GB2400497A

GB2400497A - Vehicle antenna for dual band mobile communications and AM/FM reception

Info

Publication number: GB2400497A
Application number: GB0307996A
Authority: GB
Inventors: Dedimuni Rusiru Vin Leelaratne
Original assignee: Harada Industry Co Ltd
Current assignee: Harada Industry Co Ltd
Priority date: 2003-04-07
Filing date: 2003-04-07
Publication date: 2004-10-13
Anticipated expiration: 2023-04-07
Also published as: GB0307996D0; GB2400497B

Abstract

A detachable mast antenna for a vehicle is operable in two mobile telecommunications frequency bands (eg 900 MHz, 1.8 GHz GSM), and also receives longer wavelength transmissions (eg FM, AM, DAB). A first choke 30, formed as a helical coil, delineates the extent of the highest resonant frequency (eg 1800 HMz) radiating region of the antenna. Similarly a second choke 32 delineates the extent of the second highest resonant frequency (eg 900 Mhz) radiating region. The overall length of the conductive path, which includes a metal tape 42 helically wound around a glass fibre rod 40, is one half wavelength of the mid-wavelength of the M band. A steel coil spring 36 provides flexibility. The mast can be unscrewed 12 from a mounting (see figure 1d) which contains two PCBs with circuitry (eg amplifier AGC) to process signals.

Description

MULTI-BAND ANTENNA AND CONNECTABLE

COMMUNICATION CIRCUITRY, FOR VEHICULAR APPLICATION

Background Of The Invention

The invention relates to an antenna and to communi- cation circuitry connected to the antenna in an antenna mounting on a vehicle, and more particularly to an antenna and communication circuitry that allow independent tuning of mobile communication frequency bands.

Recent years have witnessed a requirement for vehi- cular antennas that are capable of being utilized with as many frequency bands as possible. There exists a demand for a roof-mounted or fender- mounted antenna capable of being used for reception and transmission of short-wave- length mobile communication signals such as GSM (Global System for Mobile Communications) signals, as well as for reception of longer-wavelength signals such as AM (ampli tude-modulated) and FM (frequency-modulated) signals. In addition to the electrical properties of such antennas, their mechanical aspects need to be taken into account.

For instance, it is necessary that a segment of the antenna be detachable from the vehicle in simple fashion whenever the vehicle enters a car wash facility. And it is also desirable that such detachable segment include a flexure means for allowing an upper part to flex during the washing process when the detachable segment is not detached. For the detachable segment, which normally takes the form of a mast antenna, account also has to be taken of factors such as wind noise and corrosion.

Antenna length normally bears some relation to the frequency of the carrier signal being used for communi cation. With respect to AM and FM communication, the same antenna is normally used for both. In such case, the antenna length is often related to the FM wavelength, i.e. related to a divided portion (i.e. a half, quarter, eighth or even sixteenth) of the wavelength of signals in the mid frequency of the FM band. If such antenna is formed of metal, its length is equal to the divided portion of the wavelength. However, a mast antenna may instead be con- structed from an insulating rod around which a conductive radiating element, such as a length of wire, is helically wound (such mast antennas being called "helical antennas"), and in such case the length of the radiating element is normally equal to a divided portion of the wavelength.

One end of the antenna is directly connectable to an impedance-matched (normally 50-ohm) input of communication circuitry. The same FM antenna may then also be used for receiving AM signals. If an AM antenna were built to be equal to even a small portion of the wavelength of an AM signal, the antenna would be impracticably long. There- fore, means for compensating for the shortness of a FM antenna for reception of AM signals has to be built into the communication circuitry, and this normally takes the form of presenting a high impedance at the AM receiver (typically, anywhere from 100 ohms to 400 ohms, depending on average distance from AM transmitters). AS with the FM antenna input, AM antenna input is normally fed directly into communication circuitry, with no part of the communi cation circuitry being used to augment the length of the antenna.

