Over the past few months, I have been doing some experiments with my full
wavelength, 40m loop in the delta or triangular configuration.  My design
goal is to reduce the physical size of the loop, but keep the DX (vertical
polarization) performance the same.  I have been able to do this with
expedient loading practices, and I continue to  experiment with the help 
of Mel KB5ION and Maurie VK3CWB.  I still have a few other things to try, 
and will give a full account at a later date; but, in the meantime, I came 
across something that I thought would be interesting to pass on.

Did you know that on a dipole, the sinusoidal current distribution is such
that 87% of the dipole's radiation is done by the middle 67% of the dipole?
(see Figure 1).

I read this in the new Lew McCoy antenna book, and took a pen to  paper,
along with a cosine table, to see it for myself.  The end 33% (2 times 
16.5% as there are 2 ends) of the dipole only radiates 13% of the total 
energy being radiated by the antenna.  This is a great discovery which 
literally means that we can remove 33% of a dipole's physical length and 
pay a penalty in loss of only 13% or about .6dB.

Of course, when the end portion of wire is removed, it must be replaced.
This replacement is called loading.  Loading is basically the practice of
replacing physical antenna length with a lump value of inductance or
capacitance which is the electrical equivalent of the wire that has been
removed.  Since the wire was removed from the end, it is best in this case
to load it at the end.  This keeps the current distribution the same as the
full size dipole, over the middle of the antenna, where maximum radiation
occurs.  The point on the antenna at which loading must be used determines
the type of loading to use and the value of that loading.


Reactance Factor

As an antenna is shortened (or lengthened) from the resonant length,
reactance develops at the feed point in addition to the feed point
resistance.  This combination of the resistance and the reactance is
called the feed impedance.  A shortened antenna will exhibit a capacitively
reactive feed impedance, indicating that we need to add inductive reactance
(loading) to cancel this capacitive reactance.  Alternatively, when
lengthening an antenna away from the resonant length, it will develop an
inductively reactive feed impedance, indicating that we need to load it
with capacitive reactance to bring the antenna to resonance and the feed
point to a resistive value.


Capacitive Loading

End loading (or voltage node loading) requires a capacitive type of lump
value equivalent circuit component such as a capacitive hat (Fig. 2).
This type of capacitive loading is very efficient, and you will end up
with an antenna that is very close to being 100% efficient, but only 2/3rds
the size.  Of course, this antenna size reduction doesn't come free.  The
downsizing causes a decrease in radiation resistance down to about 45 ohms,
which will give us a little better match to 50 ohms, but useable (2:1)
bandwidth will also suffer very slightly.

The end loading scheme can be done in another manner for hams who don't
have the room to put up a full-size dipole, but don't want to go to the
trouble of using or building capacitive "hats."  What can be done is just
to fold the ends of the antenna to the side (method A, Fig. 3) or even fold
it back onto itself (method B, Fig. 3).  This reduces the physical length,
but keeps the efficiency fairly high.  Folding the end back onto itself
keeps the wire length the same, but reduces the physical space by making
a capacitively loaded end.

In general, antennas can be shortened at any point along their physical
length, but it is best to shorten them where the loading has least effect
on the efficiency, bandwidth and radiation resistance.  There are other
methods to perform loading, but none with as low a loss as proper,
capacitive hat end loading.


Inductive Loading

The loading method that is most familiar to the ham community is inductive
loading at the center (feed point) or towards the mid-point of each side
of a dipole (Figure 4).  As can be seen from the current distribution
(Figure 1), these points on a dipole are the high current points and since
a coil is a current operative type of passive component, it is what is used
at this point in the antenna.  The value of the coil used is dependent on
the value or reactance that is needed and the position of the coil within
the antenna.  A basic rule of thumb is that the further out from the
maximum current point that the coils are placed, the more the inductance
must be increased to provide the proper loading. 


Mobile Whips

Loading coils are most commonly used on mobile vertical whips, and rightly
so!  You couldn't drive down the highway with a 40 meter, 1/4 wave vertical
(32 feet) hanging off of your bumper.

HF mobile whips are typically base loaded or more typically center loaded,
like the bug catchers or Hustler whips (Figure 5).  This type of loading is
used to make a shortened antenna electrically resonant on the HF frequency
on which we want to QSO.

Another type of mobile use for a loading coil is in a 2 meter, 5/8 wave
length antenna (Fig. 6).  These antennas have a base loading coil.  This
coil is needed to increase the antenna electrical length by 1/8th wavelength
so that it becomes resonant at 3/4 wavelength and the feed impedance is a
low resistive value.  In fact, as also indicated in Figure 6, the loading
coil is also used to allow the feed point to be tapped to give a perfect
50 ohm match.


Summary

Loading an antenna is a very common practice to the amateur operator no
matter what amateur band or antenna type is used.  It helps reduce the
size of the antennas when either physical height or length are a problem.
Loading will also bring an antenna system into resonance, which is the
desired condition for coaxially-connected, solid state transceivers.
On full wavelength or multiple wavelength antennas, loading can help
configure the current distribution so that the currents are phased
properly and therefore provide maximum radiation in the desired direction
or desired polarization.

When experimenting with antennas, knowledge of the types of loading and
the characteristics of each is a benefit that will pay off many times over.
With this knowledge, your antennas will be the most efficient skyhooks
that can be designed and built.  Radiating a great signal on the ham bands
means more fun...and that's what it's all about!