WO2007123540A1

WO2007123540A1 - Retractable slant monopoles antenna for uhf-vhf bands with narrow bands tuning switches

Info

Publication number: WO2007123540A1
Application number: PCT/US2006/015543
Authority: WO
Inventors: Thomas John Birnbaum; Eric Andrew Dorsey
Original assignee: Thomson Licensing
Priority date: 2006-04-25
Filing date: 2006-04-25
Publication date: 2007-11-01

Abstract

A user deployable slant monopole antenna element (16) with multiple tuning stages (20, 30) comprising each an RF switch for a tuning inductor. This achieves good performance across both the 170-225 MHz and 470-850 MHz band widths. The slant monopole antenna responds to both V and H polarizations, in addition to providing both spatial and radiation pattern diversity in a multi-path environment. Through this design, a low profile antenna can be provided that is 'electrically small' (i.e. below about 650Mhz) yet provides good performance with a minimum gain of approximately -6dBi.

Description

RETRACTABLE SLANT MONOPOLES ANTENNA FOR UHF-VHF BANDS WITH NARROW

BANDS TUNING SWITCHES

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to antennae. More particularly, it relates to to a retractable slant monopole UHF antenna with Multiple Narrow Band Tuning capability. 2. Description of the prior art Typically, wide-band VHF/UHF antennae on handheld devices come in one of two sizes, large and small. An example of the large antenna would be a telescoping whip antenna. The primary problem with the large antennas is obvious, i.e., they tend to be drastically larger than the size of the handheld device to which they are connected, thus making use of the same cumbersome for the user. The small antennas available for hand held devices require very complex tuning circuits with multiple bands, thus increasing the amount of electronics, and in particular, the size requirements for the same, thus affecting the overall size of the hand held device. In addition, these complex tuning circuits increase the cost of manufacturing the hand held device. Integrating a small antenna with good performance in both the high

VHF (170 - 225 MHz) and UHF frequency bands in a hand-held device presents numerous problems of tuning and antenna efficiency. Small (electrical) size, good performance, and wide bandwidth are difficult to achieve together. Typically, 2 out of these 3 attributes are optimized at the expense of the 3rd.

i SUMMARY OF THE INVENTION

The present invention overcomes the shortfalls of the prior art by optimizing all three attributes simultaneously thru clever selection of antenna type (slant monopole) and a unique "inductor-only" tuning circuit that is tailored specifically to the antenna and the chosen frequency range.

This and other aspects are achieved in accordance with an embodiment of the invention where the antenna for portable devices includes two oppositely arranged antenna elements, and an inductor only tuning circuit within the portable device and electrically connected to the two antenna elements. The tuning circuit has multiple stages for providing maximum reception performance in a UHF bandwidth of 470-850 MHz.

According to one embodiment, the antenna elements are slant monopole elements having a predetermined length and predetermined angular position with respect to the portable device. The predetermined length may be 70mm, and is preferably less than 0.15 wavelength. The angles of the antenna elements are preferably acute angles with respect to the portable device.

According to another aspect of the invention, the tuning circuit includes a tuning inductor for each of said multiple stage, and an RF switch for each of said tuning inductors. The tuning inductors are connected to an RF signal path in the tuning circuit, and each RF switch enables selective bypassing or switching in of its corresponding tuning inductor to tune the antenna to a chosen band of operation.

According to another aspect of the invention, the antenna for portable devices includes two oppositely arranged slant monopole antenna elements, and an inductor only tuning circuit within the portable device and electrically connected to the two antenna elements for providing maximum reception performance in both a VHF bandwidth of 170-225 MHz and a UHF bandwidth of 470-850 MHz.

The tuning circuit includes a tuning inductor for each four separate bands, and an RF switch for each of said tuning inductors. The tuning inductors are connected to an RF signal path in the tuning circuit, and each RF switch enables selective bypassing or switching in of its corresponding tuning inductor to tune the antenna for maximum performance in the chosen band of operation.

According to other aspects, the tuning circuit includes a tuning inductor for each of four separate bands, where the tuning inductors are connected to an RF signal path in the tuning circuit. A first RF switch having one input and two outputs. The two outputs connected to two of the tuning inductors. A second RF switch having one input and two outputs. The two inputs connected to another two of the tuning inductors, and a third RF switch having one input connected to the antenna elements and two outputs. One of the outputs is connected to an input of the first RF switch, and the other of the outputs connected to the input of the second RF switch.

