EP0887880A2

EP0887880A2 - A multi-band antenna having a common feed

Info

Publication number: EP0887880A2
Application number: EP98111544A
Authority: EP
Inventors: Rex Leonard Niekammp; Geoffrey N. Mendenhall
Original assignee: Harris Corp
Current assignee: SPX Corp
Priority date: 1997-06-30
Filing date: 1998-06-23
Publication date: 1998-12-30
Also published as: EP0887880A3; CA2241006A1; AU7394698A

Abstract

A multi-band antenna is provided with a common feed. The antenna includes a vertically oriented elongated support structure. A first frequency selective radiating element is carried by the structure and is optimized for radiating a first RF signal having a first frequency from a first frequency band. A second frequency selective radiating element is carried by the structure and is optimized for radiating a second RF signal having a second frequency from a second frequency band. The frequencies within the first frequency band differ from the frequencies in the second frequency band. A common signal feed is carried by the structure for simultaneously carrying both the first and second RF signals for application to the radiating elements.

Description

The present invention relates to antennas and more particularly to a multi-band antenna having a common feed.

It is known in the art that antennas employed for RF broadcasting are designed to operate most efficiently at a specific frequency within a frequency band. Such an antenna would normally operate inefficiently at frequencies outside of that specific frequency band. For example, FM radio frequency broadcasting takes place at specific frequencies within the FM frequency band from approximately 88.0 MHz to approximately 108 MHz. UHF television employs radio frequency signals which are broadcast in the UHF band from approximately 470 MHz to 806 MHz. An antenna that is designed or opted for transmitting radio frequency (RF) signals in the FM band is a poor radiator of UHF signals and presents a high impedance to UHF signals. Similarly, an antenna that is designed for UHF operation is a poor radiator of FM signals and presents a high impedance to FM signals. It is known, however, that within a frequency band an antenna that is designed for a specific frequency may be employed, in a less efficient manner, with another frequency within the same frequency band. Thus, an FM antenna designed to operate at a center frequency of 99.5 MHz may be somewhat usable for transmitting a signal at 100.3 MHz.

It has been common to employ different antennas for simultaneously transmitting different frequencies from different frequency bands, in order to obtain efficient radiation. FM and UHF broadcasting antennas are typically mounted on top of a tower. A UHF antenna for example may be located on top of a tower that is 1,000 feet tall and that UHF antenna itself may be on the order of 50 feet in length. The feed system for supplying RF energy to the UHF antenna is carried by the tower and presents substantial weight and wind loading. A similar situation is true for an FM antenna also mounted on top of such a tower and coupled to its feed system which extends up along one side or inside of the tower. It is known in the art that both an FM antenna and a UHF antenna may be mounted side by side on top of such a tower or stacked vertically with each antenna being supplied with RF energy from a separate transmission feed system

At present, the TV industry is introducing high definition television (HDTV) having a digital format. As a result there is a need to accommodate installation of additional broadcasting antennas. Many of the present towers that support RF antennas are already overloaded and cannot accommodate additional antennas along with their associated RF feed systems or cables because of their weight, wind loading and physical space. It is desirable to increase the capacity of such towers by providing multi-band antennas to be mounted on such towers. This permits use of a common feed or single transmission line to be carried by the tower instead of multiple transmission lines, one for each frequency band of the antenna.

A multi-band antenna having a single feed is disclosed in the specification of U.S. Patent No. 4,356,492. This is a microstrip antenna employing a pair of radiating elements of widely separated frequencies. The radiating elements are mounted on a common ground plane. A common input point of the radiating element is fed at all the desired frequencies from a single transmission feed line. The structure, however, is not suitable for an antenna intended to be mounted on top of a tower for transmitting frequency bands such as FM and UHF. Instead, the microstrip antenna proposed by Kaloi is intended for use in applications for a low physical profile antenna, such as in missiles and aircraft. There is no need to provide a structure which will support the radiating elements as well as a common feed for energizing the elements.

