Log Periodic Antenna With Foreshortened Radiating Elements

1. Field of the Invention

This invention relates to log periodic antennas and more particularly to log periodic dipole or monopole antennas with foreshortened radiating elements.

2. Description of the Prior Art

U.S. Pat. No. 3,732,572 describes a log periodic antenna in which radiating elements {dipoles} are foreshortened in order to conserve space without adversely affecting antenna performance. This is accomplished by configuring each size-reduced dipole with the interior profile of a double ridged waveguide. By use of this technique, a reduction of 45% in the length of the monopole or dipole antenna elements is achieved. However, when these foreshortened dipoles are used as radiation elements in a practical commonly used log periodic dipole antenna, the width of such antennas cannot be reduced as much. The reason for this is that these antennas have taper angles of approximately 25.degree.. Therefore the available spacing between adjacent dipole elements is relatively small and this prevents the use of foreshortened dipole elements with large width-to-length (B/A) ratios when the planes of these elements are parallel to the antenna axis. For example, the width of a log periodic dipole antenna with a taper angle of 20.degree. can be reduced up to 30%, and one with a 30.degree. taper angle can only be reduced by approximately 25% Log periodic dipole antennas having a very small taper angle (less than 10.degree.), and therefore characterized by long structures and large spacings between adjacent dipole elements, can be reduced up to 40% because foreshortened dipoles with large B/A ratios can be used. Nevertheless the 40% reduction in antenna width is still not sufficient to enable the antenna to fit into the space available in many applications. In addition, log periodic antenna structures with small taper angles may not fit into the space available in the orthogonal direction and a structure with a larger taper angle must be used.

This invention is directed to an improved antenna construction which overcomes these limitations.

A general object of the invention is the provision of a log periodic dipole or monopole antenna having radiating elements foreshortened by up to 70% and electrical performance (VSHR) equivalent to that of a corresponding conventional log periodic antenna.

Another object is the provision of a simple technique for designing a log periodic antenna with foreshortened radiators when available space is severely limited.

These and other objects are achieved with a radiating element (monopole or half a dipole) comprising a plane rectangular conductive body and a conductive stem connected at one end to one of the feed lines and at the opposite end to the edge of the body remote from the feed line, the stem being parallel to and closely spaced from the body.

FIG. 1 is a schematic plan view of a log periodic dipole antenna of the prior art.

FIG. 2 is a view similar to FIG. 1 showing an embodiment of this invention.

FIG. 3 is an enlarged view of one of the dipoles of the FIG. 2 antenna embodying the invention and showing details of construction.

FIG. 4 is an edge view of part of the dipole taken on line 4--4 of FIG. 3.

FIG. 5 is a view similar to FIG. 3 showing a monopole embodying the invention.

FIG. 6 is an enlarged view of part of FIG. 5 showing details of the monopole feed.

FIG. 7 is a schematic plan view of half a dipole embodying a modified form of the invention.

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

FIG. 9 is a schematic plan view of a dipole embodying the invention and fabricated using the printed circuit board technique.

FIG. 10 is a section taken on line 10--10 of FIG. 9.

FIG. 11 is an enlarged view of part of FIG. 10.

Referring now to the drawings, FIG. 1 shows a the log periodic dipole antenna 10 described in U.S. Pat. No. 3,732,572 and having an axis X, an axially extending feedline 11, preferably a coaxial cable as shown, conventional rod-like dipoles 12 at the high frequency (right as viewed) end of the antenna, and foreshortened dipoles 14 at the lower frequency end. Each of conventional dipoles 12 has a length corresponding to .lambda./2 where .lambda. is the resonant frequency of the the particular dipole. Foreshortened dipoles 14 each comprise rectangular bodies 15 connected by wire-like stems 16 to feedline 11; each such foreshortened dipole has a length <<.lambda./2. The principle underlying the foreshortening of dipoles 14 of antenna 10 without significant change in operating characteristics is fully explained in U.S. Pat. No. 3,732,572. Through application of this principle to a conventional log periodic dipole antenna with a taper angle of 30.degree., the physical length of the conventional dipole has been reduced about 25%, i.e., the width W' of the low-frequency end of antenna 10 is about 25% of the width W of the conventional dipole antenna.