In contrast, antennas for receiving and transmitting mobile communication signals are often partially defined by patterning on a printed circuit board of the communication circuitry to which the antenna connects. The frequency bands used for GSM communication are normally in the ranges of 900MHz and 1. 8GHz. Since GSM mobile communication uses relatively-shortwavelength (compared to AM/FM) carrier frequencies, and because there is an interest in reducing the size of mobile devices as much as possible to both reduce weight and increase portability, the practice has grown of building a GSM external (i.e. off-the-printed- circuit-board) radiating element of overall length related to a divided portion of the shorter (1. 8GHZ) of the GSM carrier wavelengths. To obtain sufficient antenna length for the 900MHz carrier signal, the external radiating element has been serially connected to a length of internal radiating element that is patterned on the printed circuit board.

This practice of having a portion of the 900MHz radi ating element patterned on the printed circuit board of the communication circuitry has created the problem that it is difficult to independently tune the 900MHz and 1.8GHz GSM bands. Any adjustment of the effective length of the external radiating element affects the effective length of the overall (external-plus-internal) radiating element, and thereby has an effect on the acuity of the 900MHz-band carrier signal. Although GSM communication has been dis- cussed, the same considerations apply to other mobilecommunication frequencies, such as L-Band DAB (1.472GHz), UMTS (2.1GHz), and Bluetooth (2.4GHz). In the disclosure and claims of this document, it is intended for the term "mobile communications" to define all communications that utilize carrier frequencies equal to or higher than approximately 800MHz. Reference has been made to the FM and AM frequency bands as examples of longer- wavelength bands with frequencies below the frequency bands of mobile communications; another example of such longer-wavelength band is DAB ('Digital Audio Broadcasting') at 200MHz.

Likewise, whilst reference may be made to "radiating regions", the term is used for convenience only, and is intended to apply to the active signal receiving or trans- mitting regions of an antenna arranged for reception as well as, or instead of, transmission.

Summary Of The Invention

One object of the preferred embodiment of the subject invention is to provide an antenna which is connectable to communication circuitry and which includes overlapping radiating regions tuned to respective frequency bands used for mobile communications.

Another object of the preferred embodiment of the subject invention is to provide an antenna that is not only capable of being used to receive and transmit on mobile- communication frequency bands, but also is capable of being used to receive signals on lower-frequency bands such as the AM and FM bands.

A further object of the preferred embodiment of the subject invention is to provide an antenna that may be formed as a mast antenna which may be quickly dismantled in whole or part when, for instance, a vehicle to which the mast antenna is connected needs to be cleaned.

In one aspect, the invention is a mast antenna for vehicular use, one end of the antenna being adapted to be connected to communication circuitry. The antenna is configured to provide at least two overlapping radiating regions, each region terminating at the one end of the antenna. First and second such regions are each tuned to a respective different frequency band utilized for mobile communications.

In another aspect, the invention is an antenna assem- bly for vehicular use, including an antenna mounting for attaching the assembly to a vehicle and, at least partially external to the mounting, a mast antenna configured to provide at least two overlapping radiating regions, the lengths of the regions measured from an end of the antenna on or within the mounting and adapted to be connected to communication circuitry being such that each of the regions is tuned to a respective different frequency band utilized for mobile communications.

In a further aspect, the invention is an antenna assembly that includes the aforementioned antenna and a connected antenna mounting adapted to be fitted on a vehicle, or is the aforementioned antenna assembly, wherein the antenna mounting includes and houses the communication circuitry.

Preferably, the antenna is longer than the longer of the first and second radiating regions. More preferably, the length of the first and second radiating regions is determined by the position on the antenna of respective chokes, the positions of the chokes defining second ends of the respective regions. Still more preferably, the tuning of each frequency band is adjusted by varying the number of turns forming the respective choke.

Preferably, an overall length of the antenna is related to a wavelength of signals receivable by the communication circuitry and having a frequency below fre- quencies utilized for mobile communications.

Preferably, each of the radiating regions includes a respective radiating element extending on the antenna in that region, the resonant frequency for that region being determined by the length of the radiating element in that region.

Preferably, a central section of the antenna is formed of a resilientlyflexible member. More preferably, the resiliently-flexible member is a metallic coil spring.

Even more preferably, an inner section and an outer section of the antenna are threadedly-connectable to a respective opposite end of the resiliently-flexible member forming the central section, with the threaded connection allowing for dismantlement of the antenna. Even still more preferably, the first and second radiating regions are partially within the inner section of the antenna.