The invention includes a multi-band antenna, comprising a vertically oriented elongated support structure, a first frequency selective radiating element carried by said structure and optimized for radiating a first RF signal having a first frequency from a first frequency band, a second frequency selective radiating element carried by said structure and opted for radiating a second RF signal having a second frequency from a second frequency band wherein the frequencies within said first frequency band differ from the frequencies in said second frequency band, and, a common signal feed carried by said structure for simultaneously carrying both said first and second RF signals for application to said radiating elements.

Advantageously, there is provided a multi-band antenna, which includes a vertically oriented elongated support structure. A first frequency selective radiating element is carried by the structure and is opted for radiating a first RF signal having a first frequency from a first frequency band. A second frequency selective radiating element is also carried by the structure and is optimized for radiating a second RF signal having a second frequency from a second frequency band wherein the frequencies within the first frequency band differ from those in the second frequency band. A common signal feed is carried by the structure for simultaneously carrying both the first and second RF signals for application to the radiating elements.

Conveniently, the first radiating element exhibits a high impedance to RF signals in the second band and the second radiating element exhibits a high impedance to RF signals of frequencies within the first band.

Suitably, the first frequency band is a UHF band and the second frequency band is a FM band.

The invention will now be described, by way of example, with reference to the accompanying drawings wherein:

Fig.1 is a block diagram illustration of a prior art system;

Fig.2 is an elevational view of a typical prior art UHF antenna mounted on top of a tower.

Fig.3 is a graphical illustration of various frequency bands;

Fig.4 is a block diagram illustration of one embodiment;

Fig.5 is an elevational view, partly in section, illustrating one embodiment of a multi-band antenna;

Fig.6 is a sectional view taken along line 6-6 looking in the direction of the arrows in Fig. 5;

Fig.7 is a view taken along line 7-7 looking in the direction of the arrows in Fig.5;

Fig.8 is an elevational view similar to that of Fig.5 and illustrating a second embodiment of a multi-band antenna; and,

Fig.9 is a view taken along line 9-9 looking in the direction of the arrows in Fig. 8.

Fig.1 illustrates a prior art application wherein FM and UHF signals are simultaneously being transmitted. In such case, it is common for the FM transmitter 10 to apply an RF signal within the FM band (88 MHz to 108 MHz) to FM antenna 12 which may be located on top of a tower (not shown) with transmission taking place over a transmission line 14. The transmission line 14 may take the form of a coaxal cable. Similarly, a television UHF transmitter 20 typically transmits an RF signal in the UHF band (470 MHz to 806 MHz) to UHF antenna 22 by means of a coaxial transmission line 24. The FM antenna 12 and the UHF antenna 22 may each be mounted on top of separate towers which may each be approximately 1,000 feet high. Whether the

antennas

12 and 22 are mounted on separate towers or the same tower, each will have its

transmission line

14 or 24 extending from its associated transmitter up along side the tower and connected to the antenna.

Fig. 2 illustrates an application of a UHF antenna mounted on top of a tower structure. In Fig. 2 the UHF antenna 22 (schematically illustrated in Fig.1) is shown as being mounted on top of a tower structure 30 of conventional design. The tower height may, for example, be on the order of 1,000 feet and the length of the UHF antenna 22 may be on the order of 50 feet. At the ground level, the RF transmitter 20 supplies RF energy to a transmission feed system 24 which, in turn, supplies RF energy to the antenna 22. The transmission feed system 24 may include, for example, a rigid coaxial feed line including a horizontal portion and a vertical portion which extends up along one side of the tower structure 30 to feed the antenna 22 in a conventional manner.

Fig.3 illustrates various RF frequency bands including a low band VHF TV frequency band, from approximately 54 MHz to 88 MHz. This is followed by an FM band from approximately 88 MHz to 108 MHz. The FM band is followed by a high band VHF TV band which extends from approximately 174 MHz to 216 MHz. The UHF TV band extends from approximately 470 MHz to 806 MHz. Each of the FM signals within the FM band has a center frequency with variations on either side at ± 200 kilohertz. Thus, for example, a FM station having a center frequency F_c of 99.5 MHz is referred to as: F_c=99.5 MHz ± 200 kilohertz. Similarly the UHF channels are each of width on the order of 6 MHz. For example, UHF channel 14 extends from 470 MHz to 476 MHz whereas channel 16 extends from 482 MHz to 488 MHz.

