BMG Engineering announces the SuperDF Radio Direction Finder (Patented), for HAMs, Search & Rescue, government, and commercial Mobile. It is ideal for tracking down stuck transmitters, interference, or illegal transmissions. SuperDF attaches to your unmodified NBFM receiver, such as a scanner, Handi-Talkie, or transceiver. Depending on the antennas purchased it can hunt 100 to 1300 MHz. The outstanding features and capabilities of the Basic SuperDF are given below:
Young Lady with SuperDF deployed to hunt on foot (89 Kbytes).
Close up picture of SDC2-B Control Unit (57 Kbytes)
Picture of SDV1-FRB antenna shown folded up (79 Kbytes)
SuperDF uses the principle of Phase Front Detection. Because the two antennas do not work together to produce a switched antenna pattern as in some other systems (such as the Little L-Per), a phasing harness between antennas is not needed, allowing a very wide frequency range. The directional information is derived from the relative RF phase relationship as seen at the two antennas. As the antennas switch back and forth, phase steps are introduced into the RF signal fed to the receiver. An FM receiver interprets phase steps as momentary shifts in frequency. These produce voltage pulses at the FM receiver output. Using sophisticated and patented techniques, SuperDF electronically processes these pulses and displays the results as left / right information. Because SuperDF works with RF phase and not amplitude information the receiver can be saturated, and an "S" meter and attenuator are not needed.
Don't confuse SuperDF with other phase-shift units (sometimes called TDOA units) seen advertised! Some of them require the operator to listen to and interpret the antenna switching tone. This tone can be quite difficult to hear when the signal is noisy or heavily modulated, or in an acoustically noisy environment (a mobile!). These units are also ambiguous, and they operate very poorly mobile in motion. SuperDF is very much superior! Its patented electronics analyzes, interprets, and displays the RDF information.
One of the competing units advertises 7µv required for DFing. This is a fairly quieting signal in a modern receiver. SuperDF has been shown to produce a bearing on a signal strength of more than 20 dB BELOW the noise level! This is a signal so weak that your ear cannot detect its presence in your radio! SuperDF is at least 1000 times more sensitive, is non-ambiguous, and works VERY well in motion.
One advertised fold-up RDF system use flimsy folding "wing" boards and telescoping whips. These are easily damaged Also, if all the telescoping whips are not pulled out equally, the unit will have an error.
In contrast to other systems, all SuperDF antennas are VERY rugged! One hunter hit a tree at 80 mph with one top whip of his SuperDF antenna (not recommended!). The SO-239 chassis coax connector that the whip was screwed into was torn in half! After replacing the connector, and bending the whip back to being straight, the unit was as good as new.
The SuperDF SDV1-FRB antenna (for 100 to 250 MHz) folds to 1 x 4 x 24 inches, which is small enough to slide under most car seats!
In contrast with a Doppler system, SuperDF can be used on foot, and it will hunt horizontal signals. (Typically, Dopplers have a reputation for needing a strong signal.) SuperDF is 100 to 1000 times more sensitive. With a Doppler you also need a sniffer for on-foot, and a beam for weak signals. SuperDF does all three functions and does them all very well.
SuperDF is not frequency limited like the Little L-Per, which covers 6 MHz bandwidth with retuning, & is crystal controlled. Little L-Per handheld antennas are designed for backpacking, and are very lightly built. They are easily broken. SuperDF use any radio, 100 to 1300 MHz.
A beam requires a good attenuator and an S meter. Even then, when very close to the transmitter, it is very likely the receiver will be swamped by signal leaking into it by paths other than through the antenna connector. The system becomes useless at that point.
A beam cannot obtain a good bearing while mobile in motion. Multipath effects cause the S meter to swing up and down, making it difficult to peak the S meter by swinging the beam. Even standing still, the beam pattern is not very sharp.
A beam can be severely damaged by hitting tree branches.
SuperDF does not have these problems. It will work to within 1 foot of 1 Watt on 2 meters (a very very strong RF field!). It works the best when in motion; the faster the better. Its bearing is very much sharper than a beam's bearing. No S meter and no attenuator needed.