Preferably, the one end of the antenna is threadedly- connectable to a housing containing the communication circuitry.

Preferably, the communication circuitry includes two printed circuit boards, a first one of the boards being adapted to process mobile communication signals, and a second one of the boards being adapted to process signals having a frequency below frequencies utilized for mobile communications. More preferably, one of the boards has a ground plane extending substantially over all of one side.

Even more preferably, the other board also has a ground plane extending substantially over all of one side.

Preferably, the signals having a frequency below fre quencies utilized for mobile communications are FM signals or DAB signals.

Brief Description Of The Drawings

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures la, lb and lc are overlapping views of res pective inner, central and outer portions of a mast antenna in a preferred embodiment of the invention; Figure ld is a perspective view of an antenna mounting that includes communication circuitry connected to the mast antenna of Figures la, lb and lo; Figure 2a is a graph illustrating the return loss measured against frequency for the mast antenna of Figures la, lb, lc when connected to the communication circuitry of Figure Id; Figure 2b is an amplification of a portion of the graph of Figure 2a, relating to FM frequency reception; Figure 3 illustrates the radiation pattern of the antenna of the preferred embodiment for the GSM 900MHz band, the 1.8GHz band, and for FM vertical and horizontal received signals; Figure 4 illustrates a side profile of an automobile, with the antenna of the preferred embodiment at a typical mounting position on the roof at the rear of the vehicle; Figure 5 illustrates the effect on the two GSM bands of varying the number of turns of the choke positioned at the outer end of the 900MHz radiating region of the antenna of the preferred embodiment; Figure 6 illustrates the effect on the two GSM bands of varying the number of turns of the choke positioned at the outer end of the 1.8GHz radiating region of the antenna of the preferred embodiment; Figure 7 is a perspective sectional view of the antenna of the preferred embodiment; and, Figures 8a, 8b and 8c are photographs of respective inner, central and outer portions of the antenna of the preferred embodiment.

Detailed Description Of The Invention

An antenna assembly of the preferred embodiment of the invention includes the mast antenna of Figures la, lb and lo, as well as communication circuitry included in the antenna mounting of Figure id.

The mast antenna of the preferred embodiment is shown in the three overlapping views of Figures la, lb and lc, and has a threaded brass screw 12 at its one end allowing connection of the mast antenna to a complementary threaded brass socket 14 supported on a plastic cover 16 of the antenna mounting shown in Figure Id. The brass socket 14 has its base connected to a brass contact plate 18 which connects to an input port on each of two printed circuit boards (PCBs) 20 and 22, which boards define communication circuitry. The first PCB 20 is adapted to process GSM mobile communication signals, including necessary filtering for outputting such signals. The second PCB 22 is adapted to process received AM and FM signals, including amplifi cation of such signals. Other necessary circuitry may be present on PCBs 20 and 22, such as Automatic Level Control (ALC) circuitry.

The PCB 20 has a metallic ground plane that extends substantially over all of one of its sides. The PCB 22 has a similar ground plane. If part of a surface of PCB 20 were used as an internal portion of a radiating element for GSM signals received and transmitted through the antenna (as in the prior art), the presence of such ground plane would not be possible since it would interfere with the functioning of the antenna. The PCBs 20 and 22 are fixed into a diecast base 23 which can be shaped to fit a desired position on a vehicle, the plastic cover 16 being fitted to the base 23.

Integral with, and extending from one end of, the brass screw 12 is a hollow cylindrical first screw joint 24 within which is fitted one end of a first fibre glass rod 26. Soldered to an outer end of the screw joint 24 is a length of insulated wire 28. As shown in Figure la, the insulated wire 28 extends along the rod 26 to a position at which a first choke 30 is formed by winding the wire 28 four times around the rod 26 in a tight helical pattern.

The position of that one of the turns of the choke 30 that is closest to brass screw 12, which turn is approximately 4 centimetres from the end of brass screw 12, delineates an outer end of the 1.8GHz resonant-frequency radiating element of the antenna.

The wire 28 then extends further outwardly on the rod 26 for approximately a further 2 centimetres, where a second choke 32 is formed by winding the wire 28 nine-and a-half times around the pole 26 in a tight helical pattern.

