Radio Direction Finding (Foxhunting)
Radio Direction Finding is useful when trying to locate the source of a radio signal or to locate someone intentionally jamming or causing interference on a frequency.
RDF can also be used to locate a missing hiker, locate the transponder signal from a downed aircraft in the mountains, or locate a missing kayak, small boat or scuba diver lost in the ocean.
This works by triangulating the radio signal. Triangulation is where you would take a compass heading of a radio signal then move to a new location and take another compass heading from there. These compass bearings are plotted on a map and where the lines intersect you should find your transmitter. The higher number of compass bearings you have to work with, the more accurate the location results will be.
This can be performed in a couple different ways. The most common is by using a handheld FM radio with an RF attenuator or scanner attached to a handheld directional antenna and moving the antenna in a slow circle to find the direction that the radio signal is coming from by listening to where the signal drops off. When you stop hearing the signal in your turn, it will be directly behind you, also referred to as the "null zone". This technique is good for short distance location finding of low power transmitters such as during a "foxhunt" at a local park. It is not quite as practical however if you are searching a larger area such as the entire island.
Another option, which is the option that I prefer, is to use Doppler Radio Direction Finding (DRDF) equipment mounted in a vehicle, like the DDF2020T with GPS unit shown below. This system is based on the same doppler technology that the Federal Communications Commission uses to track down the source of radio signals.
This system consists of a receiver unit mounted to the dash of the vehicle which is connected to a laptop computer running mapping software. The system uses 4 identical wideband whip antennas set in a square pattern on the roof of the vehicle at a precise spacing depending on the frequency range you are trying to find a signal source on. It also uses an additional antenna in the center for improving sensitivity and accuracy. This type of system can be used to locate the source of radio signals on frequencies from below 100MHz to over 1GHz.
As the radio signal is received the system measures the minute differences in the millisecond timing that the signal reached each antenna. The first antenna that received the signal will be the direction the signal is coming from while the last antenna to receive it will be 180 degrees away from the signal source. The receiver then shows the results on a display showing the compass heading reading of the source radio signal.
For someone new to using a doppler system like this, the compass bearing may seem confusing because it does not assume that 0 degrees as being due North as a normal compass would show but instead uses the directionality of the vehicle. This system will show 0 degrees as being directly in front of the vehicle, 90 degrees will be on the passenger side of the vehicle, 180 degrees will be directly behind the vehicle, and 270 degrees would be on the driver's side of the vehicle.
The reason the system is set up this way is that it is designed to be used in a moving vehicle. The compass bearing will show in the numeric LED readout but it also lights a set of 36 LED lights arranged in a circle around the bearing display, known as a pelorus display, or compass rose, each indicating 10 degrees of a compass.
The circle is comprised of a single blue light at the top of the circle which if lit would indicate that the signal is coming from directly in front of your vehicle, and the additional 35 lights in the circle are red. As the system is searching for a signal the LED lights spin around the circle quickly at a rate of twice per second and the three-digit numeric bearing display shows dashes. As a signal is picked up by the doppler system the LED lights lock in on its direction and the bearing display shows the bearing it is coming from in relation to your vehicle.
As the signal direction changes the LED lights will show you what direction you need to head in to locate its source. This is similar to the vehicle tracking systems that you commonly see used on movies and television shows where the police follow the directions of the blinking red lights to find the bad guy's vehicle on which they placed a tracker which they are following.
These calculations are automatically handled by the system and in a matter of seconds, a plot appears on the computer screen overlayed on a map of the area which is found from the GPS in the system. Once a plot is established simply move to a new location and let the system find a new plot. The intersecting lines of the plots will show the source of the radio signal.
Doppler Radio Direction Finding systems are very fast, extremely accurate and are able to track down the offending radio transmission very quickly over a very large area. Another benefit of using a Doppler RDF system is that it can be used while moving and can also be used for finding moving radio transmitters.
