IL106274A

IL106274A - Lightning protection for antenna system

Info

Publication number: IL106274A
Application number: IL10627493A
Authority: IL
Original assignee: Israel State
Priority date: 1993-07-08
Filing date: 1993-07-08
Publication date: 1999-09-22
Also published as: IL106274A0; DE69420784T2; DE69420784D1; EP0633622B1; EP0633622A1

Description

106274/3 ΓΟ\ΜΝ mDnyo!? ρ-π TO ηιαπ LIGHTNING PROTECTION FOR ANTENNA SYSTEMS LIGHTNING PROTECTION FOR ANTENNA SYSTEMS FIELD AND BACKGROUND OF THE INVENTION The present invention relates to means for protecting electrical components from the damaging effects of lightning and, more particularly, to means for protecting a receiver or transmitter front end, which may be an amplifier, a pre-amplifier, mixer, detector, filter, limiter, T/R switch and the like, connected to an antenna, from being damaged when lightning strikes the antenna.

Typical antenna systems, such as those used in the transmission and reception of microwave signals, include an antenna dish mounted in a prominent position, usually on a mast and/or on top of a building, where the antenna dish can achieve line-of-sight contact with other antennas. The antenna is connected by a transmission line, typically a coaxial line or cable, to a front end located in the vicinity of the antenna, and from the front end to the rest of the components of the system.

The placement of the antenna in a prominent location, where the antenna is often among the tallest structures in the vicinity, makes the antenna highly vulnerable to a lightning strike. When lightning strikes the antenna dish, the coaxial line directs the high voltage impulse to the front end and, typically, destroys the front end, rendering the system inoperable and necessitating the replacement of the damaged component.

Various attempts have been made to protect the front end from lightning strikes. One such technique involves use of an appropriately rated spark gap between the inner and outer conductors. During normal operation, the two conductors are isolated from each other by the dielectric. However, when a sufficiently large voltage is encountered, a short circuit is created in the spark gap which prevents the impulse from reaching the front end. A disadvantage of the spark gap technique is that spark gaps are limited by capacitance considerations to use at relatively low frequencies.

There is thus a widely recognized need for, and it would be highly advantageous to have, means for protecting components connected to an antenna from the damaging effects of lightning, the protection means being operable over a wide range of frequencies.

ST JMM ARY OF THE INVENTION According to the present invention there is provided a wave transmission line offering protection for electronic devices connected to the line from the effects of a high voltage impulse of relatively low frequency, the line comprising a wave propagating line connected to at least one of the devices, the line characterized in that it includes at least one segment which is a metallic waveguide dimensioned to allow free passage of waves of relatively high frequencies during normal operation while substantially preventing the propagation of the relatively low frequency waves of the impulse.

According to further features in preferred embodiments of the invention described below, the wave propagating line consists of a coaxial line and a metallic waveguide. The metallic waveguide is either rectangular or circular in cross section.

The present invention successfully addresses the shortcomings of the presently known configurations by providing a simple solution to the problem of damage from lightning to electrical components connected to antennas and similar devices. While the description herein is concerned mainly with the protection of a front end connected to a microwave antenna against the detrimental effects of a lightning strike, it will be readily appreciated that the present invention can be applied in a variety of systems wherein it is desired to isolate an electronic component from the detrimental effects of a high voltage impulse of relatively low frequency.

The present invention discloses a novel wave propagating line which includes at least one segment made up of a metallic waveguide. The waveguide is characterized in that it prevents the transmission of waves below a certain frequency, depending on the dimensions of the metallic waveguide. The presence of the metallic waveguide in the transmission line, such as a conventional coaxial line, serves to isolate electronic equipment from the lightning.

BRTEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a schematic representation of one embodiment according to the present invention; FIG. 2 is a perspective partial cutaway view of a typical coaxial line; FIG. 3 is a perspective view of a metallic waveguide having a rectangular cross section; FIG. 4 is a perspective view of a metallic waveguide having a circular cross section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a means for protecting electronic equipment from the detrimental effects of certain high voltage impulses. Specifically, the present invention can be used to isolate a front end connected to a microwave antenna from the effects of lightning strikes.

Transmission lines are available in many forms. The most important and widely used of all the many possible transmission lines is the coaxial line, or coaxial cable, as it is sometimes called when used with flexible conductors. In a conventional microwave antenna system, the antenna is connected to a nearby amplifier, mixer, detector, and the like, through a suitable coaxial cable. A disadvantage of such a system is that a lightning strike of the antenna can destroy the front end.