The log periodic dipole antenna 10' shown in FIG. 2 is similar to antenna 10 of FIG. 1, like reference characters indicating like parts on the drawings, except that the two low-frequency dipoles 18 shown at the left in the drawing are constructed in accordance with this invention as described in detail below. The width W" of antenna 10' is approximately 50% of the width W of the conventional dipole antenna, thus enabling its adaptation and use in space limited applications too stringent for antenna 10.

Dipole 18 comprises identical rectangular bodies 19a and 19b, see FIGS. 3 and 4, connected by stems 20a and 20b to feed line 11 which extends along axis X of the antenna and is shown as a coaxial cable having an outer conductor 11a and an inner conductor 11b; stem 20a is connected to outer conductor 11a and stem 20b to inner conductor 11b. In this embodiment, each of bodies 19a and 19b is a plane electrically conductive sheet and each of stems 20a and 20b is an electrically conductive strip. The principal difference in the structure of dipole 18 compared to that of prior art dipole 14 shown in FIGS. 1 and 2 is that each of stem 20a and 20b extends parallel to, is slightly spaced by a distance t from the plane of the associated body 19, and is connected to the edge 21 of body 19 remote from axis X. The dimension t is relatively small, i.e. in the order of 1/10 to 1/20 of the dipole length, so that for operational purposes the antenna is essentially two-dimensional.

As explained in U.S. Pat. No. 3,732,572, the dipole length reduction factor is directly related to the ratios of parameters B/A and B/D, see FIG. 3. The larger the ratios, the larger the reduction factor obtainable. There is, however, a practical limit to the extent such ratios can be increased. Too large a ratio of B/D makes the antenna difficult or impossible to fabricate. Too large a ratio of B/A results in antenna support problems and incompatibility with log periodic dipole antenna design. For these reasons, ratios B/A and B/D in the antenna described in the above patent are limited to 0.3 and 20, respectively. Furthermore this prior art antenna has a near optimum ratio of S/A of 0.55. In comparison, a dipole foreshortened in accordance with this invention had the following dimensional ratios:

B/A--0.3

B/D--10

S/A--0.05

The latter dipole had the same resonant frequency as that of a conventional linear dipole having a length 2.5 times the dimension A in FIG. 3. This corresponds to a reduction factor of 60% as compared to the dipole described in the above patent where the maximum obtainable reduction factor is approximately 38% with the same B/A and B/D ratios. However, when these dipoles or monopoles are used as stand alone antennas, B/A ratios >0.3 can be used. In which case, a reduction factor of 70% is obtainable.

The invention may also be practiced with a monopole antenna 22, see FIGS. 5 and 6, having the same structure as one-half of dipole 18 and mounted over a planar conductor 23 such as a metal sheet or ground, like reference characters indicating like parts on the drawings. Antenna 22 is fed by coaxial cable 11 with inner conductor 11a connected to stem 20 and outer conductor 11b connected to planar conductor 23.

Another embodiment of the invention is shown in FIGS. 7 and 8 in which dipole 25, one-half of which is illustrated, has an open-faced rectangular body 26 connected by stem 27 to feedline 11. Body 26 may be formed by wire or the like as described in the above patent, stem 27 being slightly laterally spaced from body 26 and connected to the central part of the edge 28 thereof remote from feed line 11. This form of the invention is useful in outdoor applications where wind is a factor.

FIGS. 9, 10 and 11 illustrate another embodiment of the invention in which the dipole 30 is formed by the printed circuit (PC) technique, like reference characters indicating like parts on the drawings. One-half of dipole 30 is formed on one side of PC board 31, the other half on the opposite side. Each of bodies 19a and 19b of the dipoles preferably is in sheet form as shown but may also have an open configuration as in dipole 25 (FIGS. 7 and 8). Stems 20a and 20b connect to and are integral with strip feed lines 30 and 31 on opposite sides of board 31 and are connected at the outer edges, respectively, of bodies 19a and 19b by pins 32 extending through holes 33 in board 31. The thickness t' of board 31 corresponds to the lateral offset t of stems 20 from the planes of rectangular bodies 19 in the embodiments described above. The high production and precision capabilities inherent in forming such antennas by printed circuit technology make this embodiment of the invention highly cost effective. The formation this antenna by printed circuit technology enables high volume production with corresponding high quality.