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Title:
IMPROVEMENTS TO HF WHIP ANTENNA
Document Type and Number:
WIPO Patent Application WO/2003/096481
Kind Code:
A1
Abstract:
An HF whip antenna (50) is disclosed which comprises an elongate whip member (52), a main winding (58) provided in connection with the whip member (52) and connectable to a HF radio transmitter, the main winding (58) having an associated fundamental resonant frequency, and an internal assembly (70) provided within the whip member (52). The internal assembly (70) is arranged to capacitively couple to the main winding (58) of the antenna (50) such that the resonant response of the antenna (50) is modified and the load presented to the transmitter is substantially resistive over a broad resonant frequency range.

Inventors:
Ventoura, Eleftherios John (9 Kirkstone Place, Balga, Western Australia 6061, AU)
Jobse, Jasper Aart (110 Edward Street, Osborne Park, Western Australia 6017, AU)
Application Number:
PCT/AU2003/000546
Publication Date:
November 20, 2003
Filing Date:
May 08, 2003
Export Citation:
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Assignee:
Ventoura, Eleftherios John (9 Kirkstone Place, Balga, Western Australia 6061, AU)
Jobse, Jasper Aart (110 Edward Street, Osborne Park, Western Australia 6017, AU)
International Classes:
H01Q1/36; H01Q5/00; (IPC1-7): H01Q23/00; H04Q9/06
Domestic Patent References:
WO2000055940A12000-09-21
Foreign References:
GB2086662A1982-05-12
US5969684A1999-10-19
Attorney, Agent or Firm:
Starkie, Steven John (Griffith Hack Patent Attorneys, 256 Adelaide Terrace Perth, Western Australia 6000, AU)
Download PDF:
Claims:
THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:
1. An HF whip antenna comprising: an elongate whip member; a main winding provided in connection with the whip member and connectable to a HF radio transmitter, the main winding having an associated fundamental resonant frequency; and an internal assembly provided within the whip member and arranged to capacitively couple to the main winding of the antenna such that the resonant response of the antenna is modified and the load presented to the transmitter is substantially resistive over a second broad frequency range.
2. An antenna as claimed in claim 1, further comprising an inductive element disposed on the whip member at a predetermined location on the whip member for detuning or decoupling a resonant frequency of the antenna.
3. An antenna as claimed in claim 2, wherein a plurality of inductive elements are provided at spaced locations on the whip member for dutuning or decoupling a plurality of resonant frequencies.
4. An antenna as claimed in any one of claims 1 to 3, wherein said internal assembly comprises a pair of elongate capacitive elements extending at least partially along the length of the whip member.
5. An antenna as claimed in any one of claims 1 to 4, wherein said internal assembly further comprises an electrically conductive damping element extending at least partially along the length of the whip member.
6. An antenna as claimed in claim 5, wherein the whip member is of hollow construction and the antenna further includes a dielectric material spacer disposed around said damping element, said spacer and said damping element being housed within the hollow interior of the whip member.
7. An antenna as claimed in claim 5 or 6, wherein said damping element is substantially connected to ground.
8. An antenna as claimed in any one of claims 5 to 7 when dependant on claim 4, wherein said pair of capacitive elements are disposed on substantially diametrically opposite sides of the damping element of said pair.
9. An antenna as claimed in any one of claims 5 to 8, wherein said damping element is substantially centrally located relative to the whip member.
10. An antenna as claimed in claim 4, wherein a first capacitive element of said pair is of a different length to a second capacitive element of said pair.
Description:
IMPROVEMENTS TO HF WHIP ANTENNA FIELD OF THE INVENTION The present invention relates to certain improvements to high frequency (HF) whip antennas and relates particularly, although not exclusively, to an improved HF mobile helical whip antenna having a frequency spectrum of 1.8 to 26 MHz inclusive.

BACKGROUND OF THE INVENTION A number of improvements have been made to portable or mobile HF antennas compared to the models that were available 30 years ago. For example, modern HF mobile antennas are typically compact and relatively lightweight. However, due to some fundamental laws of physics, the length (or height) of an HF antenna must remain relatively large in order to achieve reasonable transmission efficiency. Previously multiple antenna elements were screwed on to a common antenna base to give multi- frequency performance. However these have generally been replaced with a single pole (whip) which has a tapped helical winding to enable a variety of frequencies to be selected with the use of a wander lead (a short piece of cable with plugs at each end).