The principles and operation of a system according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, Figure 1 illustrates a typical system according to the present invention. A transmission line 10, such as a coaxial line, is used to connect, for example, a microwave antenna 12, or portion of a compound antenna made up of a series of elements, and a front end which is made up, for example, of an amplifier 14.

In the present invention, transmission line 10 does not directly connect microwave antenna 12 and the front end, such as, for example, amplifier 14. Rather, a metallic waveguide segment, 16, of suitable dimensions, as described below, is connected in series with transmission line 10. The connection may be as in Figure 1 with metallic waveguide 16 connected at both ends to segments of coaxial lines 10. Alternately, metallic waveguide 16 may be connected at one end to either antenna 12 or amplifier 14 and to transmission line 10 at the other. In other embodiments, more then one segment of metallic waveguide 16 may be used. The combination of one or more segments of waveguide 16 connected in series with one or more segments of transmission line 10 together forms a wave transmission line.

A typical coaxial line 10 is shown in Figure 2. Coaxial line 10 includes a cylindrical inner conductor 18 of diameter dl which is wrapped by an annular dielectric 20 of outer diameter d2 insulating inner conductor 18 from annular outer conductor 22 which is, in turn, covered by an insulating jacket 24.

Two typical examples of metallic waveguides 16 are shown in Figures 3 and 4 which depict a metallic waveguide of rectangular cross section having a width 'a' and height 'b', and a metallic waveguide of circular cross section having a radius V, respectively. The metallic waveguide may be of any suitable design, including, but not limited to, single-ridged and double-ridged metallic waveguides.

Both transmission lines, such as coaxial lines 10, and metallic waveguides 16 propagate waves by modes. However, in contrast with transmission lines, such as coaxial lines 10, which operate at any frequency up to a maximum, or cutoff value, metallic waveguides 16 have minimum, as well as maximum, cutoff frequencies. The present invention takes advantage of the fact that metallic waveguides have minimum cutoff frequencies.

For example, it is known that in the common rectangular metallic waveguide (Figure 3) the lower cutoff frequency, fc, above which normal operation must occur if operation is to be at the lowest, or dominant, mode, is: fc = l/[(2a)(mI)-0.5] (1) where m and I are the permeability and the permittivity (dielectric constant), respectively, of the dielectric filling the guide. When air is used as the dielectric, Equation (1) reduces to: fc = c/(2a) (2) where c is the speed of light in vacuum.

As another example, in a circular metallic waveguide (Figure 4), the lower cutoff frequency, fc, is fc = 0.293 l/[r(mI)-0.5] (3) When air is used as the dielectric, Equation (3) reduces to fc = 0.293c/r (4) Thus, it is possible to select a metallic waveguide of a particular width or radius which will not allow waves of relatively low frequencies, such as the high voltage impulse of up to 100 MHz frequencies which are typical for lightning, to pass, yet allow the higher frequency waves encountered in normal operation to pass through unhindered.

The metallic waveguide may be of any suitable height and length, which may be selected so as to reduce losses at the transitions between the metallic waveguide and the coaxial line.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

9 10624/3 WHAT IS CLAIMED IS:

1. A wave transmission line offering protection for electronic devices connected to the line from the effects of a high voltage impulse of relatively low frequency, comprising a wave propagating line connected to at least one of the devices, said wave propagating line characterized in that it includes at least one first segment which is a metallic waveguide and dimensioned to allow free passage of waves of relatively high frequencies during normal operation while substantially preventing the propagation of the relatively low frequency waves of the impulse, said line is further characterized in that it includes at least one second segment which is a coaxial line.

2. A wave transmission line as in claim 1, wherein said waveguide is rectangular in cross section.

3. A wave transmission line as in claim 1, wherein said waveguide is circular in cross section.

4. A wave transmission line as in claim 1, wherein the wave transmission line includes a single segment of said waveguide connected at both ends to said wave propagating line. 10 10624/3

5. A wave transmission line as in claim 1, wherein the wave transmission line is connected to an antenna and said high voltage impulse of relatively low frequency is generated by lightning.

6. A wave transmission line as in claim 5, wherein said antenna is a microwave antenna.

7. A wave transmission line as in claim 5, wherein one of said electronic devices is an amplifier.

8. A wave transmission line as in claim 1, wherein said waveguide is selected from the group consisting of single-ridged and double-ridged waveguides. J / Mark M. Friedman Advocate, Patent Attorney Beit Samueloff, 1st floor 7 HaOmanim St. 67897 Tel Aviv