U.S. patent number 3,656,167 [Application Number 04/879,753] was granted by the patent office on 1972-04-11 for dipole radio antennae.
This patent grant is currently assigned to The Plessey Company Limited. Invention is credited to Charles A. Lea.
United States Patent |
3,656,167 |
Lea |
April 11, 1972 |
DIPOLE RADIO ANTENNAE
Abstract
Dipole whip antenna arrangements are disclosed which are capable
of operating over a wide frequency band without the use of
continual manual or automatic tuning. This is achieved by
connecting the dipole whip antenna to the ground plane through a
reactance network which is specially designed so as to match the
impedance at the feed point of the dipole arrangement to a desired
value over the desired frequency band. For this purpose, the
reactance network is an active network, and operates by effectively
cancelling out the reactance of the antenna.
Inventors: |
Lea; Charles A. (Ilford,
EN) |
Assignee: |
The Plessey Company Limited
(Ilford, EN)
|
Family
ID: |
25758279 |
Appl.
No.: |
04/879,753 |
Filed: |
November 25, 1969 |
Current U.S.
Class: |
343/793; 343/822;
343/853; 343/816; 343/850; 343/860 |
Current CPC
Class: |
H03H
7/383 (20130101); H01Q 9/18 (20130101) |
Current International
Class: |
H01Q
9/04 (20060101); H03H 7/38 (20060101); H01Q
9/18 (20060101); H01g 009/16 (); H01g 021/06 () |
Field of
Search: |
;343/793-798,810-816,799,850-853,845-849,825-831,700-703,792 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Microwave Transmission Circuits"-Ragan McGraw-Hill New York, 1958,
TK 6553 R34; pp. 182-184 and 311-314 .
"Antennas"-Kraus McGraw-Hill New York, 1950 TK 7872 A6K7; pp.
279-283 .
"The ARRL Antenna Book"-The American Radio Relay League West
Hartford, Conn. 1956 TK 6565 A6A6; p. 107.
|
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Nussbaum; Marvin
Claims
What is claimed is:
1. A dipole antenna arrangement, comprising
a dipole antenna having two antenna elements and a feedpoint,
a reactance network comprising a wide-band amplifier constituting
an active impedance-converting device, a series resonant circuit,
and means connecting the series resonant circuit to provide a
negative feed-back path across the amplifier, and
means connecting the reactance network in circuit with one element
of the dipole,
the gain of the amplifier and the impedance of the resonant circuit
being so chosen that the effective overall input impedance of the
amplifier varies with frequency to give the network a
reactance/frequency characteristic such as to match the impedance
at the feedpoint of the dipole to a desired value over a wide
frequency range.
2. An antenna arrangement according to claim 1, comprising
a further dipole antenna having two antenna elements and a
feedpoint,
a further reactance network comprising a wide-band amplifier
constituting an active impedance-converting device, a series
resonant circuit, and means connecting the series resonant circuit
to provide a negative feedback path across the amplifier,
means connecting the further reactance network in series with one
element of the further dipole,
the gain of the amplifier and the impedance of the resonant circuit
being so chosen that the effective overall input impedance of the
amplifier varies with frequency to give the further network a
reactance/frequency characteristic such as to match the impedance
at the feedpoint of the further dipole to a desired value over a
wide frequency range.
Description
BRIEF SUMMARY OF THE INVENTION
The invention relates to radio antennae, and is particularly though
not exclusively, concerned with whip antennae.
The invention is concerned with the problem of providing a dipole
antenna which can operate over a wide frequency band without the
need for automatic or manual continuous tuning. A simple dipole
antenna suffers from the disadvantage that it can only be used
within a narrow frequency band. Outside the narrow frequency band
for which it has been designed, a dipole antenna presents an
unacceptable impedance to the equipment to which it is connected.
In order to overcome this problem, it is known to provide a
variable antenna tuning or matching unit which transforms the
antenna impedance into a value which is acceptable to the
equipment. Such a unit may be tuned manually by an operator, in
which case this places demands on the operators's time and skill.
