Electromagnetic Filters Wherein Waveguide Walls Comprise Alternate Conductivity Sections

Abstract

A high pass filter comprising a section of waveguide having alternate sections of different wall conductivity to produce a sharper cutoff and more linear phase propagation near cutoff. Low-pass, band-pass and band-reject filters are also described.

Description

This invention relates to improved electromagnetic wave filters.

BACKGROUND OF THE INVENTION

Electromagnetic wave filters are used in a wide variety of communications systems. For example, in radio systems using several different transmit and receive frequencies, filters are typically required to separate the transmit and receive frequencies.

A typical prior art high pass filter for microwave frequency operation is simply a section of uniform waveguide having a cutoff frequency at the desired minimum pass frequency. When a signal is applied to the section, those portions of the signal having frequencies above cutoff pass through the section essentially unattenuated while those portions having frequencies below cutoff are reflected back. One difficulty with such filters, however, is the nonlinearity of the phase and attenuation propagation constants for frequencies near the waveguide cutoff frequency. Because of this nonlinearity, a significant bandwidth, on the order of 150 megahertz for a 3 gigaHertz cutoff filter, is lost for communications purposes.

BRIEF SUMMARY OF THE INVENTION

The present invention arises from the discovery that a section of waveguide having alternate sections of different wall conductivity has more abrupt cutoff characteristics than a uniform section. Thus a high pass filter comprising a section of waveguide having periodic variations in wall conductivity produces considerably less distortion of the propagation constants of signal frequencies near cutoff with a negligible increase in the loss. In addition, improved band-pass and band-reject filters can also be constructed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, the nature of the present invention and its various advantages will appear more fully upon consideration of the specific illustrative embodiments shown in the accompanying drawings and described in detail in the accompanying detailed description. In the drawings:

FIG. 1 shows an illustrative embodiment of a high pass filter in accordance with the invention;

FIG. 2 is a graphical illustration useful in understanding the invention;

FIG. 3 shows an illustrative embodiment of a low-pass filter in accordance with the invention;

FIG. 4 shows an improved band-pass filter in accordance with the invention; and

FIG. 5 shows an improved band-reject filter in accordance with the invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a high pass electromagnetic wave filter in accordance with the invention comprising a periodic structure of waveguide 10 having a cutoff frequency F.sub.c and composed of sections 11 having a first wall conductivity C.sub.1 , and length L.sub.1 , alternated with sections 12 having a second and smaller wall conductivity C.sub.2 and a length L.sub.2 . Experimentally, it has been found that there should typically be between 10 and 50 sections of each conductivity to establish periodicity without incurring excessive loss.

The values of the parameters L.sub.1 , L.sub. 2, C.sub.1 , and C.sub.2 are also chosen to establish periodicity with respect to waves of frequency near F.sub.c without producing excessive loss. For example, the total length of two sections, i.e., L.sub.1 +L.sub.2 , should be appropriate to establish periodicity with respect to wave energy of frequency F.sub.c . If the length is much smaller than a wavelength, the structure will behave much like a uniform guide while, on the other hand, if the length is much greater than a wavelength, it will behave as a plurality of serially connected uniform guides. Experimentally it has been found that the length of two alternate sections advantageously lies between 0.25 and 5 wavelengths at the cutoff frequency. In addition, it has been generally found that a sharper cutoff is produced when the length L.sub.2 of the lower conductivity region is slightly larger than that of the higher conductivity section. Experimentally a length ratio of 5:3 has been found to be particularly advantageous. The ratio between the two conductivities C.sub.1 and C.sub.2 typically exceeds 2:1 in order to have any measurable effect on the performance of the filter and is advantageously less than 15:1 in order to avoid excessive losses.

The following example further illustrates more specifically a high pass filter in accordance with the invention. The filter comprises a length of circular waveguides having an inside wall which has a 3.9 cm. radius and which is composed of alternate sections of silver and stainless steel. The silver can be coated onto a less expensive metal waveguide. The stainless steel sections are 5 cm. long and the silver sections are 3 cm. long. The entire filter comprises 20 sections of each type.

