Dual Frequency Batwing Antenna

The present invention is directed to a dual frequency batwing type antenna and more specifically to an antenna especially useful for high definition television (HDTV).

Implementation of high definition television (HDTV) in the U.S. is now undergoing testing by the Federal Communications Commission. It is contemplated that such an HDTV system will require additional channel allocations on UHF frequencies which will carry a digital signal. Thus alterations to existing transmitting antennas will be required. The existing TV signal is normally designated an NTSC signal which is analog. It is contemplated that the HDTV digital signal will be located on the same transmission tower. However, because of the normal NTSC signal is in the VHF frequency range, such VHF antenna cannot simultaneously serve UHF channels. In such cases, the HDTV operation will require a separate antenna in a different location of the tower.

Since the physical location of an antenna on a transmitting tower used by several TV stations is quite critical, to locate the HDTV antenna on another part of the tower, or even relocate the entire antenna installation is not desirable.

It is a general object of the present invention to provide an improved dual frequency antenna especially suited for HDTV.

In accordance with the above object there is provided a radio frequency antenna for radiating a pair of radio frequency signals having first and second frequencies with a ratio of substantially two to one or greater comprising a first pair of radiating elements having a radiation pattern with nulls and being resonant at the first frequency. Two or more pairs of radiating elements resonant at the second frequency are mounted in the nulls of each of the first pair of radiating elements and arranged on a support mast to minimize coupling to the first pair of radiating elements whereby the pair of radio frequency signals are radiated from a common antenna aperture.

FIG. 1 is a block diagram of a generalized transmitter system as it would drive the antenna of the present invention.

FIG. 2 is a simplified elevational view of the antenna of the present invention.

FIG. 3 is a simplified cross-sectional view taken along the line 3--3 of FIG. 2 showing the radiation pattern of a portion of the antenna of the present invention.

FIGS. 4A and 4B are diagrams illustrating the operation of the present invention.

FIG. 5 is a perspective view of the antenna of FIG. 2.

FIG. 6 is an elevational view of one of the antenna portions of FIG. 5, and

FIG. 7 is a plan elevational view similar to FIG. 6 but of an alternative embodiment.

FIG. 1 illustrates a proposed HDTV system where the transmission tower 21, with its antenna on a vertical support mast 22, will radiate or transmit both an NTSC signal received on the coaxial cable 23 and coupled to the tower and a HDTV signal on the coaxial cable or waveguide 24. In general the NTSC signal can be either in the low VHF or high VHF range which are 54-88 MHz and 170-230 MHz, respectively. Also such a signal may be in the UHF band which is 470-800 MHz. In general it is contemplated that the extra HDTV channels will be the UHF channels which were used for spacing.

The remainder of FIG. 1 are standard transmitting system blocks. From a television studio 26, one feed extends to an NTSC channel including an exciter 27, visual and aural power amplifiers 28 and a combining diplexer 29 which feeds the coaxial waveguide 23 and connects to the antenna 21, 22. The second channel from the TV studio 26 includes the encoder 31 to convert the information to digital, an upconverter 32, power amplifier 33, and a band pass filter 34 connecting to the HDTV coaxial 24. The HDTV signal as well as being digital is contemplated to be of the spread spectrum type.

FIG. 2 illustrates the support mast 22 in greater detail with the actual radiating elements mounted on the vertical support mast. This includes a first pair of batwing radiating elements 36a, 36b, and a second pair located a wavelength apart 37a, 37b. These batwings form effective conductive sheets and as will be described later may be constructed of sheet metal or rods. They are resonant and radiate at a first lower frequency f.sub.1. This is typically the NTSC signal on the coaxial antenna feed 23 (FIG. 1). Each batwing antenna includes a top, in this case horizontal termination 38, and a bottom termination 39 which circumscribes as will be discussed in more structural detail below the conductive sheet and also a structurally vacant rectangular space 41. This space which is, of course, coplanar with the remainder of the batwing antenna is utilized by the smaller high frequency batwing radiating elements 42a, 43a and 42b, 43b. Since the smaller elements must fit within the same common radiating aperture on the antenna mast 22 as the lower frequency batwing elements, it is convenient that the higher frequency, f.sub.2, (in this case the HDTV signal) be an integral multiple of the lower frequency. The antenna elements 42a and 43a are separated also by substantially a wavelength at the second higher frequency as indicated. In addition there is in between, for example, the larger low frequency batwing elements 36a and 37a, an additional high frequency element 44a, etc. This would be also spaced by the wavelength of the second higher frequency as indicated.