In different embodiments, the RF switches are single pole, single throw switches, or may be single pole, double throw switches. •

The present invention represents the optimum combination of antenna type, tuning circuit topology, and location in a hand held or portable product. Other aspects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings wherein like reference numerals denote similar components throughout the views:

Figure 1 is plan view of a hand held device incorporating the antenna system of the present invention;

Figure 2 is a block diagram of the tuning circuit according to an embodiment of the invention; and

Figure 3 is a block diagram of the tuning circuit according to another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 shows a plan view of the antenna system 10 according to an embodiment of the present invention. A hand held device 12 has a display screen 13 and two antennas 14 and 16. Antennas 14 and 16 are user deployable and are each preferably slant monopole antenna elements. Each antenna element 14 and 16 has a length L and is mounted at an angle Ai, A₂ with respect to the mobile hand held device. It is preferred that the antenna elements 14 and 16 are mounted in opposing directions with respect to each other, thus requiring the angles Ai and A₂ to be acute angles. The selection of length L and the angles Ai and A₂, in conjunction with the inductor only tuning circuit of the invention, allows the antenna system 10 to achieve good performance across both the 170 - 225 MHz and 470 - 850 MHz bandwidth.

The antenna system 10 has a relatively low profile, especially at the low end of the frequency band where typical VHF/UHF TV antennas are several times larger. The antenna system 10 can also be considered "electrically small" below about 650 MHz and yet gain performance is good with a minimum gain about -6 dBi (including the loss due to finite inductor Q).

The slant monopole antenna elements 14 and 16 respond to both V and H polarizations, while the two antennas, mounted in opposite directions, provide both spatial and radiation pattern diversity for good performance in a multi-path environment.

A connector-less, rotatable interface is made between each antenna element 14, 16 and the internal PCB of the hand held device. Spring finger antenna connections are common in today's cellular telephones and other wireless devices, however, they are often stationary.

Figures 2 and 3 shows different embodiments of the simplified inductor only tuning networks 20 and 30, respectively, according to the invention. The key feature of the tuning topology of the present invention is that when antenna dimensions L are chosen appropriately, RF switches can be used to switch in or bypass tuning inductors and provide the correct tuning to the antenna. This helps to minimize the circuit size requirements while still maintaining good performance in the selected frequency ranges.

Figure 2 shows a simplified tuning network 20 that includes RF switch components 22a, 22b, 22c which are used to tune the antenna structure in three separate bands for maximum performance. RF switches 22a, 22b, 22c are single pole single throw (SPST) switches and operate to switch in, or bypass the respective tuning inductors 24a, 24b, 24c. When a particular inductor 24 is not needed for one of the 3 bands of operation, the corresponding switch 22 is closed, bypassing the inductor. It is important to note that the inductor 24 remains attached to the RF signal path when the inductor is bypassed with a closed switch. Those of skill in the art will recognize that this is usually not an acceptable RF circuit layout. This is simply because any unused components/circuit traces of any kind attached to the primary signal path create parasitic responses that interfere with low-loss operation. However, in the present example of the invention, the parasitic response has been absorbed into the antenna tuning, thereby allowing the use of this very minimum circuit topology. The same is true of Fig 3 - the common port - "To Radio" - is not a typical RF circuit layout and would not work at all at frequencies above about 1.5 GHz, but for the antenna and freq of operation we are using, we can absorb the unwanted parasitic response and still get good performance. It becomes clear that the foregoing circuit topologies that lower cost and smaller size are obtainable without compromising on performance.

Figure 3 shows another preferred embodiment of the tuning network 30 that includes RF switch components 32a, 32b and 32c and tuning inductors 34, 35, 36, 37. The RF switch components 32 are preferably single pole, double throw (SPDT) switches and operate to switch the desired tuning inductor(s) into the circuit. Although the other inductors remain attached to the RF signal path, due to the careful selection of inductor value and PC board layout, the parasitic effects of the un-used inductors is tolerable.

Careful selection means that the antenna response is measured with the entire circuit and antenna in place, and then the tuning inductor is chosen.

Actual inductance values depend entirely on the antenna size and shape - a general range is about 10 to 100 nH but as the frequency of operation is reduced the inductor values would grow accordingly. Having said that, one of ordinary skill will recognize that the high VHF and low UHF frequency ranges being used are kind of the "sweet spot" for this antenna type and tuning circuitry. Any lower in frequency, and the antenna on a handheld product would probably be very small electrically so tuning would be more difficult. Any higher in frequency, and the antenna would not be very small electrically, and may not even require tuning.

