Thursday, November 16, 2006

I just can't get this question out of my head

A couple of weeks ago, the topic of the ADF came up in A&P school. For those of you not familiar with that acronym, and ADF is an Automatic Direction Finder. It basically points a needled at a ground-based transmitter called an NDB (Non-Directional Beacon). It's pretty old technology and really not used all that much anymore, but that is not to say that it isn't used at all. It's typically used in instrument flight by pilots that don't have access to the latest and greatest GPS technologies.

The display is very intuitive (a single arrow just points at the ground station) but the usage of it is not. Actually, the simplicity of the display is what causes so much confusion with new pilots, but once they can get their heads around it, it becomes a valuable tool. If you're interested, there's more info here.

Being mechanics school, we aren't particularly interested in how pilots use it, though. Rather, we're more interested in how it works. Very simplistically, it uses the fact that a loop antenna (think of a halo) can determine when and if it is oriented perpendicular to an AM radio signal. The new ADF loop antennas have no actual moving parts, but older antennas had a motor that would rotate the loop antenna until it detected that it was perpendicular to the ground source and send that bearing to the needle in the cockpit. Now, here's the point that I'm stuck on. The instructor said that the loop antenna is continuously rotating, much like a radar antenna. To me, that doesn't make sense.

What would make sense to me would be for the motor to rotate the antenna until it detects the bearing of the ground station. Once it senses that it is correctly oriented, it would then maintain a "lock" on the ground station by using a feedback loop to control the turning motor. That way, the antenna would always be pointing at the ground station and the positioning of the needle in the cockpit would be as simple as tying its movement to the movement of the antenna by using an autosyn. An autosyn is simply a method of electrically sending position data to a remote display using a bundle of three electrical wires, rather than a complex and heavy mechanical linkage. They are used for many, many things in airplanes, so it is a reliable and proven technology. Now, if the antenna was constantly rotating, how would the correct bearing be transmitted to the cockpit ADF indicator? Beats me! That question was not answered in class, and I didn't want to waste class time pursuing it.

But I just can't let it go! I had even talked myself out of my "solution" for awhile by considering that the needle will sometimes point in the direction of a lightning bolt. My thinking was that the antenna wouldn't be able to detect the lightning because it was "locked" on the NDB, but upon further reflection I see the fallacy of that argument. Changing the frequency of the ground station (selecting a different station in a different location) works because the orginal signal goes away, and is replaced by one that is likely on a different bearing. I think what happens with lightning is that it creates a much stronger "signal" that the ground station, and temporarily makes the antenna thinks it's pointing in the wrong direction.

Well, there you have it: this is the kind of stupid thing that occupies my mind.

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