Crossed Log-Periodic Dipole Antenna and Method of Making Same

1. Field of the Invention

The present invention relates to log-periodic antennas and more particularly to an improved crossed log-periodic dipole antenna and a method of constructing same.

2. Description of the Prior Art

The crossed log-periodic dipole antenna (LPDA) consists of two orthogonally displaced indentical log-periodic dipole antennas. The crossed LPDA provides two independent, orthogonal linear polarizations. When used in a polarimeter system the purity of the two orthogonal linear polarations, which are defined by the cross-polarization level, is very important to the performance of the system. The cross-polarization level of crossed LPDAs used in these systems should be at least 20 dB below the beam peak of the matched polarization. Circular polarization can also be obtained with the crossed LPDA with the use of a 90.degree. hybrid. Circular polarization performance also depends on the cross-polarization level which should be 15 dB or more below the matched polarization. For a well constructed crossed LPDA operating in the VHF or UHF frequency range, 15 or 20 dB cross-polarization level is relatively easy to obtain. At such wavelengths the length of the smallest dipole is much greater than the diameter of the feed booms.

A crossed LPDA designed to operate at frequencies of 12 GHz or higher cannot be assembled with this conventional construction technique. At such wavelengths the diameter of the feed boom is too large compared to the length of the dipole. For high VHF and microwave frequency LPDA operation, semi-rigid coaxial cables are often used as support booms as well as the feedlines for the antenna. The dipole halves are soldered directly to the semi-rigid coaxial cables. The 0.141" and 0.085" diameter coaxial cables are often used as the feedline/boom for LPDAs in the high VHF and microwave frequencies because of their relatively small size and rigidity. For crossed LPDAs which are required to operate at 12 GHz or higher frequencies, even the small 0.085" semi-rigid coaxial cable has too large a cross section for proper performance. At such frequencies, using conventional construction techniques, the diameter of the feed booms is so large compared to the length of the smallest dipole halves that the displacement between the two smallest dipole halves is almost the same as the length of the smallest dipole half. Performance analysis of this type of antenna is based on the assumption that each dipole has two colinear dipole halves with a feed source in the center of the dipole. In other words, the displacement between the two dipole halves is assumed to be zero, and the dipole halves are free of interference from the excitation apparatus (feedline/boom). A significant displacement between dipole halves introduces undesireable cross polarization. Moreover, close proximity of the feedline/boom structure to a significant length of the dipole halves perturbs the radiation performance of the dipoles, that is, the dipole is electrically shadowed by the feedline/boom itself.

One technique for improving the performance of this antenna is to reduce the size (diameter) of the semi-rigid coaxial cable which is also used as the antenna boom. Such an approach not only reduces the displacement between dipole halves but also reduces the shadowing effect as well. A typical diameter of such cable is 0.047". However, there are two major problems in the fabrication of crossed LPDAs using 0.047" semi-rigid coaxial cables. They are:

(1) If the crossed LPDA has a high frequency requirement of 12 GHz or higher and has an operating band of 6:1, the 0.047" coaxial cable has insufficient structural rigidity to support the larger dipole elements.

(2) Since the dipole halves are soldered directly to the antenna boom, the close proximities of adjacent dipole halves make soldering nearly impossible. This is because heat transfer adversely affects the already soldered nearest dipole element. While exotic processes such as laser welding, electron beam welding or electro-plating are available to attach dipole halves to the boom simultaneously, the required assembly fixtures are elaborate, quite expensive and a new fixture is required for each new design.

A technique was developed approximately 10 years ago to solve these problems.

(1) The radiating element, which consists of dipole halves and their supporting boom, is constructed using a chemical etching process from 0.16" thick brass sheets. FIG. 1 illustrates two types of these radiating elements, one with a straight boom 1(a) and the other with a tapered boom 1(b).

(2) Two 0.047" coaxial cables are soldered to two dipole halves booms for feedline. Two copper or brass rods having a diameter similar to the diameter of the feed coaxial cable are soldered to the other two dipole half booms for symmetry.