Alternatively, the antenna may be fed from a transmitter via a tuner device as in shipboard installations. These prior art devices are either manually or electrically controlled so as to tune the electrical length of the antenna winding artificially. More recently, several manufacturers have incorporated the tuning unit in the base of the antenna. This includes a mechanical device driven by a small geared electric motor which rotates a coil to move a contact along the coil to the appropriate position where the system is tuned, i. e. has the lowest standing wave ratio (SWR) achievable.

There are a number of disadvantages with these prior art HF antenna systems. In cases where manual adjustment is required, tuning the device can be extremely inconvenient as it is generally necessary to stop the vehicle in order to make the necessary adjustments when changing frequency. With the automatic mechanical system, there are problems with reliability due to the number of moving parts. With both types of mechanical tuning methods a substantial energy loss is evident at lower frequencies, particularly if a very broad frequency spectrum is intended to be covered. In some

cases, prior art antennas which claim broad band operation have been found to have very poor radiating efficiency with some frequencies down to less than 5% radiated power.

The present invention was developed with a view to providing an improved HF whip antenna that provides broadband frequency spectrum operation without the need for manual or mechanical adjustment.

For the purposes of this specification it will be clearly understood that the word "comprising"means"including but not limited to", and that the word"comprises"has a corresponding meaning.

SUMMARY OF THE PRESENT INVENTION According to the present invention there is provided an improved HF helical whip antenna comprising: an elongate whip member; a main winding provided in connection with the whip member and connectable to a HF radio transmitter, the main winding having an associated fundamental resonant frequency; and an internal assembly provided within the whip member and arranged to capacitively couple to the main winding of the antenna such that the resonant response of the antenna is modified and the load presented to the transmitter is substantially resistive over a broad frequency range.

Preferably, the antenna further comprises an inductive element disposed on the whip member at a predetermined location on the whip member for detuning or decoupling a resonant frequency of the antenna. Advantageously, a plurality of inductive elements are provided at spaced locations on the whip member for detuning or decoupling a plurality of resonant frequencies.

Typically, said internal assembly comprises a pair of elongate capacitive elements

extending at least partially along the length of the whip member. Preferably, said internal assembly further comprises an electrically conductive damping element extending at least partially along the length of the whip member. The electrically conductive element may be centrally located relative to the whip member.

Advantageously, the whip member is of hollow construction and the antenna further includes a dielectric material spacer disposed around said damping element, said spacer and said damping element being housed within the hollow interior of the whip member.

Preferably, said damping element is substantially connected to ground. Preferably, said pair of capacitive elements are disposed on diametrically opposite sides of the damping element on an outer surface of the spacer. Typically, a first capacitive element of said pair is of a different length to a second capacitive element of said pair.

BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a more comprehensive understanding of the nature of the invention a preferred embodiment of the HF helical whip antenna in accordance with the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figures 1 and 2 illustrate prior art mobile helical whip antennas; Figure 3 illustrates a prior art variable frequency HF antenna; Figure 4 illustrates a HF whip antenna in partial cross-section in accordance with an embodiment of the present invention; Figure 5 illustrates a cross-sectional view of part of the whip antenna shown in Figure 4 with an external decoupling inductor; and Figure 6 illustrates schematically an electrical equivalent circuit of a main winding and some of the components of the antenna shown in Figures 4 and 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates a prior art helical whip antenna having a single operating frequency.

The prior art antenna comprises a metal base 10 which may be fixed to a portion of a vehicle, for example to the bumper bar of a motor vehicle. Connected to the base 10 is a hollow fibreglass tapered rod or whip 12 on which is wound a main antenna winding 14. The main winding 14 is typically made of copper wire which may be insulated or bare. The winding may be evenly spaced or concentrated near the top of the whip 12.

An anti-corona tip 16 made of brass or aluminium is provided at the top end of the whip 12. By winding the antenna wire 14 around the whip 12 an antenna of sufficient length for HF operation can be deployed in a relatively compact configuration. However, because of the fixed length of the winding 14, this type of antenna has only a single operating frequency. Sometimes the whip 12 is interchangeable on the base 10 with one or more alternate whips with windings of different lengths to provide multi-channel operation. A coaxial cable connector 18 in the base 10 is used to connect the main winding 14 to an HF radio transmitter.

Figure 2 illustrates a further prior art HF helical whip antenna capable of multi- frequency operation. This prior art antenna is similar to that illustrated in Figure 1 and therefore like parts are identified with like reference numerals. With the antenna shown in Figure 2, the winding 20 is not evenly spaced along the length of the whip 12 but is concentrated in selected regions separated by frequency taps 22. A flexible insulated "wander"cable 24 is provided, having connectors 26 at each end, to provide an electrical connection between the frequency taps 22. The wander cable 24 enables a variety of frequencies to be selected by effectively short-circuiting various lengths of the main winding 20.

