U.S. patent number 3,742,511 [Application Number 05/153,372] was granted by the patent office on 1973-06-26 for low-loss antenna system with counterpoise insulated from earth.
This patent grant is currently assigned to Smith Electronics, Inc.. Invention is credited to John D. Musselman, Carl E. Smith.
United States Patent |
3,742,511 |
Smith , et al. |
June 26, 1973 |
LOW-LOSS ANTENNA SYSTEM WITH COUNTERPOISE INSULATED FROM EARTH
Abstract
An improved antenna system for transmitting and/or receiving
electromagnetic waves, having a counterpoise insulated from the
earth and in which the counterpoise is connected to one terminal of
a transmitter or receiver through an inductance. The inductance is
tuned in conjunction with the tuning of a conventional loading
inductance coil so as to maximize the field strength radiated by
the antenna into the far field. The power lost in the earth or
other antenna supporting surface is unusually small, so that
radiated power is large.
Inventors: |
Smith; Carl E. (Brecksville,
OH), Musselman; John D. (Brecksville, OH) |
Assignee: |
Smith Electronics, Inc.
(Brecksville, OH)
|
Family
ID: |
22546937 |
Appl.
No.: |
05/153,372 |
Filed: |
June 15, 1971 |
Current U.S.
Class: |
343/750;
343/847 |
Current CPC
Class: |
H01Q
9/38 (20130101) |
Current International
Class: |
H01Q
9/38 (20060101); H01Q 9/04 (20060101); H01q
001/48 (); H01q 009/00 () |
Field of
Search: |
;343/846,847,848,869,750 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Claims
What is claimed is:
1. In an antenna system for use upon an electrically lossy
supporting material and employing a superstructure having a
substantially non-uniform distribution of capacitance per unit area
to the material, apparatus comprising first and second antenna
terminals for connection to a source of AC excitation, a central
support device extending upwardly away from the lossy material, an
elevated conductive superstructure connected with said central
support device and sloping radially outward and downward therefrom
toward the lossy material to define an annular opening between the
superstructure and the lossy material, different portions of said
superstructure at different radial distances from said support
device having substantially different capacitances per unit area of
superstructure to respective portions of the lossy material, a
first inductance connected in a series circuit extending from said
first antenna terminal to a point on said superstructure
approximately at said central support device, a conductive
counterpoise structure located closer to the lossy material than is
said superstructure and sloping radially outward and downward from
said central support device toward the lossy material and insulated
from the lossy material, different portions of said counterpoise
structure at different radial distances from said central support
device having substantially different capacitances per unit area of
counterpoise structure to said respective portions of the lossy
material, a second inductance connected between said second antenna
terminal and the approximate center of said counterpoise structure,
and a conductive path connected between said second terminal and
the lossy material, and wherein, for the lossy material that is
generally under said superstructure, said substantially different
capacitance per unit area of counterpoise structure is proportioned
to said substantially different capacitance per unit area of
superstructure in a ratio that is approximately the same ratio for
each and every said portion of the lossy material.
2. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, apparatus as defined in claim 1 and wherein said same ratio
for each and every portion of the lossy material is approximately
equal to a ratio of said first inductance to said second
inductance.
3. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, apparatus as defined in claim 1 and further comprising a
plurality of insulating lines each having one end fixedly connected
with the lossy support material, and wherein said elevated main
conductive superstructure comprises a plurality of conductors each
extending from said central support device to the other end of one
of said insulating lines respectively, and wherein said conductive
path comprises a resistance.