Tuning inductors 34 and 35 are coupled to switch 32a, and inductors 36 and 37 are coupled to switch 32b. Switches 32a and 32b are coupled to another SPDT switch 32c. The switch matrix is essentially a 1-of-4 selection. The switch closest to the antenna port (i.e., switch 32c) selects one of the other two switches (32a, 32b) and those switches are controlled in parallel so they switch together but only one is in the circuit path due to switch 32c. Once again, this approach and design would be considered breaking the typical RF circuit rules, but it works due to the antenna type and frequency range of operation.

More conventional switching topology would require 2-4 switches per band to completely isolate the tuning inductor, thereby greatly increasing the cost, size and complexity of the tuning circuit and the requisite control interface. The antenna element type (i.e., monopole) and dimensions L are chosen so as to not require any shunt tuning components in the two series inductor locations (i.e., inductors 24a and 24b). In accordance with one preferred embodiment, Length L is 70 mm. Shunt components would load the RF path unless isolated from the RF signal path with additional RF switches. In this embodiment, the dimensions L of antenna elements 14 and 16 are chosen such that the tuning network 20 consists of series and shunt components that can be either bypassed or switched in with very simple (and inexpensive) SPST switches without adversely affecting performance.

In another preferred embodiment, L is ideally as long as possible arid the inductance values of the tuning inductors are chosen to tune the chosen length L. For ideal performance characteristics, length L is "electrically short" and is preferably less than 0.15 wavelength. Although there is no direct relationship between L and angles A, generally, the angles A should be should be greater than about 20 degrees for good efficiency. While there have been shown, described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions, substitutions and changes in the form and details of the methods described and devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed, described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

1. An antenna for portable devices comprising: two oppositely arranged antenna elements (16) an inductor only tuning circuit (20, 30) within the portable device and electrically connected to said two antenna elements and having multiple stages for providing maximum reception performance in a UHF bandwidth of 470-850 MHz.

2. The antenna according to claim 1 , wherein said antenna elements comprise slant monopole elements (16) having a predetermined length (L) and predetermined angular position (A) with respect to the portable device.

3. The antenna according to claim 1 , wherein said tuning circuit comprises: a tuning inductor (24, 34) for each of said multiple stages, said tuning inductors being connected to an RF signal path in the tuning circuit; and an RF switch (22, 32) for each of said tuning inductors, each RF switch enabling selective bypassing or switching in of its corresponding tuning inductor to tune the antenna to a chosen band of operation.

4. The antenna according to claim 2, wherein said predetermined length (L) is

70mm.

5. The antenna according to claim 2, wherein said predetermined length (L) is less than 0.15 wavelength.

6. The antenna according to claim 2, wherein said angular positions comprise acute angles with respect to the portable device.

7. An antenna for portable devices comprising: two oppositely arranged slant monopole antenna elements (16); and an inductor only tuning circuit (20, 30) within the portable device and electrically connected to said two antenna elements for providing maximum reception performance in both a VHF bandwidth of 170-225 MHz and a UHF bandwidth of 470- 850 MHz.

8. The antenna according to claim 7, wherein said tuning circuit comprises: a tuning inductor (24, 34) for each four separate bands, said tuning inductors being connected to an RF signal path in the tuning circuit; and an RF switch (22, 32) for each of said tuning inductors, each RF switch enabling selective bypassing or switching in of its corresponding tuning inductor to tune the antenna for maximum performance in the chosen band of operation.

9. The antenna according to claim 7, wherein said tuning circuit comprises: a tuning inductor (34, 35, 36, 37) for each of four separate bands, said tuning inductors being connected to an RF signal path in the tuning circuit; a first RF switch (32a) having one input and two outputs, said two outputs connected to two of said tuning inductors; a second RF switch (32b) having one input and two outputs, said two inputs connected to another two of said tuning inductors; and a third RF switch (32c) having one input connected to the antenna elements and two outputs, one of said outputs connected to an input of said first RF switch, and the other of said outputs connected to said input of said second RF switch.

10. The antenna according to claim 8, wherein said RF switches (22) are single pole, single throw switches.

11. The antenna according to claim 9, wherein said RF switches (32) are single pole, double throw switches.

12. The antenna according to claim 8, wherein said predetermined length (L) is 0mm.

13. The antenna according to claim 8, wherein said angular positions comprise acute angles (A) with respect to the portable device.

14. The antenna according to claim 8, wherein said predetermined length (L) is less than 0.15 wavelength.