(3) The four dipole halves, two with coaxial cables feedlines and two with brass or copper rods, are then arranged to form a crossed LPDA. The four dipole halves can be formed into two different configurations of crossed LPDAs which are shown in FIG. 2(a) and 2(b). The configuration shown in FIG. 2(a) have the coaxial cable feedlines on the inside of the dipole halves. In this configuration, the two dipole halves (which form each planar LPDA) are considerably spatially displaced. If this antenna is designed to operate up to 18 GHz, the smallest dipole half is approximately 0.16" and the displacement between the two smallest dipole halves is in the neighborhood of 0.14" which is almost the same as the smallest dipole half. The crossed LPDA structure shown in FIG. 2(b) is characterized by a straight or tapered dielectric square rod which is used to support the four dipole halves, one on each side. The size of the square rod is determined by the outline of the dipole supporting boom. The four dipole halves and supporting booms form two orthogonal pairs of balanced microstrip transmission lines. The size of the square rod and the dielectrical constant of the material used to make the rod determine the characteristic impedance of the microstrip transmission lines. In this configuration, the spacing between the smallest dipole halves is acceptable, but the proximity effect or electrical shadowing effect of the feed cable is quite large. It can be seen from FIG. 2(b) that the 0.047" coaxial cable is shadowing almost of 50% of the dipole halves. The shadowing effect pertubates the performance of this antenna. Thus, this configuration is also undesirable.

This invention is directed to an improved LPDA construction and method of making it which overcome the above mentioned disadvantages.

A general object of the invention is the provision of a microwave or higher frequency crossed LPDA construction in which displacement between two dipole halves is substantially minimized.

A further object is the provision of such an antenna construction in which the electrical shadowing effect of the feedline/boom on the dipole halves is substantially reduced.

Still another object is the provision of such an antenna construction that has sufficient rigidity to support larger dipole elements present in a broad frequency band crossed LPDA.

A further object is the provision of a method of making such an antenna which achieves the foregoing objects efficiently using conventional low cost soldering techniques.

These and other objects of the invention are achieved with an LPDA construction in which each half-dipole structure comprises a boom, and half-dipole assembly formed from a single conducting sheet with a central boom portion of reduced thickness and press fitted against and permanently bonded to the boom. The curved boom portion of the one-piece conducting sheet reinforces the boom throughout its length to provide the desired rigidity. The invention also includes a method of making such an antenna by forming half-dipole assemblies from a single conductive sheet with a central boom portion from which the half-dipoles project, press fitting the boom portion of each assembly tightly against the adjacent boom, and permanently bonding the boom portion to the boom.

FIG. 1 is a plan view of a portion of two different types of radiators of a log-periodic prior art antenna.

FIG. 2 is an enlarged transverse section of a crossed LPDA's showing two prior art feedline/boom constructions.

FIG. 3 is a perspective schematic view of a portion of a crossed LPDA of the prior art.

FIG. 4 is a slightly enlarged transverse section of the prior art antenna of FIG. 3 showing the feed point detail of a typical feedline/boom construction.

FIG. 5 is a transverse section of an LPDA embodying this invention showing the boom/feedlines as coaxial cables.

FIG. 6 is an enlarged end view of a boom/feedline and half-dipole assembly connected thereto.

FIG. 7 is a plan view of a portion of the half-dipole assembly forming part of this invention.

FIG. 8 is an end view taken on line 6--6 of FIG. 7.

Referring now to the drawings, FIGS. 3 and 4 show a prior art crossed LPDA construction 10 having a longitudinal axis A and consisting of two orthogonally displaced identical axially extending LPDAs 11 and 12, antenna 11 being vertically polarized, as viewed, and antenna 12 being horizontally polarized. Since the LPDAs are identical, except for their polarizations, only one, antenna 11, will be described, like reference characters and their primes indicating like parts on the drawings. Antenna 11 has two laterally offset booms 13 and 14 to which half-dipoles 15 and 16, respectively, are connected; the half-dipoles are arranged in pairs, the half-dipoles of each pair being identical in length, transversely aligned and laterally offset for functioning as a single dipole element. The lengths and axial spacings of the dipole elements increase from the high frequency end to the low frequency end of construction 10 in increments of a predetermined ratio as is well-known in the art of log-periodic antennas.

