GB2201550A

GB2201550A - Antenna with signalling device

Info

Publication number: GB2201550A
Application number: GB08702621A
Authority: GB
Inventors: Barbara Christine Angelou
Original assignee: Antiference Ltd
Current assignee: Antiference Ltd
Priority date: 1987-02-05
Filing date: 1987-02-05
Publication date: 1988-09-01
Also published as: GB8702621D0

Abstract

An antenna arrangement comprises a signalling device, eg a lamp 18, mounted on the antenna and means for superposing a powering current for the signalling device onto a radio frequency signal to be transmitted by the antenna. The arrangement further comprises means, eg. a capacitor 20, for filtering the signal to be transmitted and the powering current so that a substantial portion of the signal to be transmitted does not pass through the signalling device. The superposed signal to be transmitted and powering current are fed from a transmitter 30 via an impedance network 34. The antenna may be a folded monopole 10, 12. <IMAGE>

Description

LOW POWER ANTENNA WITH SIGNALLING DEVICE This invention relates to the provision of a signalling device on an antenna.

It is sometimes necessary, or desirable, to mount a signalling device such as a light source, for example a lamp or an infra red source or an audible warning device, at or near the top of an antenna. For example, if an antenna exceeds a certain height the owner is obliged to ensure that the antenna is viable at all times especially to pilots of low flying aireraft.

Such antennas are commonly high powered devices transmitting signals over large distances.

It has been proposed that it would be desirable to have a light source mounted on the antenna of a smaller transmitter. For example, a lamp mounted on a portable transmitter would prove invaluable as a distress beacon should the user find himself in difficulty. This has applications in both land and sea rescue where the rescue services could be aided in finding an injured party or a distressed vessel at short range once radio tracking is no longer effective in precisely locating the injured party or the distressed vessel.

A problem arises when trying to supply electrical power to a light source, or indeed any other electrically powered signalling device since a feed and a return path will be required to supply power to the lamp and any conducting elements in the electromagnetic field radiated by the antenna will distort the radiated field with undesirable and sometimes unpredictable consequences. This problem is overcome when the power in the Radio Frequency (RF) signal is substantially larger than the power required to drive the signalling device by using the RF signal to drive both the antenna and the signalling device . However, the problem still remains if the antenna is supplied with a low power RF signal.

The invention includes a method of powering a signalling device mounted on an antenna comprising the step of superposing a powering signal onto a current for the signalling device being transmitted by the antenna.

Preferably the method further comprises the step of inputting the superposed current and signal at one end of the antenna.

Preferably the method further comprises the step of filtering the signal being transmitted and the powering current so that a substantial portion of the signal being transmitted does not pass through the signalling device.

The powering current may be constant or it may be pulsed.

The invention also provides an antenna arrangement comprising: a signalling device mounted on the antenna, and means for superposing a powering current for powering the signalling device onto a radio frequency signal to be transmitted by the antenna.

In d preferred embodiment the arrangement further includes filter means for filtering the signal being transmitted and the powering current so that a substantial portion of the signal being transmitted does not pass -through the signalling device.

Preferably the antenna is a folded monopole.

In the preferred embodiment the folded monopole comprises a first and a second pillar mounted substantially perpendicular to a ground plane and substantially parallel to each other.

Preferably the signalling device and the filter means are connected across the free ends of the first and the second pillar.

In the preferred embodiment the end of the first pillar nearest the ground is insulated from the ground plane and the end of the second pillar nearest the ground plane is electrically connected to the ground plane.

Preferably the arrangement includes means for inputting the signal to be transmitted and the powering current at the electrically isolated end of the first pillar.

Preferably the input means includes an impedance network for tuning the antenna to the frequency of the signal to be transmitted.

In the preferred embodiment the filter means comprises a capacitor and the light source comprises a lamp.

In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawing which shows a schematic view of an antenna arrangement.

Referring now to the drawing; an antenna arrangement generally indicated at 8, comprises a first pillar 10 and a second pillar 12. The pillars 10, 12 are constructed from a conducting material, for example brass or copper. The pillars 10, 12 may be of tubular form of differing or substantially similar widths and are substantially parallel to each other and substantially perpendicular to a ground plane 22. The pillars are positioned in spaced relation as described in more detail herein below. A lamp 18 and a capacitor 20 are connected in parallel across the free ends 14, 16 of the two pillars 10, 12. The lamp 18 is typically a low voltage tungsten filament type, having low resistance and inductance.The capacitor 20 is of a capacity suitable to provide substantially a shortcircuited path across the free ends 14, 16 of the pillars to a radio frequency (RF) signal and an open circuit across the free ends 14,16 of the pillars to a DC signal. Thus, the RF signal is directed to act only on the pillars 10,12 which form the radiating elements of the antenna and the DC signal provides power to drive the lamp 18. Since the capacitor 20 acts as a short circuit to the RF signal, the two pillars are connected together electrically to form a monopole type antenna.