Instead, the antenna tuning unit may be designed to set itself
automatically, but this inevitably results in a large increase in
cost and complexity and a corresponding reduction in reliability.
Even with such an automatic antenna tuning unit, the antenna can
only be used for a narrow band of frequency at any one time: it
cannot be used for transmission or reception of wide band signals
nor for transmission or reception of a number of widely spaced
frequencies simultaneously.
Accordingly, the invention provides a dipole antenna arrangement
comprising a dipole antenna having a reactance network connected in
series with one element of the dipole, the reactance/frequency
characteristic of the network being such as to match the impedance
at the feed point of the dipole to a desired value over a wide
frequency band.
The use of the reactance network, in accordance with the invention,
thus enables the antenna arrangement to operate over a wide
frequency band without the necessity for manual or automatic
tuning, and in addition enables the antenna arrangement to be used
for transmission or reception of wide band signals or of a
plurality of widely spaced frequency simultaneously.
BRIEF DESCRIPTION OF THE DRAWINGS
Dipole whip antenna arrangements embodying the invention will now
be described, by way of example only, with reference to the
accompanying drawings in which:
FIGS. 1 to 13 each show, diagrammatically, a different embodiment
of the invention, the circuit connections of which are shown
schematically; and
FIG. 14 is a schematic circuit diagram of a reactance network for
use with the antenna arrangements.
An item in an Figure which corresponds to a similar item in any
other Figure is given the same reference number.
DETAILED DESCRIPTION
In Figure 1, the dipole whip antenna comprises two elements 5 and 6
having a feed point 8 by means of which the energy to be radiated
can be fed in or by means of which radiation energy received by the
antenna can be fed out, and the lower element 5 of the dipole is
connected to a ground plane 9 through a two-terminal reactance
network 10 having a reactance X(f) which varies in a predetermined
manner with frequency.
The impedance, Zf, at the feed point 8 depends on the lengths
l.sub.1 and l.sub.2 of the two elements 5 and 6 of the dipole. The
lengths l.sub.1 and l.sub.2 control Zf because they determine the
way in which the amplitude and phase of the current flowing in the
dipole varies along its length. The two-terminal reactance network
10 is so designed that the reactance at its terminals so varies
with frequency that the impedance Zf maintains a desired value over
a wide range of frequencies. In this way, the dipole whip antenna
of FIG. 1 can be used over a wide frequency range without the need
for a manually or automatically tunable antenna tuning unit, and
the value of Zf is automatically maintained at a value which is
acceptable to the equipment to which the antenna is to be
connected. The antenna can thus be used for transmission or
reception of any frequency over a wide range, or for transmission
or reception of wide-band signals or simultaneously occurring
signals having a number of widely spaced different frequencies.
FIG. 2 shows how a connection may be made to the feed point 8 from
or to the associated equipment, not shown.
In FIG.2, the lower element 5 of the dipole whip antenna is formed
by a hollow tube 12 which is electrically connected, at its upper
end 14, to the outer conductor 16 of a coaxial feeder 18 whose
central conductor 20 connects to the upper element 6 of the
antenna. The tube 12 is connected through the two-terminal
reactance network 10 to the ground plane 9. Therefore, the coaxial
feeder 18 connects the feed point 8 of the dipole to the associated
equipment, not shown.
In FIG.2, the annular space between the tube 12 and the outer
conductor 16 of the coaxial feeder acts as a short-circuited length
of transmission line and thus presents a reactance in parallel with
the two-terminal network 10. This parallel reactance is taken into
account in designing the two-terminal network.
The impedance presented to the associated equipment, not shown, by
the coaxial feeder 18 of FIG.2 will not be equal to the feed point
impedance Zf; the coaxial feeder, of characteristic impedance Zo,
will transform the feed point impedance Zf to a new value, Z. By
appropriately dimensioning the coaxial feeder 18, so as
correspondingly to determine the characteristic impedance Zo, the
impedance Zf can be transformed to Z in such a way as to provide a
corresponding reduction in the effect, on the impedance Z, of
variations in Z(f) with frequency. Thus, the impedance transforming
action of the coaxial feeder 18 augments the effect of the
reactance 10 in increasing the operating band width of the
antenna.
