Topband: . Re: 160 metre vertical with 'top loading'

[W9UCW at aol.com]

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 
capacitance. Thus, currents  measured above a loading coil will be notably 
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. 
 
The current reduction in lumped inductance and  capacitance loading 
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.
 
An ultimate example, somewhat related to  "umbrella" wire loading and 
linear loading is the "Meandered Line" antennas  published in the IEEE 
Transactions, December, 1998. It's performance  can be best likened to a large, 
unshielded dummy load.
 
I'm hoping to soon finish and publish the  complete results of a dozen or 
so extensive measurement programs done over the  last 40 years with the 
involvement of many cohorts. All of the work deals with  loaded, shortened 
monopoles and related issues.
 
73, Best DX,  Barry