SuperDF works incredibly well Mobile-In-Motion! This is true because of two factors. First, amplitude variations do not affect SuperDF. Second, in a multipath environment, the average phase front is always the same as the phase front of the strongest path*. Because SuperDF can take up to a 1+ second average, the bearing is that of the strongest path, and is very accurate and stable. The faster the vehicle is traveling, the more accurate the bearing. The weaker reflections are ignored. Not having to stop to take bearings provides two very significant benefits. First, the bearing taken while moving will usually be more accurate than one taken standing still. When standing still, all radio direction finders become susceptible to contamination by multipath signals. Second, the transmitter will be reached sooner.
* (For a very good explanation of this behavior read the article, "Direction Finding with the Interferometer", page 33, QST magazine for Nov. '85. SuperDF behaves Mobile-In-Motion in the same way the authors describe for their technique, used on-foot in a field.)
Because it ignores signal strength, there is no need to have or adjust an attenuator, or to have and read an "S" meter. These items would add to the complexity of the operation and act as distractions while hunting. There is no reason to pull off the road to take bearings, thus eliminating the need to be alert for a good site from which to take bearings. Also eliminated are the hazards of getting off & back onto the road. Further adding to the ease (and safety while mobile) is the audible display feature. Left / right information is indicated by the pitch of a tone. Low pitch indicates the signal is to the left of the antenna heading, and high pitch indicates to the right. The operator can keep his eyes on the road. Used on foot this display allows the operator to watch his step on rough ground, thereby preventing falls.
For repeated short-duration signals (Ker-Chunkers), each time the signal appears there will be a left / right indication. Starting with the antenna pointed south, the first signal places the direction within a 180 degree arc. Pointing to the center of the arc, the second event reduces the unknown to within a 90 degree arc. Continuing in this fashion soon pinpoints the bearing.
SuperDF is in use by many government agencies from cities up through the U. S. State Department, the U. S. Army, and FCC Field Offices. HAMs use the system for sport and self-policing. Common Carrier companies use SuperDF to protect service to their customers.
The Basic SuperDF can be purchased either in kit form or assembled and tested. Documentation is very complete and includes:
Parts are mounted on quality glass-epoxy circuit boards. There is very little hand-wiring. The antennas mount with supplied U-Bolt on masts up to 1.5 inches in diameter. They are very rugged!
Common hand tools and a low wattage soldering iron are needed to assemble a control unit kit. For either kits or assembled Basic Units the purchaser must also provide the following:
The antenna is connected to the back of the control unit with (customer provided) male-male barrel connector or any length of coax line. The control unit has cables that will plug into the owner's radio antenna connection and earphone jack. Customer provides connector to mate with the radio.
Display a figure showing this SuperDF Hookup. Down load figure as a GIF Image. Its name is SDHookUp.GIF.
Kits are guaranteed for 90 days from purchase, as follows: If you build a kit and cannot get it working properly, first call us for help. Follow the suggestions we give. If it still doesn't work, call again to get a return authorization. Ship it back to us with a $10.00 bill (to cover return shipping), and we will make it work for you, and ship it back. (Kits built with acid core solder, or otherwise grossly damaged will be returned unprepared.) Also see "Warranty" & "Service" below.
The SuperDF is covered by a 1 year replacement warranty on parts for assembled units & kits, and for factory repair labor on assembled units. Factory repair is available for a fee for units out of warranty.
Technical information and advice is available free of charge. If you have a hardware or a hunting question, contact us (telephone charges on your telephone number).
SuperDF is available by mail. Follow the instructions on the Price List & Order Form sheet. The following information may help you.
You will need a Control Unit and at least one Antenna Unit. Any Control Unit will work with all Antenna Units. Any antenna unit which is cut to one frequency will hunt over its specified range, although it will pick up more energy at the frequency for which it is cut. Extra or replacement whips are available and can be ordered separately. Please call.
Model numbers ending in K are kits. Kit can be assembled in 4 to 8 hours. Those ending in B are assembled and tested unit.
For 100 to 250 MHz the most popular antenna is the SDV1-FRB (or -FRK). Its advantage is that the whips can be loosened and folded down parallel to the boom and retightened when not in use. Thus the whips cannot become separated from the boom while the unit is in storage. It also makes for convenient and safe carrying on foot before or after hunting.
George R. Andrews (Russ, K6BMG)
BMG Engineering, Inc.
9935 Garibaldi Avenue
Temple City, CA
91780, USA
Voice 1(626)285-6963
Fax 1(626)285-1684 (24 hour automatic)
America OnLine: Grandrews
Web: http://members.aol.com/bmgenginc
(6 Mar 1996)
Send E-mail to grandrews@aol.com.
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