The fixed end 24, of the first pillar 10, is insulated from the ground plane 22 by an insulating member 28 which, in this embodiment, is made from nylon. The fixed end 26 of the second pillar 12 is secured to the ground plane 22 completing the electrical path and thus providing a return path for a signal applied to the antenna 8.

A transmitter 30 generates an RF signal to be transmitted. The transmitter 30 also contains means, not shown, for producing a powering current to power the lamp 18 and for superposing the powering current onto the signal to be transmitted. The superposed current and signal are fed from the transmitter 30 along a cable 32, which is preferably a co-axial type cable, to be input at the fixed end 24 of the pillar 10 via an impedance network 34. The impedance network 34 may be omitted if only an RF signal at a single frequency is transmitted and the information carried by the RF signal occupies a narrow frequency band about the RF frequency.

Changes in the electrical "length" or impedance of the antenna are caused by variations in the mounting position of the pillars 10,12 both in relation to each other and in relation to other influencing objects as well as by production tolerances, so the impedance network 34 is included to provide an easy way of adjusting the electrical "length" of the antenna so that it corresponds to the wavelength of the RF signal or a multiple, or a fraction thereof. The impedance network 34 may also be arranged to present a resistive impedance at its input, as seen from the co-axial cable 32, when an RF signal is applied at the transmission frequency.The impedance network 34 may also be adapted to match the impedance of the pillars 10,12 which form the- transmitting element of the antenna arrangement, with the impedance of the co-axial line 32. The impedance network 34 is arranged to be resonant at desired radio frequencies thus ensuring maximum power transfer between the co-axial cable 32 and the pillars 10,12 during transmission.

The impedance network 34, comprises at least a parallel tuned LC circuit designed to resonate at a predetermined radio frequency. Thus, if the antenna is to transmit, or receive, at a number of frequencies the network is provided with a further number of resonant circuits to compensate for changes in the reactance of the antenna caused by alterations to the transmission frequency. In the event that a plurality of RF signals are to be transmitted at different frequencies the network may be adaptedto present a resistive impedance at. each of the plurality of RF signal transmission frequencies. This facility is of- particular importance where, for example, three RF signals are to be transmitted, and the highest RF signal frequency is three times greater than the lowest RF signal frequency, since the change in the reactance and other characteristics of the antenna over this range of frequencies will be substantial. The network is also arranged to pass the powering signal, which may be either a constant voltage level or a pulsed voltage, from the co-axial cable 32 to the lamp 18 mounted on the pillars 10,12. The RF signal is radiated from the antenna element 8 in a manner that is well known to those skilled in the art. The transmission and reception characteristics of the antenna are determined by, inter-alia, the diameter of the vertical pillars 10, 12, the relative spacing of the pillars, and the length of the pillars in relation to the wavelength of the signal transmitted.For example, the ratio of the currents in each of the vertical pillars constituting the antenna element is directly proportional to the respective diameters of the pillars. The density of the field radiated by each pillar is also proportional to the current in each pillar. Thus, the pillars may be arranged to contribute equally to the overall radiation pattern of the antenna element by making the pillars of substantially equal diameters.

The capacitor 20 is optionally provided across the terminals of the lamp 18 to minimise the effects of the impedance of the lamp 18. Ideally, the lamp and capacitor should co-operate to produce a short circuit across the free ends 14, 16 of the pillars when a signal at the desired radio frequency is input to the antenna. In practice, the choice of capacitor will involve a trade-off between the physical size of the capacitor 20 and the value of capacitance, which should generally be as large as possible.

The powering current which provides power to the lamp is supplied to the lamp along the same path as the signal to be transmitted, that is along the co-axial cable 32 through the impedance network 34 to the fixed end of the first pillar 10. The powering current may be in the form of a constant DC current or a pulsed DC current. In the case where the powering current is pulsed, the frequency and duration of the pulses may be adjusted to a low frequency where the lamp is seen to flash on and off. Alternatively the frequency may be increased so that the lamp appears to be continuously on. Typically the resistance in the lamp will be of the order of 10 ohms, the voltage required to drive the lamp may be as low as 2 to 3 volts and the power required to drive the -lamp will be approximately 500mw.

A low power transmitter will typically transmit at less than 5 watts.

Commonly, such low power transmitter devices are provided in a single case with the antenna mounted on the case and the ground plane being provided either by the body of the case or by a suitably adapted printed circuit board. Of course, the antenna may be sited at a remote location in which case the ground plane may be provided by any of the methods which are well known.