Fig.4 illustrates one embodiment. The FM transmitter 10 and the UHF transmitter 20 are connected to a common antenna 40 by way of a common transmission line 42. In addition, a UHF filter 44 is interposed between the FM transmitter and the transmission line 42 and an FM filter 46 is interposed between the UHF transmitter 20 and the common transmission line 42. The UHF filter 44 serves to prevent the UHF RF signal from transmitter 20 from being fed back into the FM transmitter 10. Likewise, the FM filter 46 serves to prevent FM RF signs from transmitter 10 from reaching transmitter 20. Eoth the FM signals and the UHF signals transmitted by

transmitters

10 and 20 are fed through the common transmission line 42 and, thence, upward along a tower structure to feed the common antenna 40, which is mounted on top of such a tower.

The common antenna 40 is illustrated in Fig.5. This antenna 40 is mounted to a tower structure 30' (similar to that of tower structure 30 in Fig. 2) and is coupled to a common transmission feed 42 by way of

annular flanges

44 and 46 which are fastened together and to a portion of the tower structure 30' as with suitable fastening means, such as nuts and bolts.

The antenna 40 includes a vertically oriented elongated support structure 50 which is mounted to the tower 30', as described above. The elongated support structure 50 includes a plurality of vertically aligned slots 52 each extending in a vertical direction and located in the peripheral wall of a cylindrical mast 54. Mast 54 is cylindrical in a cross section, as is seen from Fig.5, and is made of an electrically conductive material, such as steel or aluminum. The mast 54 coaxially surrounds an inner conductor 56 which extends throughout the height of the mast. The structure 50 includes a horizontally oriented section 60 which couples the mast 50 to the coaxial common transmission feed line 42 that extends upwardly along one side of the tower 30'.

The vertically oriented slots 52 in mast 54 are spaced from each other by a distance S. This distance S is preferably of a length on the order of 1 wavelength at the operating frequency. The vertical length L of each slot 52 is preferably on the order of 1/2 a wavelength. These slots are optimized in size and slot spacing for a particular UHF channel such as channel 14 or channel 16 and so forth. As is known in the art, a change in the operating frequency requires a change in the slot length L and the slot spacing S.

In Fig. 6, each slot has associated therewith a coupling probe 55 which is suitably mounted to one side of the slot, at essentially the mid-point thereof, and is located on the interior side of the mast. The probe may be secured to the mast as with a suitable weld. Although the coupling probe is illustrated as having a rectangular cross-section, it may also have a circular cross-section. It is constructed of metal such as stainless steel. Such a coupling probe assists in coupling the energy within the mast so that the radiating field appears across each slot at which a coupling probe is associated.

Whereas the antenna 40 in Fig.5 is illustrated with five bays with one vertically oriented slot per bay, it is to be appreciated that several additional bays may be provided as desired. The pattern that is generated may, for example, take the form of an omni, cardioid or peanut shaped pattern for UHF television transmission. A plastic radome (not shown) may extend coaxially around mast 50, as is convention in the art.

The description of the mast 50 is that of a conventional UHF multiple slot antenna suitable for broadcasting frequencies within the UHF frequency band. This antenna includes at least one radiating element taking the form of slot 52 which is carried by the support structure 50 and optimized for radiating an RF signal in the UHF frequency band.

The structure 50 also carries a second frequency selective radiating element taking the form of a dipole 70 having

antenna portions

72 and 74, as best seen in Figs.5 and 7. The dipole 70 is carried by an extension 76 of the support structure 50 with the extension taking the form of a rigid coaxial cable having an inner conductor 78 which is coaxially surrounded by an outer conductor 80. The inner conductor 78 is electrically coupled by suitable means to the inner conductor 56 in mast 54 and the outer conductor 80 is mechanically and electrically secured to mast 54. Antenna element portion 72 is secured, as by welding, to the terminal end of conductor 80 and antenna element portion 74 is secured, as by welding, to the terminal end of the inner conductor 78. The extension 76 is structured so that the distance X from the dipole 70 to the center line of mast 54 is on the order of 1/4 wavelength of the FM operating frequency of the dipole 70.