Thus the ideal situation is where the higher HDTV frequency is an integral multiple such as 2, 3, or 4 times the NTSC signal. However it is not necessary because at non-integral multiples, the spacing is adjusted to the best fit. And well known techniques of non-uniformly spaced arrays are used to shape the radiation pattern at f.sub.2 frequency. This provides for optimum spacing within the same antenna aperture on the same transmitter tower. Typically in a batwing type installation there may be a vertical array of perhaps 3 to 4 batwing elements. And within the vacant rectangular space provided by the low frequency batwings, there may be multiples of 2, 3 or 4 UHF antennas 42a, 43a, etc.

In order to provide two different operating frequencies within the same antenna aperture, both physical blockage and electrical interference must be minimized. This is accomplished by the present invention as illustrated in FIGS. 3 and 4A and 4B.

First referring to FIG. 3, the radiation pattern of the batwing antennas 36a and 36b are illustrated as ellipses 46 and 47. The mast of course is indicated at 22. An electromagnetic null is effectively provided between the two radiation patterns 46 and 47 as indicated at 48. Within this null are placed batwings 42a and 42b as indicated without causing any interference. A location such as the dashed batwing 50 would in effect provide a corner reflector.

Besides avoiding interference with the radiation characteristic, the two antennas of high frequency and low frequency must not electrically couple. The ideal arrangement is for the antennas to be co-linear as illustrated in both FIGS. 3 and 4A. Here for the larger batwing antenna 36a, the current flows over the surface of the effective conductor sheet in opposite directions. And the same is true of the higher frequency batwing 42a. With these currents being colinear, the radiation intensity in the plane of the wings goes to zero. As a result the two elements do not radiate toward each other. And as indicated by the dashed line 51, even if the element 42a is slightly angled, there will still not be a significant cross coupling.

FIG. 4B illustrates both antenna units, designated 36'a and 42'a adjacent each other and not co-linear. Here there is a significant amount of unwanted coupling because both elements radiate strongly toward each other. Thus in summary by mounting the smaller radiating elements within the null of at least the first pair of lower frequency radiating elements and also arranging them in a noncoupling mode as illustrated in FIG. 4A, a pair of radio frequency signals may be radiated from a common aperture.

The arrangement shown in FIGS. 2 and 3 is of course very simplified and FIG. 5 shows a commercial antenna. Besides the initial batwing elements 36a and 36b, another pair of batwings 36c and 36d, provides for an additional radiation pattern such as 46, 47 shown in FIG. 3, but rotated 90 degrees so that a full circular pattern is provided. At the same time, the additional quadrature batwings will not be either interfered with by the, for example, batwings 42a and 42b for the same reason that the larger batwings 36a, 36b do not interfere with quadrature batwings 36c, 36d.

FIG. 6 shows an even more enlarged view of, for example, batwing 36b which is constructed of rods with an upper terminating rod 38 and a lower terminating rod 39 attached to the support tower 22. The substantially vacant rectangular space, of course, is illustrated generally at 41 in which the smaller batwings 42b, 43b are again fastened to the tower 22 and the intermediate conductive rod 56. Batwing 36b is fed through coaxial cable 23 (see FIG. 1) from its center conductor 57 and the intermediate rod 58. Then the smaller batwings 42b and 43b are fed either collectively or individually in the same manner by the coaxial cable 24.

FIG. 7 illustrates an alternative embodiment of the invention where rather than the batwings being constructed of rods, they may consist of conductive planes indicated in the case of the larger batwing by plane 1 connected to the pole 22 by rods 10 and 11 and the smaller batwing antennas 2, 3 and 4. Conductive plates 6 and 7 connect or attach both the rods 10 and 11 to the ends of the smaller batwings 2 and 4. The rods 7 and 8 offer additional support for the panel 1. The vertical dimension 15 and the horizontal dimension 14 are such that the element 1 will resonate at the lower frequency. The same is true of the higher frequency batwing elements 1, 2, 3 and 4.

Thus an improved dual frequency antenna has been provided where within the same radiating aperture two different frequencies are accommodated. This is done without any major reconstruction of the existing transmitting tower.