Alternatively the antenna may be incorporated into the printed circuit bo-ard as may the impedance network, the lamp or other signalling device, and the capacitor acting as a filler.

It will be obvious to those skilled in the art that many modifications of the described embodiment are possible. The principle of superposing a power signal onto the transmission signal may be applied to many types of antenna other than the exemplary monopole described hereinabove. For example, the antenna may be a folded dipole adapted to accommodate the signalling device.

Similarly the described embodiment may be adapted for use as a "loaded" antenna or a "trapped" antenna in which at least a parallel resonant inductor-capacitor circuit is inserted into the antenna to divide the antenna into portions proportional to fractions or multiples of the wavelength of the signals to be transmitted. In this way the resonant circuit or circuits can be used to present an open-circuit to a plurality of RF signals at different frequencies, effectively altering the electrical "length" of the antenna as is well known to those skilled in the art.

The antenna arrangement may be adapted in many ways which are well known -to increase the gain of the antenna in a predetermined direction. For example, the antenna may be provided with a plurality of passive elements positioned in a predetermined spaced relation to the antenna to form a Yagi antenna. Similarly, the antenna arrangement may be provided with additional intersecting conducting planes in a predetermined angular relationship to each other and a predetermined spaced relationship to the antenna to form a corner reflector with desired predetermined characteristics.

Moreover a plurality of antenna arrangements may be combined to form, for example, a multi-element broadside array with predetermined directional characteristics.

In a transceiver the antenna acts as both the transmitting element and the receiving element. The embodiment is also not limited to use only when the antenna is transmitting a signal. The power carrying signal may still be delivered to the lamp when the antenna is acting as a receiving element should this prove advantageous to the user.

Claims

CLAIMS:

1. A method of powering a signalling device mounted on an antenna comprising the step of superposing a powering current for the signalling device onto a signal being transmitted by the antenna.

2. A method according to claim 1 further comprising the step of inputting the superposed current and signal at one end of the antenna.

3. A method according to claim 1 or 2 further comprising the step of filtering the signal being transmitted and the powering current so that a substantial portion of the signal being transmitted does not pass through the signalling device.

4. A method according to any preceding claim in which the powering current is constant.

5. A method according to any preceding claim in which the powering current is pulsed.

6. A method according to any preceding claim in which the antenna is a low power type.

7. An antenna arrangement comprising: a signalling device mounted on the antenna, and means for superposing a powering current for powering the signalling device onto a radio frequency signal to be transmitted by the antenna.

8. An arrangement according to claim 7 which further includes filter means for filtering the signal to be transmitted and the powering current so that a substantial portion of the signal to be transmitted does not pass through the light source.

9. An arrangement according to claim 7 or 8 in which the antenna is a folded monopole.

10. An arrangement according to claim 9 in which the folded monopole comprises a first and a second pillar mounted substantially perpendicular to a ground plane and substantially parallel to each other.

11. An arrangement according to claim 10 in which the signalling device and the filter means are connected across the free ends of the first and the second pillar.

12. An arrangement according to any of claims 9 to 11 in which the end of the first pillar nearest the ground plane is insulated from the ground plane.

13. An arrangement according to any of claims 9 to 12 in which the end of the second pillar nearest the ground plane is electrically connected to the ground plane.

14. An arrangement according to claim 12 or 13 further including input means for inputting the signal to be transmitted and the powering current at the electrically isolated end of the first pillar.

15. An arrangement according to any of claims 7 to 14 in which the signal to be transmitted and the powering signal are at low power levels.

16. An arrangement according to claim 14 or 15 in which the input means includes an impedance network for tuning the antenna to the -frequency of the signal to be transmitted.

17. An arrangement acc-ording to any of claims 8 to 16 in which the filter means comprises a capacitor.

18. An arrangement according to any of claims 7 to 17 in which the signalling device comprises a lamp.

19. An arrangement according to any of claims 7 to 17 in which the signalling device comprises an infra red source.

20. An arrangement according to any of claims 7 to 17 in which the signalling device comprises an audible beacon.

21. An arrangement according to any of claims 7 to 20 in which the ground plane is provided by a printed circuit board.

22. An arrangement according to claim 21 in which the impedance network is provided on the printed circuit board.

23. An arrangement according to claim 21 or claim 22 in which the antenna is provided on the printed circuit board.

24. An array antenna comprising a plurality of transmitting elements in which at least one of the transmitting elements is arranged in accordance with any of claims 7 to 19.

25. An antenna . arrangement comprising at least an antenna arrangement in accordance with any of claims 7 to 23 and a plurality of passive RF electromagnetic wave reflecting or directing elements.

26. An antenna arrangement substantially as herein described with reference to the accompanying drawing.

27. A method using an antenna arrangement substantially as herein described with reference to the accompanying drawings.