Figure 3 illustrates a further type of prior art HF mobile antenna which is also capable of multi-frequency operation but does not require manual adjustment. The prior art antenna shown in Figure 3 has a radiator 28 which may consist of a stainless steel solid rod or may be a wound fibreglass whip as in the antennas of Figures 1 and 2. In either

case, the radiator 28 is tuned to the highest operating frequency of interest. In a base 30 of the antenna, a rotatable drum 32 formed of insulating material is wound with a copper wire winding 34, the rotatable drum 32 being usable for tuning the antenna to different operating frequencies. A sliding connector 36 is used to connect one end of the winding 34 to the transmitter via coaxial cable connector 38. The sliding contact 40 is used to connect the winding 34 to the radiator 28. The sliding contact 40 automatically moves up or down the winding 34 depending on the direction of rotation of the drum 32. The drum 32 is rotated by a small DC motor 42 via a gearbox 44 also provided in the base 30 of the antenna. The motor 42 must be connected to an external power supply and controller (not shown) via an electrical connector 46.

As noted above, there are several problems with these prior art HF mobile antennas, including the number of moving parts and/or the need for manual adjustment. Also, with the type of antenna shown in Figure 3, there are substantial losses in transmission efficiency at certain frequencies due to the methods of tuning the antenna to different operating frequencies.

Figures 4,5 and 6 illustrate a preferred embodiment of a HF mobile whip antenna 50 in accordance with the present invention. The present invention seeks to address the problems caused by resonance which is a characteristic of all antennas. Without wishing to be bound by theory, it is generally well understood that an antenna can be described as a resonant circuit. All circuits which resonate have a Q (quality factor) which is a measure of the selectivity of the circuit. At resonance, an ideal antenna exhibits a purely resistive impedance. At frequencies above and below the resonant frequency, there will normally be a reactive component (either capacitive or inductive) in the antenna impedance, which introduces a phase shift in the relationship of the voltage and current. This condition causes additional power to be drawn from the transmitter to overcome the reflected current, with the result that the effective power of the transmitter is reduced. If the current reflected is equal to the drive current there is no transmitted signal. It is therefore desirable that the antenna be coupled to the transmitter and driven by frequencies substantially corresponding to the resonant frequency.

However, if the antenna impedance could be made to be almost purely resistive over a

wide range of frequencies, then it should be possible to shift the frequency of the transmitter over a broadband of the frequency spectrum without having to tune the antenna. It is thought that if the resonant response of the antenna could be modified so as to increase the range of resonant frequencies, then the Q would be low and the load presented to the transmitter would be substantially resistive over a larger frequency range than with whip antennas known hitherto. In the present invention, this is achieved in the first instance by employing an internal capacitance coupled to the main winding of the antenna.

As shown in Figures 4 and 5, the preferred embodiment of the antenna 50 includes an elongate whip member 52 having a base 54 shown in partial cross-section which is adapted to be mounted on a vehicle or other apparatus. The whip member 52 in this example is a hollow fibre glass whip former and carries a main winding 58 of insulated copper wire wound around an external surface of the whip member 52 over substantially the whole length of the whip member 52. The base 54 of the antenna comprises a metal base member 60 of generally cylindrical configuration connected to the whip member by means of a supporting conical top section 62. The top section 62 is constructed of insulating material sufficiently strong to securely hold and support the whip member 52.

A threaded rod 64 is used to mount the base 54, for example on a motor vehicle. A coaxial cable connector 66 provided on a lower side of the base cylinder 60 is used to connect the antenna 50 to a HF radio transmitter (not shown).

As shown more particularly in Figure 5, an internal assembly 70 is provided within the whip member 52 and is arranged to capacitively couple to the main winding 58 of the antenna 50. In this embodiment, the internal assembly 70 comprises a pair of elongate capacitive elements 72 and 74 which extend internally along the length of the whip member 52. The internal assembly 70 also includes damping element 76 located centrally within the hollow interior of the whip member 52 and around which is disposed a dielectric material spacer sleeve 78. The spacer sleeve 78 isolates and spaces the damping element 76 from the first and second capacitive elements 72 and 74. The damping element 76 is connected substantially to ground. The capacitive elements 72, 74 are attached to the spacer sleeve 78 on diametrically opposite sides of the damping

element 76. The capacitive elements, 72,74 and the damping element 76 are typically made from copper wire.