FIG.3 shows an antenna in which the augmenting effect provided by
the coaxial feeder 18 is further increased. The arrangement of
FIG.3 is generally similar to that of FIG.2, except that the
diameter of the inner conductor 20 of the coaxial feeder is varied
over its length. Thus, the diameter has one value over the length
l.sub.3 , another diameter over a length l.sub.4 , and another
diameter over a length l.sub.5 . In this way, the coaxial feeder
acts as three impedance transformers connected in cascade thus
further reducing the value of Z as compared with Z(f) and thus
further reducing variations in Z resulting from variations in Z(f).
In the embodiment of FIG.4, the outer conductor 22 of a coaxial
feeder 24 itself forms the lower element 5 of the dipole whip
antenna and, in a manner analogous to that shown in the previous
Figures, is connected to the ground plane 9 through the reactance
10. In order to prevent the coaxial feeder from short-circuiting
the reactance 10, the coaxial feeder itself is coiled up, over a
region 26, to form an inductor and thus provides r.f. isolation of
the bottom of the dipole from the ground plane 9. The inductance
provided by the coil region 26 of the coaxial feeder, and its
variation with frequencies, has to be taken into account in
designing the reactance 10. The coiled up region 26 of the coaxial
feeder 24 of the embodiment of FIG.4 does not effect the
impedance-transforming action of the coaxial feeder which, as in
the embodiment of Figures 2 and 3, again augments the impedance
matching effect of the reactance 10; as in the case of FIG.3, this
augmenting effect can be increased by varying the relative sizes of
the two conductors of the feeder along its length.
FIG.5 shows an arrangement corresponding to that of FIG.4, but the
reactance network 10 and the coiled region 26 of the coaxial feeder
24 are enclosed in a screening tube 28 below and connected to the
ground plane 9. Such an arrangement can be used on vehicles and the
like where it is desirable for physical protection purposes. The
reactance presented at the bottom of the dipole by the network 10
and the inductive region 26 is transformed by the length of
transmission line formed between the outer conductor 22 of the
coaxial feeder and the screening tube 28, and this has to be taken
into account in designing the reactance network 10.
In the embodiment of FIG.6, the external equipment is connected to
the feed point 8 of the dipole whip antenna by means of a twin-wire
or twin-strip transmission line 30, one conductor 32 of which
provides the lower element 5 of the dipole and the other of which
is connected to the other element 6. The conductor 32 of the
transmission line 30 is connected to the ground plane 9 through the
reactance 10, and the bottom of the dipole is isolated, at radio
frequencies, from the ground plane 9 by coiling up both conductors
of the transmission line 30 over a region 34 to form a bifilar
inductor. The operation is generally similar to that described in
connection with the preceding Figures, and again the impedance
matching effect of the reactance 10 is augmented by the impedance
transforming effect of the transmission line 30. In a manner
analogous to that shown in FIG.3, the impedance transforming effect
of the transmission line 30 of FIG.6 can be further increased by
varying the sizes of the two conductors of the transmission line
along their lengths as shown.
FIG.7 shows an embodiment analogous to that of FIG.5 in which the
twin-wire transmission line 30 is arranged within a screening tube
28.
FIGS. 8 and 9 show embodiments of the invention for use where no
ground plane 9 is available (such as on the mast of a ship). FIG.8
corresponds to FIG.4 and consists of two dipole whip antennae
arrangements, each as illustrated in FIG.4, which are combined to
form a balanced pair. The coaxial feeders 24 of each of the two
antennae of Fig. 8 are connected together to a common feeder 34
which is arranged to feed both antennae in phase so as to provide
maximum radiation at right angles to the axis of the antennae.
FIG. 9 shows an arrangement analogous to FIG. 8 in which each of
the two dipole whip antennae is an antenna as shown in FIG.6.
