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
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|