Introduction to DFing.
The art of Direction Finding (or "DF'ing") has been around since at least 1921, when the US Coast Guard built special receivers to track and intercept rum runners. It has evolved into an exciting sub-hobby within Amateur Radio. A hidden transmitter, or "Fox," may become the centerpiece of a grown-up game of hide-and-seek at many amateur radio gatherings. All of these fun-and-games have a purpose; as hams we want to be prepared to use our equipment and our services to aid or community. DF'ing will continue to be used in such very real situations as:

  • A local public service has a "stuck mike" and needs to know which unit is involved. Until located the repeater supporting that public service is typically unavailble for other use.
  • A person is sending out "harmful or malicious signals." Several govermental agencies considered "pirate radio" to be such a source.
  • When airplane goes down, an automatically actuating the ELT (Emergency Locator Beacon) starts transmitting. DF'ing is how they find it.

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It would seem straightforward; you turn the antenna until the signal gets stronger. At that time you are pointing to the signal. That would be easy enough but there are two problems that transform DF'ing from a basic technique into a complex art form.

First, the audio volume won't necessarily get stronger as you are pointing to it the transmitter. Most radio signals vary quite a bit, even second-by-second, and to take care of this they have an ALC (Automatic Limiting Circuit) built in. This keeps the volume pretty much steady and much more enjoyable to listen to. However, this also makes it much more difficult for DF'ers.

If your radio has an signal strength meter (or "S-meter") you can use that instead. The S-meter circuitry is tapped out before the signal reaches the ALC stage and it can show the needed variation.

Otherwise you will have to rely on your ears, on experience or on more specialized equipment.

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An inexpensive method is to use body-block with almost any scanner. First, tune in the signal and hope for a steady audio, such as a tone. If your scanner has different modes so much the better; switch to continuous wave (or "CW") or single side band (or "SSB") to get a continuous tone. Next, hold the receiver right next to your chest, in an upright position, and turn around slowly. It will be very difficult to hear where the signal is loudest, so instead you are listening for the "null," a sharp point where the signal fades. At the null you body is blocking the signal, so the transmitter is precisely 180 degrees away, or exactly behind you. A little practice might be in order. It is recommended that you find a continuous, weak signal such as the weather station at the local airport. At the Ardmore Downtown Executive Airport there is an AWOS (or "Automatic Weather Observing Site") that operates on 118.15 MHz. These stations broadcast continuously and are easily read from the air, but have very short range on the ground. At the Ardmore Industrial Airport try the ATIS (or "Automated Terminal Information Service") transmitter on 125.6 MHz. These are both great for DF practice. So are the National Weather Service transmitters around 162 MHz.

You might also have more success if you decrease the sensitivity of your scanner. If you have an attenuator switch, you're in luck! Get familiar with it. If not, try replacing the Rubber Duck with a straight paper clip. When you get really close to the transmitter remove the antenna altogether. We often will race our teenagers to a hidden 121 MHz transmitter; we will use scanners and body-bock and let the kids use exotic L-PERS and other exotic equipment. As often as not cunning and experience of old age can beat the exuberance (and better equipment) of youth!

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DFing as Art

There is a second, and more complex problem. The signal will reflect off of many metal objects and travel down others, hiding its true location. Pretend the transmitter is a light source, (flashlight?) and that the entire city is otherwise dark. If you were to point the light source at a radio tower or a water tower, it would look like the light was actually coming from the tower, right? DF'ers would say you "have lit up the tower." This masks the true location of the transmitter. The signals can also travel down fences, power lines and railroad tracks. It takes quite a bit of practice to get used to it.

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One of the purposes of Civil Air Patrol is to locate downed or disabled aircraft. Most aircraft carry an Emergency Locator Beacon, or ELT. The ELT will begin transmitting on 121.5 MHz (or 243 MHz if military) if jarred or if the battery goes down. A satellite will pick it up and relay an approximate location to CAP/HQ, which will dispatch a ground team and an air crew. Both teams have DF equipment. This equipment uses some fairly sophisticated circuitry.

The CAP unit in most areas provides continuous training to its members on DF techniques including the peculiarities of "urban DF'ing." A few groups are even becoming equipped with modern Doppler DF rigs, which are an advanced technique.

So get on out there and try some body-blocking! Read about CAP on the web and be sure to search for DF'ing and Radio Foxhunts, too. Also look for sites about "ARDF." You'll enjoy it!

 

- this page by KM5OX