Unlicensed and malicious radio operators beware, just because the Federal Communications Commission is not out there constantly tracking radio signal sources does not mean that you will get away with what you are doing. Amateur Radio is a self-policing hobby. We monitor the bands ourselves, and when we have a situation where an unlicensed or malicious radio operator is interfering with the legitimate communications of licensed amateur radio operators and preventing us from enjoying the radio frequencies that we are licensed to use, we will use our equipment and the skills that we have honed in the hobby to track you down and find you. We also make audio recordings of the offensive radio transmissions and forward all of the evidence and the offender identification to the FCC for prosecution.
There are several different formats or styles of hidden transmitter hunting described below.
ARDF - Amateur Radio Direction Finding - This is the official radiosport form of hunting used in national and international competitions. The competitors hunt on foot, usually in a large forested area. 2-meter and 80-meter hunts are common. More information about ARDF style hunting can be found here.
Walk in the Park - This is a less formal form of ARDF used by many amateur radio clubs. The hunters are on foot and the hunt takes place in a park. Walk in the Park style hunts use 2 meter FM.
Automotive - In this type of hunt the hunters are in an automobile. Obviously, they can cover a much greater distance than on foot hunts, in extreme cases, they may cover 100 miles or more to find all of the hidden transmitters.
QRP - In a QRP hunt, the transmitter power is very low, so low that it may not be heard in many locations in the search area. The transmitter may be low power, e.g. less than 1 watt, the antenna may be low gain, e.g. a rubber duck, the location may be poor, e.g. behind a hill or a combination of all three.
The hunters may start at any location in the hunt area and could adopt a few different strategies. For example, they could start at a high spot in the search area, try to get an initial bearing then move toward the transmitter. As the hunter moves toward the fox they may go through low areas where the fox cannot be heard but eventually, they will come within range. Another strategy could be to traverse the search area listening for the fox.
The first hunter to find the fox wins.
Collaborative or Turkey Shoot - In a cooperative hunt all of the hunters cooperate to find the fox rather than compete against each other. The fox makes very short transmissions at random intervals to make the hunt as difficult as possible. For example, I might just give their call sign on the first transmission and “Fox” on the second and so on. Rather than transmitting on a regular schedule such as every 60 seconds the first two transmissions might be 15 seconds apart and the next two 100 seconds apart. The idea is to create a situation where the hunters attempt to locate the source of random transmissions that could be inadvertent or malicious. There are many examples of such transmissions; malicious transmissions intended to disrupt normal radio activities (hence the name turkey), inadvertent transmission because of an equipment failure, power line noise, it could even be the family pet. Look here for an article about such a situation.
The hunters select one or two coordinators who would act as a net controller and “general” who deploys the troops. Each mobile hunter begins at a different location within the search area and after each transmission the hunters, at least those who heard the fox, report to the coordinator with some indication of signal strength and direction. The coordinator assembles the reports and instructs the hunters to move to a new location hopefully closer to the fox. Over time they should be able to tighten the search area until the fox is finally found.
One nice aspect of this type of hunt is, for some of the hunters at least, no special equipment is required. In fact, you do not even need to leave home.
The coordinator can sit at home with a base station, one or two maps of the search area, and APRS running to follow the hunters.
People at home with an omnidirectional antenna can report signal strength which will provide useful information, at least at the beginning of the hunt. For example, if someone in Ewa Beach reports a stronger signal than someone in Honolulu the fox is probably in the western part of the island.
People at home who have a beam can provide even better information by providing a beam heading.
Hidden Receiver - In a hidden receiver hunt the fox uses a receiver and directional antenna and the hunters use standard 2-meter mobile gear. The hunters start at random locations and periodically give a short transmission then ask the fox for a bearing and signal strength. The hunter then proceeds toward the fox, asks again for bearing and signal strength, and so on until the fox is found. The hunter with the fewest “asks” is the winner. The advantage of this type of hunt is the hunters do not need any special type of gear, an ordinary mobile unit will do.