The position of that one of the turns of the second choke 32 that is closest to the first choke delineates an outer end of the 900MHz resonant-frequency radiating element of the antenna.

The wire 28 then extends further along the rod 26 to a point where it is soldered to an end of a brass second screw joint 34. The other end of the first rod 26 is fitted into a hollow cylindrical opening in the second screw joint 34. One end of a steel coil spring 36 is threaded into the screw joint 34, and the other end of the spring 36 is threaded into a brass third screw joint 38.

One end of a second fibre glass rod 40 is fitted into a hollow cylindrical opening in the third screw joint 38.

Soldered to an end of the screw joint 38 is one end of a metal tape 42, which is approximately 0.5 millimetres wide and 0.1 millimetres thick and is wound in a loose helical pattern on the second rod 40, as shown in Figure lb. Fitted over an outer end of the second rod 40 is a hollow brass cylinder (extending within the end knob 44 shown in Figure lc, and shown as 46 in the perspective view in Figure 7). An outer end of metal tape 42 is soldered to the brass cylinder 46. The length of the conductive path, i.e. radiating element, that extends to the outer end of the hollow brass cylinder 46, and which includes the length of insulated wire 28 and the metal tape 42, approximates one-half wavelength of the mid-frequency of the FM fre quency range. The overall length of the mast antenna of this embodiment could be anywhere between about 28cm and 44cm, depending on factors such as the length of the coil spring 36 and the helical angle at which the metal tape 42 is wound on second rod 40.

A heat-shrinkable protective plastic sheath (numbered as 48 in Figure 7) covers the outside of the mast antenna over its whole length.

Before the plastic sheath is inserted over and heat- shrunk into place, the tuning of the antenna is adjusted by incrementally adjusting the number of turns of the first choke 30 and of the second choke 32 on the first fibre glass rod 26. Also prior to the plastic sheath 48 being applied, a length of plastic cord 50 is wound in a loose helical pattern over the metal tape 42 that extends heli cally on the second fibre glass rod 40. The plastic cord extends approximately the whole length of the second fibre glass rod 40. The plastic sheath 48 is then inserted over the antenna and is heat-shrunk; the heat-shrinking holds the plastic cord 50 in position. The plastic cord 50 serves a mechanical function of reducing wind noise on the antenna.

The coil spring 36 provides flexibility for the mast antenna in cases where an outer end of the antenna is flexed. For instance, a car wash facility may have brushes which flex the outer end of the mast antenna. The mast antenna may alternatively be totally removed from the vehicle by unscrewing the connection between the screw 12 and the socket 14.

The graphs in Figures 2a and 2b, illustrating return loss versus frequency, were obtained using the embodiment described above (with four turns of the first choke 30 and nine-and-a-half turns of the second choke 32). For tuning of the antenna in other jurisdictions, where mobile communication frequencies differ from 900MHz and 1. 8GHz, the number of turns of chokes 30 and 32 will have different values. Figure 5 illustrates the effect on the two GSM frequency band ranges of varying the number of turns of the second choke 32; the effect on the lower GSM frequency band can be seen to be more pronounced in terms of a frequency shift than on the upper GSM frequency band. As shown in Figure 5, each additional turn of the second choke 32 shifts the centre of the lower GSM frequency band downward by approximately 30 MHz. Similarly, Figure 6 illustrates that increasing the number of turns of the first choke 30 has virtually no effect on the lower GSM frequency band but shifts downward the centre frequency of the upper GSM frequency band.

Besides the number of turns of the chokes 30 and 32, the length of that portion of the wire 28 that extends between the two chokes is a factor that enters into the tuning of the antenna. Ultimately, experimentation is required to obtain optimized results.

In all three bands (the two GSM bands and the FM band), good omnidirectional coverage is obtained. The antenna provides a significant gain across the GSM bands.

The position shown in Figure 4 for mounting the mast antenna on a vehicle is just one possible position. The mast antenna may be mounted at the front of the roof, on a fender or near one of the side mirrors. A position on the roof is most desirable because the whole length of the mast antenna is there most unobstructed. The mast antenna is normally mounted at an angle of anywhere from 45 to 60 degrees to the horizontal.

Figures 8a, 8b and 8c are photographs of respective inner, central and outer portions of a hand-built prototype of the mast antenna of the preferred embodiment.