The capacitive elements 72,74 interact with the main winding 58 to reduce the Q of the antenna. The damping element 76 functions as an absorber so as to further reduce the Q of the antenna by reducing the inductance of the main winding 58. A certain amount of energy is absorbed by the damping element 76. However, the addition of the two radiating capacitive elements 72 and 74 limits this effect. Additional capacitive elements may be employed if required to improve the efficiency of the antenna. As shown in Figure 5, the lengths of the capacitive elements 72 and 74 are not the same, but their length can be shown to be related to the physical length of the main winding 58 and dependent on the lowest frequency employed. They are driven elements but 180° out of phase with the main winding 58 and are dependent on the lowest frequency employed. They are also capacitively coupled to the main winding 58 due to their close proximity inside the whip member 52.

Extensive testing of a prototype antenna revealed that problems could arise with the internal assembly 70 at some frequencies. These problems were overcome by the addition of respective tuning inductors 80 and 82 located in the base 54 of the antenna and connected in series with the first and second capacitive elements 72 and 74 respectively. The tuning inductors 80,82 limit the effective current passing to the capacitive elements 72 and 74 at higher frequencies, and some capacitance drive may be added to increase their effectiveness at low frequencies by choosing appropriate inductor cores.

Figure 6 illustrates an electrical equivalent circuit of the main winding 58 and internal components of the antenna 50. A toroidal balun transformer 84 is provided to act as an impedance transformer between the coaxial connector 66 and the main antenna winding 58. The Balun transformer 84 also acts as an impedance transformer between the coaxial connector 66 and the capacitive elements 72 and 74. For this purpose, the balun transformer 84 has a first secondary winding 86 connected to the main winding 58, and a second secondary winding 88 connected in series. The second secondary winding 88

supplies 180° out of phase energy to the first and second capacitive elements 72 and 74 via the tuning inductors 80 and 82. The damping element 76 is connected to a common ground terminal 90, optionally via an inductor 83, as are the coaxial connector 66, the first and second primary windings 92 and 94 respectively of the balun transformer 84 and the first and second secondary windings 86,88 of the balun transformer 84.

Since whip sections may vary in thickness, it is envisaged that the tuning inductors 80 and 82, which limit the current supplied to the capacitive elements 72 and 74, will need to be custom made in order to match the spacing of the capacitive elements from the main winding 58 on the external surface of the whip member 52. Other spacing methods may be adopted to obtain the best consistency in manufacture. As the capacitive elements 72 and 74 are in close proximity to the central damping conductor 76, there are some losses, but controlling the separation with the dielectric spacer sleeve 78 has proved to be satisfactory at most frequencies. The losses of the antenna system 50 have proved to be comparable or less than those of the base tuning variety of prior art antenna.

As can be seen most clearly in Figures 4 and 5, the whip member 52 is also provided with three series connected external decoupling inductors 96,98 and 100 disposed at spaced locations along the whip member 52. Each of the decoupling inductors is in the form of an inductor wire 102 wound around a toroidal core 104, although it will be understood that any other suitable inductor configuration may be used.

The illustrated embodiment of the antenna 50 is a full spectrum antenna (1.8 to 30 MHz), however it is envisaged that models of narrower bandwidth (e. g. 2.0 to 16 MHz range) would not require as many decoupling inductors for this purpose.

In the illustrated embodiment, the whip member 52 has an overall length of approximately 1.8 metres, although the electrical length of the main winding 58 determines the locations of the decoupling inductors on the whip member 52. In the event that a shorter or longer whip member 52 is employed, the decoupling inductors would be positioned differently.

At the top of the whip member 52, a tip 106 of solid brass (or other metal) approximately 20 mm in length is added to prevent (or minimise) the effect of corona discharge when used with high powered transmitters.

Now that a preferred embodiment of the HF mobile whip antenna has been described in detail, it will be apparent that it has a number of advantages over the prior art HF antennas, including the following: (a) it is capable of performing across substantially the full HF spectrum (of the order of 1.8-30 Mhz) without requiring manual adjustment; (b) there are no moving parts and hence improved reliability; (c) the SWR (standing wave ratio or reflected energy) is maintained to an acceptable level throughout the HF band and the transmitted energy is also radiated efficiently; (d) the same design principles can be applied to HF antennas with a narrower bandwidth; (e) the antenna is usable with rapid frequency hopping equipment as there is no time delay due to the need to wait for frequency taps to be changed or for the mechanical settings to occur automatically.

Numerous variations and modifications will suggest themselves to persons skilled in the radio and communications arts, in addition to those already described, without departing from the basic inventive concepts. For example, the internal capacitance provided within the whip member of the antenna may take a different form from the conductive wires illustrated in the described embodiment. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description.