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

Background of Invention

Implementation of high definition television (HDTV) in the United States 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.

Obiect and Summary of Invention

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 arranged as a dipole panel antenna having a pattern with nulls and being resonant at the first frequency. Two or more dipoles resonant and radiating at said second frequency are spaced on the panel between the first pair of radiation elements with the second frequency dipoles having a radiation pattern to provide noninterference with the first pair of dipoles.

Brief Description of the Drawings

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

Figure 2 is a perspective view of a panel antenna system embodying the present invention.

Figure 3 is a top view of Figure 2.

Figure 4 is a diagrammatic view of Figure 2.

Figure 5 is a radio frequency propagation pattern illustrating Figure 5.

Figure 6 is a simplified elevational view of Figure 3 taken substantially along the line 5-5 of Figure 3.

Figure 7 is a simplified plan view of Figure 5 showing an alternative embodiment.

Detailed Description of Preferred Embodiments

Figure 1 illustrates a proposed HDTV system where "he transmission tower 21 which 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 Figure 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.

Figure 2 illustrates one of the four panel antennas 40 which would be mounted on the transmission tower 21. Referring also to Figure 3, it includes a first pair of dipoles 41a, 41b, resonant at a first lower frequency, f _1# (for example, the NTSC frequency) and spaced a half wavelength apart. A second set of smaller dipoles 42a, b and c, are resonant and radiating at a second higher frequency, f ₂ . These are all mounted on the panel 40. Then to provide circular

polarization, in addition to the larger dipoles 41a, 41b, there are a 90 degree quadrature pair 43a, 43b.

Figure 4 illustrates the various dipoles and their physical arrangement on the panel. This includes the dipoles 41a, 41b, being spaced a half wavelength apart along with the quadrature dipoles 43a, 43b. The latter are driven, of course, by the first frequency f at a 90 degree angle compared to 0 degrees for 41a, 41b. Then for the higher frequency f ₂ , the dipoles 42a, 42b, 42c, are again arranged a half wavelength apart and the frequency is at least double, or in this case, triple the frequency of f . This provides intermediate space between the first pair of dipoles 41a, 41b.

The radiation pattern of the vertical dipoles 42a, 42b, 42c at frequency f ₂ is illustrated in Figure 5. This elevation radiation pattern is, of course, formed by three high frequency dipoles spaced one-half wavelength apart. The first null of the elevation pattern of the array falls at an angle which is indicated as 42 degrees. This is by a known radiation formula, the arc sine of 2/3. If there are more than three high frequency elements than in accordance with the above formula, the first null point may decrease to an angle approaching 32 degrees. On the other hand, if only two dipoles are used, the null angle is approximately 45 degrees. With a frequency f ₂ which is approximately three times f- _L used here, any of these null angles will provide a radiation pattern which, as illustrated in Figure 6, will not interfere with the lower frequency dipoles 41a, 41b. As illustrated in the drawing, the angle is the angle at which the dipoles 41a, 41b are viewed by the array of the three higher frequency dipoles, which in this specific context works out to 45 degrees. With the radiation pattern shown in Figure 5 and the null located at 42 degrees, the radiation pattern of the higher frequency dipoles will not interfere with the dipoles 41a, 41b. That is the radiation

from the three dipole array to the lower frequency dipoles is essentially zero or close to zero.

From a reverse standpoint, any interfering radiation from the lower frequency dipoles 41a, 41b, is also zero or close to zero because the radiation patterns of these two lower frequency dipoles tend to cancel each other. Thus in general the radiation pattern, with respect to physical interference from the higher frequency dipoles is less than the slant angle which is the angle from the center axis of the panel dipole antenna to either of these first pair of dipoles 41a or 41b.

With respect to the high frequency dipoles 42a, 42b and 42c, although the half wavelength spacing indicated is optimal to provide an effective pattern, it may not be necessary in all contexts. In general the spacing and excitation of this higher frequency array can be designed to minimize the coupling between this array and the f ₁ dipole array.

Referring to Figure 4 there is, of course, no interference with the dipoles 43a, 43b because of the cross polarization of the two. Specifically the dipoles 43a, 43b are both vertically polarized while the three dipole array 42a-42c is horizontally polarized.

Figure 7 illustrates an alternative embodiment with regard to the dipole array 42a, 42b, 42c where a conductive box 51 surrounds the three-dipole array. Box 51 forms the cavity 52 having the conductive side walls 53 indicated in dashed outline in Figure 6. The height of the side walls is approximately the height of the dipole antennas. The dimension a (the width dimension) of the box 51 determines the radiation patterns' three decibel points. Use of such of the above technique is especially effective if the panel antenna is designed to achieve maximum circulation based on the shape of the cross section of the tower on which

generally several panels would be mounted (see Figure 2 which indicates the orientation of one panel) .

Thus in summary the invention provides a dual frequency radio antenna where within the same aperture two different frequencies can be accommodated.