4. In an antenna system for use upon an electrically lossy
supporting material and employing a superstructure having a
substantially non-uniform distribtion of capacitance per unit area
to the lossy material, apparatus comprising mechanical spacing
means connected with the lossy material for establishing a distance
from said lossy material, a main conductive superstructure
connected with said mechanical spacing means and spaced away from
said lossy material thereby for producing electromagnetic waves,
different portions of said superstructure being capacitively
associated respectively with different portions of the lossy
material to form distributed capacitances, said different portions
of said superstructure having substantially different capacitances
per unit area of superstructure to said respective portions of the
lossy material, conductive counterpoise means disposed intermediate
said main conductive superstructure and said lossy material for
forming distributed capacitances between said superstructure and
said said counterpoise means, different portions of said
counterpoise means also being associated respectively with said
different portions of the lossy material to form distributed
capacitances, the capacitance per unit area between said different
portions of said counterpoise means and said respective different
portions of the lossy material being of substantially different
capacitance values, each of said substantially different
capacitances per unit area of counterpoise means being proportional
to said substantially different capacitance per unit area of
superstructure for the same portion of said lossy material in a
common ratio that is substantially equal for each of said different
portions of the lossy material, means for energizing said main
superstructure with a first AC signal at a point on said
superstructure to produce first displacement currents flowing from
each portion of said superstructure to the respective portion of
the lossy material, and means for energizing said counterpoise
means with second AC signal at a point on said counterpoise means
to produce second displacement currents flowing from said
counterpoise means to each of said portions of said lossy material,
and wherein for each portion of said lossy material said first
displacement current is equal in magnitude and of opposite phase to
said second displacement current.
5. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, apparatus as defined in claim 4 and wherein said means for
engaging said main superstructure with a first AC signal comprises
an AC source and first phase shift means connected intermediate
said AC source and said point on the superstructure for shifting
the phase of said first AC signal, and wherein said means for
energizing said counterpoise means comprises said AC source and
second phase shift means connected intermediate said AC source and
said point on the counterpoise means for shifting the phase of said
second AC signal.
6. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, apparatus as defined in claim 5 and wherein said first phase
shift means comprises a first inductance and said second phase
shift means comprises a second inductance.
7. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, apparatus as defined in claim 6 and wherein said first and
second inductances are connected with different terminals of said
AC source having signals phased 180.degree. apart, and wherein an
inductance ratio of said first inductance to said second inductance
is approximately equal to said common ratio.
8. In an antenna system for use upon an electrically lossy
supporting material and employing a superstructure having a
substantially non-uniform distribution of capacitance per unit area
to the lossy material, a method for reducing electrical losses from
currents induced in the lossy material, comprising the steps of
providing a counterpoise between the superstructure and the lossy
material having a substantially non-uniform distribution of
capacitance to the lossy material that, for each portion of the
lossy material which capacitively communicates with the
superstructure, is proportioned by a constant ratio to the
non-uniform capacitance between the superstructure and the lossy
material, exciting the superstructure at a first point with a first
AC signal, and exciting the counterpoise at a second point with a
second AC signal, said AC signals being established relative to a
potential of said lossy material, and adjusting the relative phases
and amplitudes of said first and second AC signals to maximum
antenna efficiency.
9. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, a method as defined in claim 8 and wherein exciting the
superstructure at a first point comprises exciting said first point
from one terminal of an AC source through a first series
inductance, and wherein exciting the counterpoise at a second point
comprises exciting said second point from another terminal of the
AC source through a second series inductance, and wherein adjusting
the relative phases and amplitudes comprises establishing relative
values of said first and second inductances in approximate
proportion to said constant ratio.
10. In an antenna system employing a superstructure having a
substantially non-uniform distribution of capacitance per unit
area, a method as defined in claim 9 and further comprising a step
of providing a resistive current path from said other terminal of
the AC source to said lossy material.
Description
This invention relates to an apparatus and method for radiating (or
receiving) electromagnetic waves from an antenna. It is suitable
both for antennas having mechanical dimensions that are small
compared with a free space wavelength of the wave to be radiated
and for those having dimensions that are large.
Electrically small antennas, for which the invention is especially
suitable, often employ an umbrella-like system of conductors called
a top hat connected to the top of a main vertical radiating element
and extending outward or else outward and downward from the top
center of the antenna toward a supporting surface which is often
the earth or a surface of a vehicle. The top hat is a capacitive
top structure that is employed in order that a standing wave of AC
current flowing in the antenna will not be zero at the top center
of the antenna but rather will have some larger value at the top
center and will be zero only at the outer periphery or ends of the
top hat. The current in the main vertical element is therefore
larger than it would be without a top hat, and it radiates energy
better.
