What's so special about the so-called "Shorted
Quarter Wave Stub"?
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
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.
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.