Booms 13 and 14 are electrically connected to a feed source 17, such as a transmitter or receiver, and function as feedlines for the associated dipole halves. As shown in FIG. 6, booms 13 and 13' of antennas 11 and 12 may consist of coaxial cables having outer conductors 19 and 19' connected to the associated dipole halves by soldering or the like and inner conductors 20 and 20' electrically connected to opposed booms 14 and 14', respectively, shown as solid bars. Antennas 11 and 12 are supported in appropriate laterally spaced relation by one or more dielectric spacers 21. The disadvantage of such conventional construction techniques at frequencies above 8 GHz is electrical shadowing of the dipoles by the booms as is readily apparent in FIG. 6; such shadowing adversely affects performance of the antennas.

Referring now to FIGS. 5 and 6, a crossed LPDA construction 22 embodying this invention comprises four half-dipole structures 24, 25, 26 and 27 arranged in opposed pairs to constitute two orthogonally displaced LPDAs. Structures 24 and 26 are mirror images of each other as are structures 25 and 27. Each of structures 24, 25, 26 and 27 comprises a boom/feedline 28 and a half-dipole assembly 30. As its name implies, boom/feedline 28 functions both as a supporting boom and as a feedline for the antenna, and for sake of convenience will be referred to herein by the abbreviated name "boom." Booms 28 preferably are identical and each comprises a semi-rigid coaxial cable as shown having an inner conductor 31 and an outer conductor 32. Inner conductor 31 of one boom is electrically connected to the outer conductor 32 of the opposite boom.

Assembly 30 of each structure is formed by chemical milling or the like from a single sheet of electrically conductive material such as half-hard brass or beryllium copper and comprises a central longitudinally extending boom portion 34, see FIGS. 7 and 8, and log-periodic half-dipole elements 35. In order to connect assembly 30 to boom 28, boom portion 34 is press-formed into a semi-cylindrical cross-sectional shape with a diameter equal to the outside diameter of boom 28 and is permanently bonded to boom 28 throughout its length by soldering or the like. Such connection additionally contributes to the rigidity of the boom. Since a brass sheet of the desired length having a thickness sufficient to make elements self-supporting is difficult to press-form into such a shape, the wall thickness of the brass sheet along boom portion 34 is reduced by chem-etching or by conventional machining techniques by about one-half prior to the press-forming operation. Such reduction of wall thickness of boom portion 34 not only makes the shaping operation thereof easier to perform but also aids in minimizing the lateral offset spacing between the halves of the dipoles.

By way of example, semi-rigid coaxial cables having outside diameters of 0.047" have been used successfully as booms 28. A brass sheet having a thickness of 0.016" is used for the half-dipole assembly 30, the boom portion 34 thereof having a thickness of 0.008" or less. For a crossed LPDA construction embodying this invention and designed to operate up to 12 Ghz, the chemical-milling technique is satisfactory to form elements 35. For applications requiring operation at frequencies above 12 GHz, electronic discharge machining (EDM) is preferred for forming elements 35. In the latter process, the thinning of boom portion 34 is performed first using conventional machining techniques and elements 35 are thereafter formed by EDM.

It will be noted that practice of this invention enables use of relatively small diameter coaxial cables as the booms to minimize electrical shadowing. Also any compromise of rigidity in boom 28 because of its desirably small diameter is more than compensated by bonding of the semicylindrical boom portion 34 thereto throughout its length.

The invention also includes the method of making the crossed LPDA construction consisting of forming each of the half-dipole assemblies from a single piece of electrically conductive material and thereby providing a central longitudinally extending boom portion and associated half-dipole elements projecting therefrom, reducing the thickness of the boom portion, forming the boom portion to conform to the shape of the exterior of the boom, pressing the boom portion tightly against the outer surface of the adjacent boom, and permanently bonding the boom portion to said adjacent boom.