It is also common practice to employ a counterpoise which is a
plate or system of wires near the base of the main vertical element
of an antenna and extending outward to a sufficiently great radius
that the capacitance between the top hat and the earth or other
supporting surface is as small as practical compared with the
capacitance between the top hat and the counterpoise. The top hat
and the counterpoise form two plates of a capacitor. The purpose of
the counterpoise is to provide a surface on which the displacement
current that originates on a charge on the top hat can terminate
near the earth without having to terminate on the earth itself
which often has conductivity inferior to that of the counterpoise
and is therefore susceptible to power losses when electron current
flows in it. Such power losses reduce the efficiency of the antenna
so that part of the power output of the transmitter is wasted in
supplying the losses and only the remainder is used for launching
an electromagnetic wave into the intended propagating medium such
as the air or free space. The losses that are reduced by the
present invention are those that occur in portions of the earth
that lie under and near the counterpoise.
The strength of the radiated electromagnetic wave from an
electrically small antenna depends upon and increases with the
current flowing in the main vertical element of the antenna. For
this reason such antenna systems ordinarily employ an inductance
coil called a loading coil connected in series with the transmitter
and the main vertical element of the antenna having a valve
appropriate to cause electrical resonance with a self-capacitance
of the antenna. When the resulting series resonant circuit,
comprising the loading coil and the capacitance from the vertical
radiating element and top hat to counterpoise, is tuned so as to be
electrically resonant at the frequency of excitation of the
antenna, a greater alternating current flows in the antenna and in
particular in the main vertical element thereof, whose current
determines the field strength of radiated electromagnetic wave.
Many electrically large antennas do not require the top hat, and in
those cases a distributed capacitance of the main vertical element
is sufficient to resonate the antenna. Counterpoises are used by
both electrically small and electrically large antennas, and the
present invention is applicable to both but especially to the
electrically small types.
In some systems of the prior art and in some versions of the
present system, counterpoises are constructed of wires near the
earth radiating outward from the center of the antenna and spaced
sufficiently closely to each other as comapred with their height
above the earth so that they effectively collect most of the
displacement current (sometimes called electric field flux) that
exists in the space between the top hat and the counterpoise and
between the vertical radiating element and the counterpoise. Any
displacement current that terminates on the earth instead of on the
counterpoise ordinarily causes losses in the earth due to a flow of
electron current that is induced in the earth by the alternations
of displacement current.
It is not economical to increase beyond a certain number the number
of counterpoise wires or, if a grid is employed, the closeness of
the grid spacing, in order further to increase the share of
displacement current that is captured by the counterpoise as
compared with that captured by the earth beneath it, because of the
additional cost of the extra counterpoise wires. Neither is it
economical to increase the share of displacement current that the
counterpoise takes by raising the counterpoise very high above the
earth so as to change the ratio of (a) spacing between grid wires
at any particular radius to (b) the distance from the grid wires to
the earth, because raising the counterpoise shortens the effective
portion of the main vertical element of the antenna and thereby
reduces the radiated energy for any particular fixed value of
antenna current. Design compromises are ordinarily made between the
height of the counterpoise, the angular spacing between
counterpoise wires, and the radius to which the counterpoise
extends, in order sufficiently to conceal electrically the earth
from the top hat by means of the counterpoise, so that the earth
will not have excessive power losses.
The present invention reduces power losses caused by currents
induced in the earth, by reducing the currents induced in the earth
under the counterpoise.
An advantage of the present invention is that a counterpoise can
have its wires somewhat wider spaced than in antennas of the prior
art or somewhat closer to the ground and have the same degree of
efficiency as systems of the prior art. Alternatively, for a given
spacing betweeen wires and a given height of counterpoise above the
ground, the antenna system of this invention is somewhat more
efficient than those of the prior art and, therefore, radiates
greater field strength for a given transmitter power than do prior
systems.
