Eric R Snow said:
Why antennas made that are 1/4 or 1/2 the wave length?
A short-form, somewhat-simplified explanation is as follows:
Consider what happens on a quarter-wave long length of transmission
line, open at the far end. Or better, at multiples of a quarter-wavelength
back from the open end on such a line. The "standing wave"
pattern on such a line, resulting from the total reflection of the
forward signal by the "open," has a voltage maximum (and
obviously, a current minimum) at the open end, and conversely
a voltage minimum (and current maximum) at a quarter-wavelength
back (with this pattern repeating from there).
Now, think about a half-wavelength antenna - a simple "dipole"
- as being nothing more than such a quarter-wave section of
line, "opened up" such that one conductor points this-a-way and
the other conductor points that-a-way (in the opposite direction).
Our first guess might be that the standing-wave situation described
above remains in effect - the tips of this "opened-up" structure are
points of high voltage (and opposite polarity, with respect to one
another), while the center is a low-voltage/high-current (read: low
impedance) point. This is precisely what you want for making EM
radiation - an electric field created "across" the structure (i.e., between
high-potential ends), plus a magnetic field created "around" it and in the
correct phase relationship. So what we've just described ought to be just
fine for launching EM radiation, while providing a reasonably decent
impedance at the point we wish to drive (the center of the structure).
So, with at least very minimal math, this is a workable model or
visualization as to why half-wave dipoles work well for transmitting
radio waves. (And fortunately, what works well for sending such
signals works just as well for receiving them.)
A quarter-wave antenna results from noting that the two pieces of
the above structure are really just quarter-wave-long mirror
images of one another, and either of them will work in the same
manner if the other is replaced by a "ground plane" normal to the
remaining element and passing through the center point of the original
dipole structure. This plane, from the standpoint of determining the
fields produced by the remaining element, really DOES act like a
"mirror," although obviously nothing is really emitted by the "mirror
image" viewed as being on the other side of that plane.
In reality, various factors make for neither the half-wave dipole or
the quarter-wave derived from it exactly a half- or quarter-wave
long, relative to the wavelength of the signal in question in free
space. They're generally going to turn out being a bit shorter, to get
the proper "resonance" and a decent (resistive only, in the ideal case)
feedpoint impedance. But they're close enough that these names
work just fine.