INTRODUCTION – HOW OUR DF’S WORK IN “PLAIN” ENGLISH
All our permanently installed direction finders have two antennas which are mounted on the aircraft, a left antenna and a right antenna. The signal from both of these antennas goes into the DF unit. Here a switch alternately makes connection to the left antenna and the right antenna. This switching is done approximately 80 times per second. The composite signal (50% of the time connected to the left antenna and 50% of the timeconnected to the right antenna) is fed to the normal antenna input of your COM receiver. The COM receiver works in the regular way except that instead of being connected to your existing COM antenna, it is alternately connected to the left, and right DF antenna. A connection is made by the radio installer to take the signal out of your COM receiver. The pickoff point is just after the detector but before the squelch and noise limiter. The reason that you have to do this and cannot take the signal from the speaker or head phone output is that the receiver noise limiter clips the signals in an attempt to diminish noise pulses. For DF signal strength between the left and right antenna since this is where we get our directional information. So, as you can see, if the signal from the left and right antenna were both clipped to the same size, we would lose our directional information. This signal feeds into the DF where it is filtered and extraneous information, such as audio and noise, is removed from it, and then it is amplified. This amplified signal goes to an electronic switch which is not only identical to the one that is used to switch between the left and right antenna, but is so connected that it switches exactly synchronously with the left/right antenna switch. The output of this switch goes to the left terminal of the DF meter. So, as you can see, if the airplane is pointed straight at the transmitter, the signal will be equally strong in the left and right antenna. The signal, as it comes out of the receiver, will be identical and so the voltage applied to the left and right side of the meter will be equal and the meter will remain in the enter of its range. If, on the other hand, the airplane is turned so that the transmitter is off to the right, the signal will be strongest in the right antenna and the voltage applied to the right side of the meter will be larger than that to the left and the meter will deflect to the right and point at the transmitter.
There is no problem determining whether you are flying “To” or “From” the transmitted signal. For example, if the needle is deflected to the right and you turn the aircraft to the right, and the needle centers, you are going “To” the transmitter. If the needle is deflected to the right and you turn The aircraft to the right, and the needle moves farther away from center, you have passed the transmitter. However, by continuing the turn and following the needle, the DF will cause you to turn back in the direction of the transmitted signal. Just follow the needle and the DF will always lead you to the transmitter.
If this has been explained clearly enough, you can see that the meter will point in the direction of any transmitted signal that is being received by the COM receiver to which it is connected. For this reason it is important to listen to the signal so as to properly interpret the reading. For example, if you were homing in on the unicom of a small airport, you would want to take your bearing when THAT Unicom is transmitting, and not a neighboring one. When a plane transmits, the needle will point at it. This feature is very useful for telling you which direction to look in to spot a particular aircraft.
Benefits of a Direction Finding System
- Low fuel? Emergency Situation? Fly direct to the nearest airport without the use of other navigation aids.
- Home in on a small field without straining for landmarks
- Home in on a controlled field by monitoring the tower frequency
- Home in on other aircraft
- Makes VFR night flying easier
- Locate remote camping sites using a portable transmitter
- Participate in search and rescue missions to locate emergency signals from aircraft, boats and people
- For offshore operations: Navigate via a direct path to offshore platforms
- For surveying: Using portable transmitter to locate remote survey sites
- For fishing fleets: Spotter aircraft is able to home in on factory ship
- For commercial spray applications: With handheld transmitter, mark end points of spray runs
- For law enforcement: Monitor distress signals and police cars.