Accordingly, it is a principal object of the present invention to
provide a new and improved antenna structure which is capable of
adjustment to reduce losses in the earth, or other supporting
structure such as a vehicle, so as to increase the efficiency of
the antenna system for radiating and receiving electromagnetic
waves.
Another object of this invention is to provide a new and improved
method of connecting and adjusting an antenna system so as to
reduce wasted power and thereby to increase radiated field strength
or receiving gain of the antenna.
Still another object of the present invention is to provide an
antenna system which is convenient for use in various and differing
settings, such as irregular earth surfaces and earth surfaces
having very poor electrical conductivity as well as those with good
conductivity, so that the antenna system can be portable and yet be
efficient.
Still another object of the present invention is to maximize by
means of the invented apparatus the ratio of the power transferred
from an antenna to a desired radiation transmission medium and the
power transferred from the same antenna to an undesired neighboring
medium such as the earth.
Still another object is to minimize the losses in the earth or
mounting vehicle or ship or other such mounting surface of an
antenna used either for transmitting or for receiving
electromagnetic waves.
A further object is to provide an apparatus and a method for
isolating the effects of an antenna from its harmful environment
such as a lossy earth despite the unavoidable existence of a
significant partial capacitance to the harmful environment from
components of the antenna structure.
Another object of the present invention is to provide an antenna
system in which an impedance is connected in series with the
counterpoise and adjusted so as to reduce the earth losses.
A further object of the present invention is to teach the use of an
impedance to buffer a transmitter from an earth ground for the
purpose of minimizing loss.
Still another object of the present system is to provide an antenna
whose counterpoise structure is small and yet sufficient for
achieving a required degree of efficiency of operation, hence, a
counterpoise system that is economical to construct.
Although the drawings show only a transmitting application of the
present invention, the objects of the invention include both
transmitting and receiving applications. In receiving applications,
the object of the tuning of the counterpoise network is
preservation of available signal-to-noise ratio; reduction of the
antenna losses is sought in a receiving application in order to
maintain a good signal-to-noise ratio at the receiver's input
terminals.
Other objects and features of the invention will become more
apparent upon a consideration of the following description taken in
conjunction with the accompanying drawings wherein:
FIG. 1 is an elevational view of an electrically small embodiment
of the inverted antenna and its ancillary equipment. FIG. 1 also
shows some capacitance symbols which represent the partial
capacitances from top hat to counterpoise, from counterpoise to
earth, and from top hat to earth.
FIG. 2 is a simplified plan view showing only one of four quadrants
of an embodiment of the present invention which is, as to this
view, the same as the prior art. The top hat conductors, their
insulators and the counterpoise conductors and their insulators are
shown in FIG. 2.
FIG. 3 is a schematic electrical circuit diagram depicting the
antenna system in an equivalent circuit form similar to an
open-circuited artificial transmission line for purposes of
explaining the operation of the invention.
FIG. 4 is a phasor diagram which illustrates the great amount of
current flowing in a lossy earth which results from even a small
phase imbalance.
A transmitting application will be described; receiving
applications are similar by reciprocity.
An improved electrically small antenna system 10 constructed in
accordance with the present invention is illustrated in FIG. 1. The
particular embodiment shown is a top-loaded 25 foot high monopole
antenna with 12 umbrella-like top hat conductors and 24 insulated
counterpoise conductors which is portable and supported by an
inflatable mast. The principal working component of the illustrated
antenna is a main vertical element 12 in which an alternating
current flows. The current causes radiation from the antenna
structure 10 into the surrounding air, free space, water, or other
medium of radiation transmission. The main vertical element is
driven by a transmitter 14 having a first terminal 15, through a
series-connected loading inductance coil 16. At the top of the main
vertical element 12 are connected a plurality of top hat conductors
18 which collectively resemble the stays of an umbrella.
