Radio Direction Finding
Radio direction finding or RDF has been around since before World War One. From the time of the invention of radio, there has been a desire to know from what direction a radio signal was arriving at the listener’s radio receiver.
Amateur Radio has found several uses for RDF:
• Hunting down interfering radio signals, both accidental and malicious interference to repeaters (affecting both ham and commercial communications, including emergency services).
• Helping to locate downed aircraft by DFing their emergency locator beacons (ELT).
• The entertaining sport of “fox”, “bunny” or T-hunting.
It is “fox hunting” that has spread through many ham radio clubs around the world as a very exciting and fun aspect of the hobby. Fox hunting can take many forms of transmitter hunting, from a person hiding within a few blocks of the starting point with his handheld and periodically making a transmission while others try to find him on foot using directional antennas; to a competition with multiple unmanned automatic transmitters scattered over a course that can be several hundred kilometers long – the entrants being required to find each transmitter in proper order with a minimum number of kilometers driven. Another variation includes jogging or running from one low power fox transmitter to another while carrying RDF equipment.
What makes fox hunting so popular?
• The social aspect of getting together with others with similar interests.
• Anyone can take part – you don’t need a ham license since only a receiver is required.
• The satisfaction of building your own equipment such as an antenna for use in RDFing.
• The fun and competitiveness of the hunt, which also can involve both physical and mental exercise (walking while searching, and the calculations and map plotting required to determine where the fox may be located).
• The outdoor aspect of the sport (sunshine and fresh air).
The “fox” has several basic requirements:
• Be able to move to a location unobserved by those who plan on taking part in the hunt.
• Be able to hide well enough at the location he has chosen so he will not be accidentally spotted. The hunters should have to almost stumble over him in order to find him.
• Be equipped with enough handheld battery capacity, water, lunch etc. for the expected duration of the hunt – it could be one or two hours or more in length, depending on the distance the fox is from the starting point and how well he is able to confuse the hunters as to his probable location.
An unmanned, automatic fox transmitter is the better choice since it can be hidden hours or even a day before the hunt and can be turned on remotely via a sequence of touch tones.
The “hunter” needs several things to make it possible for him to find the fox.
• A 2 meter portable receiver with some sort of signal strength indicator.
• A portable antenna with directional characteristics so that the signal peak or null can be used to determine the direction to the fox.
• A means to attenuate or reduce the signal strength from the fox transmitter so that it will remain near the halfway point on the receiver’s signal strength indicator. As you get nearer to the fox transmitter, the signal will keep getting stronger – it will have to be attenuated to enable you to be able to locate a signal peak or null.
A fox frequency of 146.565 MHz in the 2 meter ham band is most popular in Canada and the U.S., although any open simplex frequency could be used.
Any 2 meter handheld transceiver can be used, or a portable scanner covering the 2M band. During the mobile part of the hunt when you are driving toward the probable location of the fox, a mobile 2M rig could be used, but eventually you are going to have to get out of the car and complete the final stage of the hunt on foot – so a portable receiver is a requirement.
Some of the best antennas for fox hunting are homemade ones such as the quad and yagi designs included here. There are many other types of RDF antennas such as the loop or ZL special in addition to many electronically augmented designs such as the TDOA (time difference of arrival) and switched antenna system.
Occasionally you can find a bargain at a flea market on a suitable RF attenuator for use on 2M. It can be either a switched type or a slider type like the design included here, but in order to be useful for RDF and fox hunts, it should be capable of 60 to 100 dB of attenuation (of course 2 attenuators can be connected in series to obtain this value). Just remember that you cannot transmit through an attenuator without damaging either it or your transmitter.
Al Duncan – VE3RRD
updated March 2006