From the foregoing it is seen that the embodiment of Fig. 5 includes a common transmission line 42 which carries both the FM RF signal from transmitter 10 and the UHF signal from transmitter 20 and these signals are commonly fed by the transmission line upward along the tower 30' then supplied to a common feed carried by the support structure 50. This common feed in support structure 50 includes the cylindrical mast 54 as the outer conductor and the inner conductor 56. The inner conductor 56 is coupled to the inner conductor 43 in the transmission line 42. This energizes the UHF multiple-slot antenna for radiating the UHF signal and additionally energizes the dipole 70 for radiating the FM signal.

The embodiment disclosed in Figs.5,6, and 7 employs two different types of radiating elements. One radiating element takes the form of a UHF slot antenna including at least one slot 52. The other radiating element takes the form a dipole 70. Consequently, the embodiment of Fig.5 employs a first radiating element, which is a different type of radiating element from that of the second radiating element.

The embodiment in Figs.8 and 9 employs radiating elements employed for radiating RF signals from different frequency bands and wherein all of the radiating elements are the same type, such as dipoles.

Reference to the embodiment illustrated in Figs.8 and 9. This embodiment is similar to that illustrated herein at Figs.5 through 7 and consequently like components are identified with like character references.

Fig.8 illustrates a multi-band antenna 40' which includes a vertically oriented elongated support structure 50'. Structure 50' includes a hollow mast 54' which serves as an outer conductor which coaxially surrounds an inner conductor 56'. Mast 54' does not include slots 52, as in the embodiment of Fig.5. Instead, mast 54' is provided with

coaxial extensions

100, 102 and 104 which extend radially out from the mast and then feed

dipoles

110, 112 and 114 respectively. Each of the dipoles has a first element portion 120 and a second element portion 122. Element portion 120 is connected to the inner conductor 124 as by welding and element portion 122 is connected to the outer coaxial extension 102. The dipoles are spaced horizontally from the conductor 56' by a distance Y and this distance is on the order of 1/4 wavelength at the operating frequency of the frequency band being employed for

dipoles

110, 112, and 114.

In Figs.8 and 9, the

dipoles

110, 112, and 114 may be employed in a high-band VHF TV frequency band, such as 174 MHz to 216 MHz. In this embodiment, dipole 70 is employed as an FM radiator having a frequency within the FM band from 88 MHz to 108 Mhz.

Claims

A multi-band antenna, comprising a vertically oriented elongated support structure, a first frequency selective radiating element carried by said structure and optimized for radiating a first RF signal having a first frequency from a first frequency band, a second frequency selective radiating element carried by said structure and optimized for radiating a second RF signal having a second frequency from a second frequency band wherein the frequencies within said first frequency band differ from the frequencies in said second frequency band, and, a common signal feed carried by said structure for simultaneously carrying both said first and second RF signals for application to said radiating elements.
An antenna as claimed in claim 1 wherein said first radiating element exhibits a high impedance to RF signals of frequencies in said second band and said second radiating element exhibits a high impedance to RF signals of frequencies within said first band, in which said first radiating element is of a different type of radiating element from that of said second radiating element.
An antenna as claimed as in claim 2 wherein said first frequency band is a UHF band from approximately 470 MHz to 806 MHz, and said second frequency band is an FM band from approximately 80 MHz to 108 MHz.
An antenna as claimed in any one of clams 1 to 3 wherein said support structure includes an elongated hollow tubular member and wherein said common signal feed includes said tubular member and an elongated center member carried within said hollow tubular member.
An antenna as claimed in claim 4 wherein said tubular member is cylindrical in cross-section and coaxially surrounds said center member, including coupling means for coupling said common feed to said first and second elements.
An antenna as claimed in claim 5 wherein said first element includes at least one slot in said tubular member.
An antenna as claimed in claim 6 wherein said second element is a dipole.
An antenna as claimed in any one of claims 2 to 7 wherein said first radiating element is of the same type of radiating element as said second radiating element.
An antenna as claimed in clams 8 wherein said first element includes a dipole and said second element includes a dipole.