FIG.10 shows an embodiment of the invention providing a wide band
directional array. It comprises two dipole whip antennae 36 and 38,
each of which is shown symbolically only but which may be of the
form shown in FIGS. 1 to 7. The two antennae 36 and 38 are spaced
apart by a distance equal to one quarter of the wave-length at the
mid band frequency. They are connected to be driven through a
common feeder 40 which feeds through a phase shifting circuit 42 to
provide a 90.degree. phase difference between the signals supplied
to each of the antenna.
FIG.11 shows a directional array comprising a first pair of
balanced antennae 44 and 46, and a second pair of balanced antennae
48 and 50. The antennae 44 to 50 are shown symbolically only, but
each is of the form shown in FIGS. 2, 3, 4 or 6. The antennae are
fed through a common feeder 52 and a phase shifting network 54, the
latter ensuring that there is a 90.degree. phase difference between
the signals supplied to the pair of antennae 44, 46 and the signals
supplied to the pair of antennae 48, 50.
Each of the arrangement of FIGS.10 and 11 may be modified by
incorporating more than two antennae, or two pairs of antennae, in
order to obtain higher directivity, and the phase shift between
adjacent elements or adjacent pairs of elements may be made to
change with frequency so as to maintain the directivity
approximately constant over a wide frequency band.
Instead of the arrangements of Figs. 10 and 11, a wide-band
directional array can be constructed from a plurality of dipole
whip antennae, each as disclosed herein, in which only one of the
antennae is driven and the remainder parasitic. FIG.12 shows such
an arrangement in which a dipole whip antenna 56, which is shown
only symbolically but which may take the form of an antenna as
disclosed in one of the FIGS. 1 to 7, is driven through a feeder
58, while another dipole whip antenna 60, which again may take the
form of an antenna disclosed in one of the FIGS. 1 to 7, is
parasitic and has its feeder terminals short-circuited. FIG.13
shows an arrangement comprising a balanced pair of dipole whip
antennae 62, 64, connected to be driven through a common feeder 66,
and a further pair of antennae 68, 70 which are parasitic and which
have their feeder terminals cross-connected. Each of the antennae
62, 64, 68, 70 is shown symbolically only but may be as disclosed
in any one of the FIGS. 2, 3, 4 and 6.
It will be appreciated that each antenna shown symbolically in
FIGS. 10 to 13 includes its respective network 10.
In the arrangement of FIGS. 12 and 13, the directivity of the array
can be altered by adjusting the reactance/frequency characteristics
of the two terminal reactance networks 10 associated with the
parasitic elements of the arrays.
The method of designing the reactance network 10, as used in the
embodiments described, is first to calculate or measure the value
of reactance required at each frequency. In one example of a
centre-fed whip antenna with a bandwidth of approximately one
octave, the reactance required is inductive at the lowest
frequency. The reactance falls with increasing frequency, becoming
zero, i.e. a short-circuit, at mid-band. At higher frequencies the
reactance becomes capacitive and increases steadily up to the top
frequency. This reactance variation is a mirror-image of the
reactance of a series resonant circuit. Because it has a negative
slope, it cannot be realised by a passive network. It can, however,
be derived from a series-resonant circuit by means of an impedance
converter. One example of impedance converter which can be used is
shown in FIG. 14 and consists of a wide-band amplifier 70 with a
voltage gain of +2, a high input impedance and a low output
impedance. If an impedance Z is connected as a feed-back path
between output and input, the input impedance is-Z.Thus, a suitable
series resonant circuit 72 is connected as the feed-back path, so
that the input impedance of the amplifier has the desired
mirror-image reactance variation. The reactance network produced in
this way may be said to produce the desired wide-band propoerty by
cancelling out the reactance of the antenna. In order to do this it
has to supply the reactive power associated with this reactance.
Hence an active, or power-conversion device is necessary.
It will be appreciated that many modifications may be made to the
embodiments described without departing from the scope of the
invention. As an example, it may be noted that the arrangement of
FIG. 1 may be modified by connecting the reactance network 10 to
the feed point 8, and feeding the antenna between the bottom end of
the element 5 and the ground plane.
Although the specific embodiments described are all whip antennae,
the invention is not limited to whip antennae.
* * * * *