What's So Special About the So-called Shorted Quarter Wave Stub?

09/03/2001

First Considerations

• Well, to begin with, we need to remember that a full cycle (starting at 0) goes through a peak at 90 degrees, which is simply a "quarter of a cycle".
  • If we think about this for a moment, that simply means that if we are dealing with voltage, then we are starting with zero volts and then increasing until we reach the maximum voltage at 90 degrees.
  • We can also take another side of that same picture, where if we start at the top of that maximum point (at 90 degrees), and continue for another "quarter of a cycle", we will find ourselves at zero volts again.
• The important concept here is that every "quarter of a cycle" we find that we are at the opposite extreme of where we started.
• Now, if a signal is traveling down a conductor (say like an antenna wire) for one quarter of a cycle of the frequency involved, we will discover that at one end of this quarter wavelength conductor we find exactly the opposite of the other end of that same quarter wavelength conductor.
• In simple terms, we can say that this is like as if we have an open circuit at one end of our quarter wavelength conductor, while we have an short circuit at the other end of that quarter wavelength conductor.
  • We will find an interesting phenomenon for this quarter wavelength conductor, in that if one end represents exactly the opposite of the other, then we could deliberately place one end at a zero potential, and the other end would therefore appear to be (or represent) an open circuit!!
  • Understand though that this is only for just the one frequency that this conductor represents a "quarter wavelength".
  • Here we discover that we can use conductors for insulators! ... but again, only for that one frequency.
• These are referred to as "Shorted Quarter-Wavelength Stubs", being shorted on the one end, but appearing as open circuits on the other end.

There's more!

• We said earlier that one end was just the extreme opposite of the other.
• Well now, suppose we had an "Inductive" component on one end of our quarter wavelength conductor, what do you suppose would appear on the other end? If you guessed "Capacitive" you are correct!
• Likewise a "Capacitive" component on one end would appear as "Inductive" on the other!
• We could even have a variable component and make it appear on the other end as a variable opposite.