The top hat electrical conductors 18 terminate in insulators 20 at
their outer ends and are supported from that point onward to the
periphery of the antenna by non-conducting nylon support guys 22.
The purpose of the non-conducting support guys is to hold the
conducting top hat wires in place without interfering with
radiation of energy from the main vertical element to regions
outside the antenna.
A counterpoise structure 24 comprising radial electrically
conductive wires is located below the top hat conductors and
extends outward beyond the radius at which the top hat insulators
20 are located. All of the counterpoise conductors 24 are connected
to each other near the center of the antenna structure. The
counterpoise structure is not, however, connected at its center to
the main vertical element 12, but is connected through a
counterpoise variable inductor 26 to a second terminal 27 of the
transmitter 14. At its outer end, each counterpoise conductor
terminates on an insulator 29 which supports it. Thus, the
counterpoise is electrically insulated from the earth 32 and is
conductively connected to other elements of the system only through
the counterpoise variable inductor 26.
The angular relationship of top hat conductors to each other and to
the counterpoise conductors 24 can be seen in FIG. 2 which is a
plan view of the antenna system. In the embodiment depicted here,
there are twice as many counterpoise conductors 24 as there are top
hat conductors 18; top hat conductors are located at 30.degree.
intervals of the full circle and counterpoise conductors are
located at 15.degree. intervals. It should be noted in FIG. 2 that
the portions of the counterpoise conductors 24 which are closest to
the ground, namely, those portions at the greater radii, are the
portions which are farthest spaced from adjacent counterpoise
conductors. This is one of the problems inherent in an antenna
system of this type which is alleviated by the present invention.
The wide spacing between counterpoise conductors at their outer
ends together with the close spacing of the counterpoise conductors
to the earth causes displacement current which flows from the top
hat conductors downward to tend to go to the earth at some places
between counterpoise conductors instead of terminating on the
counterpoise conductors, which would be much more desirable from
the standpoint of minimizing losses. In FIG. 2 it may be seen that
the insulators 20 form the boundaries between electrically
conductive top hat lines 18 and electrically non-conductive nylon
support guys 22.
The antenna top hat structure 18 is resonated by adjusting the
loading coil 16. This is a reactance tuning network which comprises
a tapped coil in series with a continuously adjustable variable
inductor, such as a variometer, for fine turning between discrete
tap positions.
A second reactance network 26 is also tuned, so as to resonate with
the counterpoise system 24. Both of the reactance tuning networks
are adjusted so as to minimize the power losses and maximize the
useful radiation from the antenna.
A distributed partial capacitance exists between the wires of the
top hat 18 and the wires of the counterpoise 24. This distributed
capacitance 28 is shown in FIG. 1 where it is represented as a
lumped capacitance for simplicity of discussion. Another partial
capacitance 30 exists from the top hat conductors 18 to the earth
32, capacitance 30 ordinarily being smaller in value than
capacitance 28. A third distributed capacitance of importance in
this system is the capacitance from the counterpoise wires 18 to
the earth 32, this capacitance being shown in FIG. 1 for simplicity
as a lumped capacitance 34.
The value of the loading inductance coil 16 is approximately such
as to resonante at the frequency of excitation with the capacitance
28. The capacitance 30 could be small by comparison with
capacitance 28, but capacitance 30 is nevertheless significant for
other reasons as will be seen below. The inductance of the
counterpoise variable inductor 26 is preferably of such value as to
resonate at the electrical frequency of excitation with capacitance
34 existing from counterpoise wires 24 to the earth 32.
Resistor 36 is a static drain which serves to connect one terminal
of the transmitter to earth ground.
When the antenna is used as a transmitting antenna, a transmitter
drives current through inductor 16 and up the main vertical element
12 to the top hat 18. Conduction current flows out along the top
hat conductors 18 but does not flow beyond the terminating
insulators 20. From that point displacement current flows
downward.
