If you go back to an elementary science class, someone once told you that if you wave a magnet around a wire, that wire will produce an electric current. They also probably told you that if you pass a current through a wire it will produce a magnetic field around the wire. This was the amazing discovery of Michael Faraday and upon which all electrical and radio theory is based.

Some alternators or generators do indeed use a permanent magnet whirling around inside a coil of wire to produce power but the output is directly proportional to how fast you spin it. If it makes more output than you need you must find a way to get rid of the excess. This is not a problem if the output is relatively small but if you want something that can produce a lot of output for the times when you need a lot of output, it produces way too much when you don't need it all. Hence permanent magnet alternators, officially known as "dynamos", tend to be small things.

But if you want one that can produce a lot of output when needed but not much output when not needed you need a way to vary the effectiveness. If you remember the two things that our buddy Mike discovered, i.e. that moving magnetism generates an electric current and moving electrons generate magnetism, you have the basic components you need. If you want to increase the output of your alternator at a given rotational velocity you need more magntism and vice versa. So how can we turn the magnatism up and down as needed? Why, we use a coil of wire with a current flowing through it. If we increase the current, the magnetism increases and the output of our alternator increases. If we reduce the current, the output of our alternator decreases. This electromagnet is the rotating part of the alternator. It is called the rotor but it is also called the field winding from the olden days when we used generators.

A generator has the power-producing windings on the spinning part called the armature and the magnetic field producing part, the field windings, around the outside. An alternator has the magnetic field windings on the spinning part (rotor) and the power-producing coils (stator) are around the outside. You see I keep using the term "magnetic field producing part" over again. That just got shortened over time to the word "field".

So the way this whole thing works is to have an external sensor determine if the alternator is producing as much power as needed. It does this by measuring the voltage on the bus. If the voltage is too low it allows more current to flow through the field winding. This increases the magnetism in the center of the alternator and that then induces more output in the stator winding. The voltage rises. If the voltage gets higher than we want the VR reduces the current in the field, the magnetic field is decreased, the output of the stator windings is less, and the voltage at output is reduced. To me this represents PFM (Pure f'n magic) and is also PFN (pretty f'n neat). Thanks Mike!

Brian Lloyd

For more, check out Wikipedia's Alternator entry: http://en.wikipedia.org/wiki/Alternator

Where is the source for a 40" MB coax antenna? All I find in the Connection is a 75" length for a quarter wave balun.

Answer:

Antennas don't necessarily need to be resonant and/or efficient. If you have a lot of signal you might not need a lot of antenna. Consider that a marker beacon transmitter may run 50W of power or so into a directional antenna beaming it straight up at your airplane. Now consider that you need to receive it from only a couple thousand feet away at most. It doesn't take a lot of antenna at the receiver to pick up that signal.

Frankly, just about any piece of wire will suffice as a marker beacon antenna.

So, take a piece of coax, strip the shield off of about 40" of it (length is not critical) and connect the other end to your MB receiver. Put the unshielded 40" or so where it might be able to see the ground without too much shielding from the airframe. There is your MB antenna.

Brian Lloyd