W9UCW at aol.com W9UCW at aol.com
Tue Apr 26 11:39:33 PDT 2011

```This subject comes up often. That should be no  surprise. It's very
important to those who need to load shortened monopoles, and  there is a great deal
of conflict in the available literature on the subject.  The same seems to
be true of "conventional wisdom," as one might expect. Some of  us have been
working to sort it out for decades.

The simplified explanation of RF current  measurements made at various
points along a quarter wave monopole fed against a  ground system is the same
for a "sized" or a "loaded" antenna. First let's  consider a full-sized
monopole, "cut" to resonate at a particular  frequency. The monopole conductor
will have an inherent inductance and  capacitance and their opposing reactances
will be equal... thus,  resonance.

A monopole is a standing wave antenna. Two RF  currents exist when it is
powered. One is the forward current and the other is  the current reflected
from the open end. These are AC currents at an RF  frequency. The RF currents
will be of opposite phase at the top of the monopole,  thus the current will
be near zero. Just the opposite will be true of the RF  voltages, thus the
voltage will be maximum at the top end.

The RF current  at any point along the  monopole will be the "vector sum"
of the two currents whose phase angles  are changing in opposite directions.
Thus, the sinusoidal curve depicting the  antenna current.

If the monopole is shortened and steps have been  taken to resonate it,
then some amount of "lumped" inductance and/or capacitance will be present to
replace that lost in  the shortening. The phase angles of both forward and
reflected currents will  change more rapidly in the lumped inductance and/or
less than below the coil. The same  is true of a loading capacitance.

The result of this phenomenon is most severely  illustrated in very short
monopoles fed against a poor ground plane,  like a 75 meter mobile antenna.
The far-field field-strength of a 9 foot mast  with a coil and hat on top
measures 16 db better than the same mast resonated  with a base loading coil.
The difference for a 160 vertical of, say, 60 feet  over an extensive ground
system is much less, but still very significant, like a  4 times power
differential.

It should be remembered that the  electromagnetic/electrostatic field
between the antenna mast and the ground  plane is the part of the antenna system
that "loses" energy that we call radio  signal radiation. The more current
in the mast, the stronger the radiating  field, thus the more radiation.

explains why we have found no significant difference between  High-Q vs. Low-Q
loading coils, and very minute difference between  coil vs. hat wire top
loading. In fact, hat wires less than 90 degrees to  the vertical mast cause
notably less field strength depending on the angle  and length. Obvious field
canceling is the culprit.