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Editor's note: The following question comes from email and the response from this site.

Question about DF tracking: Why does the DF have only a left / right indicator instead of a pointer like an ADF unit in an airplane? It seems that it would be much easier to find an ELT if the indicator always pointed at it.
Rory Holmes
Catskill Mtn Group DOS

The question is a very good one. As such, the answer is not simple or easy. There are two answers, though: the long and the short. I'll give you the short answer first:

Sensitivity. An ELT is not powerful enough to use an ADF type receiver.

The long answer is simply an explanation of the short. Let's begin by comparing the power output of an ELT versus that of a typical nondirectional beacon (NDB). The power output on an ELT is a nominal 100 milliwatts (mW). That's 0.1 Watt (W). That's about comprable to, say, a flashlight. An NDB, on the other hand, outputs power on the scale of THOUSANDS of Watts. A few AM-band radio stations, which we can of course use on the ADF, output one million Watts of power--a Megawatt (MW). A megawatt station is thus putting out 10,000,000 or 1x10^7 times the power of an ELT. We could compare this kind of power output to, say, the combined lighting of an outdoor stadium.

Which is easier to find? As a hint, both are easier to find at night. (humor)

An Automatic Direction Finder (ADF) is designed to locate these relatively strong signals. An ADF antenna is a short coil--something like the edge of a spiral-bound-notebook--that is able to rotate within a disk. This entire antenna assembly is about 6" across and an inch thick. It is very directional but not very sensitive. That is, it can point very accurately, but only to a very strong (relatively) signal. We all know that the ADF is pretty good at its job but that it is subject to interference from thunderstorms and other distant stations at night. The little coil in there rotates around electrically until a peak signal is obtained. The pointer needle in the compass rose corresponds to the position of the antenna, and thus gives a relative bearing to the station. This information is obviously of navigational use to us.

An ELT-DF unit, such as the L-Tronics LA-series ('A' for aircraft), utilizes antennas of much higher relative gain. In reality, the antennas aren't really different from the VHF communications antennas that we use on aircraft. Note: the way they work on the DF unit is outlined in one of my powerpoint presentations. These antennas are able to pick up a much weaker signal than an ADF antenna. Unlike an NDB, ELT signals are dependent upon line-of-sight in order to be received. This is simply due to the frequency band--ELTs are VHF (very high frequency) whereas NDBs are LF or MF (low or medium frequency, respectively). It is the property of VHF signals to act by line of sight whereas LF or MF frequencies will "bend" around objects better. The LORAN navigation system utilizes these properties to the maximum extent. In any case, the higher relative gain allows us to determine direction to the ELT but not a relative bearing. This is why an aircraft is an ideal vehicle for direction finding. It allows a position that presents an unimpeded path to receive the signal, and thus the best chance of 1) receiving a signal and 2) avoiding reflections from objects on the ground.

I might add, however, that in DF mode the aircraft (and ground) DF DOES always point directly to the station--it will indicate a right or left turn, of course, instead of a relative bearing.

The type of antenna that is used in an ADF could be theoretically used to locate ELTs when the signal is strong enough. Amateur radio operators use "quad" antennas that are similar in function and directionality to the helical (spiral) antenna of an ADF. The problem, however, is that to be a good antenna for the VHF band, the antenna needs to be a couple FEET across. The impracticality of this type of an antenna on an aircraft is obvious.

So the DF unit we use in aircraft is a compromise of space, function, and direction. It is very sensitive and very directional at the expense of ease of display.

I'll finish by saying that I'm a pilot, NOT an engineer. The things I've related here are as I understand them and may or may not stand up to an engineer's slide-rule scrutiny. But they work.

This page of the CAP Emergency Services Resources website was last updated 07/02/2008

1998 - 2006 Scott E. Lanis.  All Rights Reserved.