While the present invention has been described in terms of a preferred embodiment, it is to be understood that the words which have been used are words of descrip- tion rather than limitation, and that changes may be made to the invention without departing from its scope as defined by the appended claims.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The text of the abstract filed herewith is repeated

here as part of the specification.

A multi-band antenna for vehicular use is adapted to provide signal input to communication circuitry and is configured to provide at least two overlapping radiating regions, each of the regions terminating at an end of the antenna connecting to the circuitry. Each of first and second such regions are associated with the wavelength of a respective frequency utilized by the circuitry for mobile communications. The circuitry connected to the antenna does not require any additional antenna section to be formed on internal printer circuit boards, and as a result may have a ground plane formed over substantially all one side of such boards. One part of the antenna may be a mast antenna connectable to a further part of the antenna housed within a mounting adapted to fit on a vehicle. The further part of the antenna connects to the communication circuitry which is also within the housing.

Claims

CLAIMS: 1. A mast antenna for vehicular use, one end of the antenna being

adapted to be connected to communication circuitry, the antenna being configured to provide at least two overlapping radiating regions, each region terminating at the one end of the antenna, first and second such regions each being tuned to a respective different frequency band utilized for mobile communications.
2. An antenna assembly for vehicular use, comprising an antenna mounting for attaching the assembly to a vehicle and, at least partially external to the mounting, a mast antenna configured to provide at least two overlapping radiating regions, the lengths of the regions measured from an end of the antenna on or within the mounting and adapted to be connected to communication circuitry being such that each of the regions is tuned to a respective different frequency band utilized for mobile communications.
3. An antenna assembly comprising the antenna of claim 1 and a connected antenna mounting adapted to fit on a vehicle, or the antenna assembly of claim 2, wherein the antenna mounting includes and houses the communication circuitry.
4. The antenna or antenna assembly of any preceding claim, wherein the antenna is longer than the longer of the first and second radiating regions.
5. The antenna or antenna assembly of any preceding claim, wherein the length of the first and second radiating regions is determined by the position on the antenna of respective chokes, the positions of the chokes defining second ends of the respective regions.
6. The antenna or antenna assembly of claim 4, wherein the tuning of each frequency band is adjusted by varying the number of turns forming the respective choke.
7. The antenna or antenna assembly of any preceding claim, wherein an overall length of the antenna is related to a wavelength of signals receivable by the communication circuitry and having a frequency below frequencies utilized for mobile communications.
8. The antenna or antenna assembly of any preceding claim, wherein each of the radiating regions includes a respective radiating element extending on the antenna in that region, and wherein a resonant frequency for that region is determined by the length of the radiating element in that region.
9. The antenna or antenna assembly of any preceding claim, wherein a central section of the antenna is formed by a resiliently-flexible member.
10. The antenna or antenna assembly of claim 9, wherein the resilientlyflexible member is a metallic coil spring.
11. The antenna or antenna assembly of claim 9 or 10, wherein an inner section and an outer section of the antenna are threadedly-connectable to respective opposite ends of the resiliently-flexible member forming the central section, the threaded connection allowing for dismantlement of the mast antenna.
12. The antenna or antenna assembly of claim 11, wherein the first and second radiating regions are within the inner section of the antenna.
13. The antenna or antenna assembly of any preceding claim, wherein the one end of the antenna is threadedly- connectable to a housing containing the communication circuitry.
14. The antenna assembly of any preceding claim, wherein the communication circuitry comprises two printed circuit boards, a first one of the boards being adapted to process mobile communication signals, and a second one of the boards being adapted to process signals having a frequency below frequencies utilized for mobile communi- cations.
15. The antenna assembly of claim 14, wherein one of the boards has a ground plane extending substantially over all of one side.
16. The antenna assembly of claim 15, wherein the other board also has a ground plane extending substantially over all of one side.
17. The antenna or antenna assembly of any preceding claim, wherein the signals having a frequency below fre- quencies utilized for mobile communications are frequency- modulated (FM) signals or Digital Audio Broadcasting (DAB) signals.
18. A antenna substantially as herein described with reference to and as shown in the accompanying drawings.
19. An antenna assembly substantially as herein des- cribed with reference to and as shown in the accompanying drawings.