The graphical illustration of FIG. 2 compares the transmission propagation characteristic of this filter with those of a uniform silver waveguide having the same cutoff frequency, F.sub.c , at about 2.95 gigahertz. Curve 1 shows the phase propagation constant, B, measured in radians per meter, of the transmitted wave as a function of frequency for the above-described periodic filter. Curve 2 shows the same characteristic for a uniform waveguide. It will be noted that in the region between 3.04 and 3.1 gigaHertz, the characteristic of the periodic filter is more nearly linear than that of the uniform filter, and that the periodic filter has a much sharper cutoff at 3.03 gigaHertz

Curve 3 shows the attenuation propagation constant, .alpha., measured in nepers per meter for the periodic filter; and curve 4 shows .alpha. for the uniform waveguide. It will be noted that .alpha. for the periodic filter is more linear than that for the uniform waveguide and is only slightly greater.

In addition to having more linear characteristics for the transmitted wave, the periodic filter also has more linear characteristics for the reflected wave as can be inferred from FIG. 2 by examining the curves below cutoff frequency. Thus the structure can also be used as a low-pass filter by selectively extracting the reflected signal as is illustrated in FIG. 3. In the figure there is shown a low-pass filter comprising a three-port circulator 31 with a terminated high pass filter 32 at one of the ports. In particular, a signal enters the device at the input port 1, passes to port 2 where it enters a periodic high pass filter 32 as described above. The frequencies above cutoff pass through the filter to termination 33 while the frequencies below cutoff are reflected back and leave through the output 34 at port 3.

As is well known, band-pass and band-reject filters can be made from high pass and low-pass filters by selectively combining the outputs of a pair of filters having different cutoff frequencies.

FIG. 4 illustrates an improved band-pass filter in accordance with the invention comprising a high pass filter as described in connection with FIG. 1 in series with a low-pass filter as described in connection with FIG. 3. The low-pass filter is designed to pass only frequencies below a certain frequency, F.sub.1 , and the high pass filter is designed to pass only frequencies above a certain frequency, F.sub.2 . If F.sub.1 is larger than F.sub.2 , only frequencies in the band between F.sub.2 and F.sub.1 will pass to the output 45.

FIG. 5 illustrates an improved band-reject filter according to the invention. The device comprises a pair of circulators 51 and 53, a pair of periodic filters 52 and 54, a termination 56 and a directional coupler 55. A signal enters the input 50 at port 1 of circulator 51, passes through port 2 to filter 52. The portion above the cutoff frequency F.sub.1 passes through the filter to port 1 of the second circulator 53 where it passes to port 2 to a second filter 54. Frequencies above the cutoff frequency, F.sub.2 , of the second filter pass through to directional coupler 55. Frequencies below cutoff are reflected and dissipated at termination 56. Referring back to circulator 51, those frequencies reflected by filter 52 pass to port 3 and then enter the directional coupler 55. Thus at the output of the directional coupler are all frequencies below F.sub.1 and above F.sub.2 .

In all cases, the above-described arrangements are merely illustrative of a small number of the many possible specific embodiments which can represent applications of the principles of the invention. For example, while a specific example using circular waveguide has been described, rectangular waveguide can also be used, and while silver and stainless steel were given as examples of metals having the required conductivity ratios, many other pairs of metals, such as, for example, brass and silver or brass and copper, can also be used. Thus numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

Claims

What I claim is:

1. An electromagnetic wave filter comprising a waveguide of essentially uniform diameter having conductively bounded walls, said walls comprising alternate sections of conductive material having different conductivities, the ratio of the conductivities being between two and fifteen.

2. A device according to claim 1 wherein there are between 10 and 50 sections of each conductivity.

3. A device according to claim 2 wherein said waveguide has a cutoff frequency and the length of two of said alternate sections lies between 0.25 and 5 wavelengths at the cutoff frequency.

4. A device according to claim 3 wherein the length of a section of the lower conductivity is greater than that of a section of the higher conductivity.

5. An electromagnetic filter comprising a filter in accordance with claim 1 and means for selectively extracting the transmitted and the reflected portions of an input signal.

6. An electromagnetic wave low-pass filter comprising a three-port circulator including an input, a filter according to claim 1 terminating the second port and an output port.

7. A band-pass filter including a pair of filters according to claim 1 having different frequency characteristics.

8. A band-reject filter including a pair of filters according to claim 1 having different frequency characteristics.