Almost all of the displacement current that flows from the top hat
wires 18 and the top of the main vertical element 12 through
distributed capacitance 28 terminates on the counterpoise wires 24.
The return path for this current is electron conduction through the
counterpoise wires 24 to the counterpoise inductor 26 and through
it to the second terminal 27 of the transmitter 14.
When the loading coil 16 is properly adjusted, loading coil 16
resonates with the capacitance 28 and a great current flows in the
series circuit comprising those two elements and the counterpoise
24 and the variable inductor 26. A reactive voltage drop is created
across tuning coil 26 by the flow of current through coil 26. This
determines the potential of the counterpoise with respect to
terminal 27 of the transmitter. It is believed that when the system
is properly adjusted, the potential of the counterpoise is such
that displacement current flowing from the counterpoise 24 through
capacitance 34 to earth is equal in magnitude but opposite in sign
to the displacement current flowing from top hat 18 through
capacitance 30 to earth. Consequently, capacitor 34 supplies the
current demanded by the capacitor 30 and very little current need
flow through the lossy earth under the counterpoise.
The large current flowing in the main vertical element 12 causes
electromagnetic radiation, which travels radially away from the
main vertical elements, continues through the annular opening
between the top hat 18 and the counterpoise 24 below it, and
propagates outward from the antenna as a whole. For the antenna
described, the radiation is uniform in all azimuthal directions and
is vertically polarized, having its electric field vector vertical
and its magnetic field vector horizontal.
A procedure for tuning the antenna and counterpoise system to a
specified frequency is as follows: First, the loading coil 16 is
adjusted to a tap which is known to be appropriate for resonating
with capacitance 28 at the frequency to be radiated. Second, the
counterpoise tuning coil 26 is adjusted to a position at which coil
26 will be approximately resonant with capacitance 34 at the
desired frequency. Third, the transmitter is operated so as to
excite the antenna at the desired frequency, and the variometer
which is part of loading network 16 is tuned for maximum output as
indicated by a meter on the transmitter or in the far field. This
procedure may be repeated if desired for finer tuning.
For one system which was tested and found to result in great
improvement over the prior art, the adjustable loading coil 16 in
series with the main vertical element has an inductance range of
0.17 to 1.6 millihenries. The variable inductance 26 in series with
the insulated counterpoise has a range of adjustment of 0.04 to 0.2
millihenries. Capacitance from the main vertical element 12 and the
top hat 18 to the counterpoise 24 for the antenna tested was
approximately 320 picofarads. The approximate capacitance from the
counterpoise structure 24 to ground 32 was 2440 picofarads. The
static drain 36 was a 100 ohm resistor. This range of parameters
permits tuning the antenna over the frequency range of 260 to 530
kilohertz. The antenna's effective input resistance ranges from
approximately 7 ohms at 530 kilohertz to 10 ohms at 260
kilohertz.
It is necessary to treat the antenna as a distributed parameter
system even through it is electrically small because the losses
which it is sought to minimize arise in consequence of the
distribution of the parameters. The capacitance from the top hat to
the counterpoise, from the counterpoise to the earth, and from the
top hat to the earth are distributed throughout a circular area
centered on the antenna. Also, the resistivity of the earth or
other support surface under the antenna causes power loss, due to
currents of small magnitude, to be distributed throughout the earth
under the counterpoise and somewhat beyond it. The operation of the
invention can nevertheless be described by reference to a lumped
parameter equivalent circuit diagram shown in FIG. 3 wherein the
distributed parameters are shown as three groups of partial
capacitances similar to those of an artificial transmission line.
In FIG. 3, the top hat is shown as the upper line 18 of the
transmission line. The counterpoise structure is shown as the
lowest line 24 of that equivalent transmission line. The earth 32
is shown between lines 18 and 24 although mechanically the
counterpoise is not located there unless it is buried underground,
which it may be in some cases. The line 32 representing the earth
has series resistors to symbolize that it is a lossy path. The
lines 18 and 24 are shown as lossless because they are good
conductors; no resistances are included in their equivalent circuit
because they would be negligibly small for purposes of this
discussion.
