not surprisingly, the manual recommends they be replaced by stronger magnets. Good idea; so why didn’t they do that at the factory?!
Like its little brother, the DF1, antenna length and spacing are critical for performance, depending upon the frequency. Fortunately, unlike the DF1, the antenna spacing can be easily adjusted, supporting a wide frequency range of operation.
The control unit is handsome and functional, with every adjustment you will need at your fingertips. The advertising says the DDF1 can be used from 130-1300 MHz; at a nearby shopping center I tested the DDF1 from 30-470 MHz with excellent results. We have no communications systems in the area above that to test its upper limit.
Aiming the front of my car north and calibrating the LED readout, I found the bearings for our NOAA weather broadcasters, a local Taco Bell kiosk, nearby Wal-Mart handy-talkies, and the sheriff’s repeater. And they were all correct.
This is a nice RDF at a reasonable price. Properly adjusted, it will provide excellent reliability and accuracy for determining bearings on target radio transmissions in the VHF/UHF spectrum. And if you replace the antenna magnets, you can even do it while you’re moving!
DF1 “Foxhound” kit, $69.95 less case and knob, $84.90 with case and knob, plus shipping. DDF1 Doppler RDF, $149.95 kit, $269.95 factory wired, plus shipping from Ramsey Electronics. Call (800) 446-2295 or visit their website at http://www.ramseyelectronicscom.
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