A transmitter 14 connects to the input terminals of this equivalent
transmission line, which is a tapered line because of the variation
in values of the parameters with varying radius from the center of
the structure. Current driven into the transmission line by the
transmitter passes through the loading coil 16 and out to various
partial capacitances 28a, 28b, 28c in the equivalent circuit. These
capacitors represent the distributed capacitance between top hat 18
and counterpoise 24, and have a total value equal to that of
capacitance 28.
Distributed capacitance between the counterpoise 24 and earth 32 is
designated in FIG. 3 as 34a, 34b and 34c. Capacitance from top hat
18 directly to the earth 32 is represented by capacitors 30a, 30b
and 30c. A main current flows from the transmitter 14 through
loading coil 16, then through capacitors 28a, 28b and 28c to the
counterpoise 24 and then through variable inductor 26 and back to
the transmitter. The transmission lines 18 for top hat and 24 for
counterpoise are not equally balanced electrically to earth 32;
instead, they have a relationship based on a ratio of the
capacitance 30 to the capacitance 34. This relationship to earth is
established by the unavoidabe presence of the capacitance 30 and
the capacitance 34. Inductor 26 permits the counterpoise 24 to
assume a voltage corresponding to this asymmetrical relationship to
earth of the lines 18 and 24. It is believed that no voltage of
appropriate value could develop on line 24, the counterpoise, if
inductor 26 were replaced by a short circuit to ground.
In the antenna of this invention, the ratio of the inductive
reactance of loading coil 16 to the capacitive reactance of
capacitance 30 is approximately equal to the ratio of the inductive
reactance of counterpoise tuning coil 26 to the capacitive
reactance of capacitance 34. Thus, the inclusion of tuning coil 26
permits current from the counterpoise to earth to be controlled in
the area under the counterpoise.
Failure to control the potential of line 24 would cause circulating
currents to flow through the partial capacitances 30a, 30b, 30c and
34a, 34b and 34c into the earth, whose resistivity is symbolized by
the resistances 38a, 38b, 38c, 38d, 38e, 38f. In the absence of
inductor 26, circulating currents would exist in the earth.
The potential of counterpoise 24 is controlled with respect to
earth in the present invention. The counterpoise 24 is not
symmetrically disposed to earth as compared with top hat 18; that
is, the earth's potential is asymmetrically related to those of the
top hat 18 and the counterpoise 24 but the counterpoise potential
is maintained so as to minimize the flow of circulating currents in
the earth. It is a flow of these earth currents that would
otherwise cause heating loss in the earth and which would reduce
the efficiency of the antenna. FIG. 4 is included to show how in
the absence of an inductor such as 26 a small phase imbalance 42 in
two phasor currents 44 and 46 induced in the earth by capacitances
30 and 34, respectively, causes a significant imbalance current 48
to flow in the lossy earth.
The antenna system of this invention can be used equally well for
receiving radio waves as for transmitting them. When used as a
receiving antenna, the signal-to-noise ratio is improved by
comparison with antennas of the prior art because the usable
signal-to-noise ratio depends upon the power losses in the
receiving antenna, and smaller losses correspond to greater usable
signal-to-noise ratio.
Although this invention has been described by reference to one
particular embodiment thereof, it is clear that various other
antennas may be constructed that utilize the inventive concept
described herein and it is intended that such variations be
included in the scope of this patent. One such variation is to
control the alternating potential of the counterpoise by connecting
a source of alternating voltage to the counterpoise instead of
using the inductor 26. The source of alternating voltage could then
be synchronized with the transmitter with an appropriate phase
relationship thereto, by conventional techniques, in order to
minimize earth losses.
Both electrically large and electrically small antennas are
included.
While the invention relates to antennas of all electrical size, it
is particularly useful for those whose mechanical dimensions are
small in terms of a wavelength of frequency of excitation, this
class being known as electrically small antennas.
* * * * *