US3355740A - Log-periodic zig zag antenna - Google Patents

Description

Nov. 28, 1967 MAYES 3,355,740

LOG-PERIODIC ZIG ZAG ANTENNA Filed April 4, 1966 2 Sheets-Sheet 2 RADIAHON PATTERN l 1 3Q ZIG-ZAG ELEMENT United States Patent 3,355,740 LOG-PERIODIC ZIG ZAG ANTENNA Paul E. Mayes, Champaign, Ill., assignor to University of Illinois Foundation, a non-profit corporation of Illinois Filed Apr. 4, 1966, Ser. No. 549,084 4 Claims. (Cl. 343792.5)

This application constitutes a continuation-in-part of the previous applications of this inventor, namely, Serial No. 238,666, filed November 19, 1962, now abandoned and Serial No. 190,624 filed April 27, 1962, now abandoned.

This invention relates to log periodic antennas and in particular to a zig-zag antenna of this type having a unidirectional radiation pattern and a relatively high level constant impedance which is substantially independent of frequency over a wide bandwidth. Such an antenna is especially advantageous for minimizing signal losses in the commercial television frequency range.

The log periodic class of frequency independent antennas have now come into general use in the art primarily because of their ability to provide a substantially constant radiation pattern and impedance level over a wide frequency band. Log periodic antennas in the form of coplanar dipole arrays are presently in use for the reception of frequencies in the television band in view of the above desirable operating characteristics. Another form of log periodic antenna includes a series of zig-za-g elements having teeth-like portions protruding from both sides of a central conducting member which interconnects all of the elements throughout the antenna length. This type of antenna has not been generally used for the reception of television signals because of its low value of input impedance. In order to maintain losses in signal strength between the antenna and a television receiver at a minimum, the antenna impedance should be closely matched to the receiver input impedance and the impedance of the transmission line coupling the antenna and the receiver. Commercially available transmission lines have a characteristic impedance of approximately 300 ohms. The input impedance of log periodic zig-zag antennas according to the prior art is approximately 100 ohms and such a difference in impedance levels produces an impedance mismatch which results in substantial loss of signal strength.

According to the present invention, a pair of conducting members are positioned to continuously extend away from a feed point, with at least one of the conducting members having a continuous conducting element in a spreading substantially zig-zag pattern, such that the conducting element is formed of reversing V-shaped sections electrically coupled solely in an end-to-end manner. It has beendetermined that an antenna constructed according to the principles of the present invention provides a significant increase in input impedance over that available in the prior art. The present antenna provides high directive gain and yet is extremely simple in its configuration and arrangement of components since antenna operation is not greatly dependent upon exact dimensions of the elements. Fine adjustments are therefore not essential to proper operation.

tor as will be more particularly hereinafter described,

the antenna presents an input impedance of between 260 and 320 ohms over the desired operating band. In particular, a balanced array of two zig-zag elements according to this invention provide a compact antenna for efiiciently receiving signals in the UHF-TV band. Due to the relatively small and compact configuration of this antenna, an array of two or more of such balanced zig-zags increase the antenna gain and yet maintain the overall structural size of the antenna within reasonable limits.

The invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying drawings in which:

FIGURE 1 represents in plan view a log-periodic zigzag antenna extending above and over a ground plane;

FIGURE 2 is a side elevational view showing the an tenna of FIGURE 1 in elevation with respect to the ground plane;

FIGURE 3 is a modification showing the antenna invention in plan somewhat like FIGURE 1 but showing two zig-zag units in a balanced configuration fed from a two-wire line at substantially a common point;

FIGURE 4 is an elevation view of the antenna of FIGURE 3;

FIGURE 5 is a modification showing the zigzag elements generally similar to the arrangement of FIGURE 3, but wherein one zig-zag element takes the form of a coaxial line which is also used as a feed for the balanced antenna structure, the second element being a zig-zag formed from the continuation of the center conductor of the coaxial cable;

FIGURE 6 is an exemplification of the radiation pattern of the antenna of FIGURE 1, assumed to be fed from a coaxial feed line or cable and providing a backfire radiation; and

FIGURE 7 is a typical radiation pattern of the antenna arrangement shown by FIGURE 3 with a single central lobe indicating the pattern where the antenna zig-zag elements are in balanced or adding relationship, this being shown by the solid line, and the dotted line patterns showing the split line radiation pattern when the halves of the antenna structure are turned relative to each other in an unbalanced configuration. Both of these patterns can be obtained simultaneously from the structure of FIGURE 3 by appropriate connections to the twowire feeder which are standard in the art.

Referring now to the drawings for a further understanding of the invention, reference may be made first to FIG- URE 1. In this figure, one member of the antenna is shown as comprising a conducting element 11 which is bent back and forth generally in a zig-zag arrangement. The zig-zag arrangement of the antenna is fed from its smaller end at a terminal 13 which is connected, as schematically shown particularly by FIGURE 2, to a central conductor 15 of a coaxial line 17. The outer conductor of the coaxial line is connected to a ground plane, forming the second antenna member and represented at 19 in the fashion indicated, at the connecting point 21.

As is shown particularly by FIGURE 1, the width of the conductor is defined by the angle 3. The angle which the zig-zag element assumes relative to a line centrally positioned with respect to the antenna as a whole is represented by the taper =angle oz. It may be assumed that the lower portion of the antenna shown as adjacent to the solid line represents this amount of offset.

Also, as can be seen from the showing of FIGURE 1, the antenna is so constructed that each complete zig-zag section represents what may be termed as a single cell. This axial length of the n cell is shown for reference purposes as d,,. It thus may be considered, illustratively, as related particularly and specifically to one particular frequency within the selected range for which the antenna is designed to function. As seen by the showing of FIG- URE 1, the conductors 23a, 23b, etc. :are all positioned substantially parallel with respect to each other. Similarly,

conductors 24a, 24b, etc. are substantially parallelly positioned with respect to each other, thus making the zigzag cells substantially similar. Furthermore, adjacent cells are electrically coupled solely end to end without a central conducting element as in the prior art. The difference in conductor thickness represented by the angle continues, of course, from one end of the antenna to the other. This is not a rigid requirement, but is a requirement which should be given consideration as will be hereinafter described.

The maximum width (w) should be at least equal to /z the wave length at the lowest frequency to be received. Further, the scaling factor (T) establishes the change in the cell width. If the index of the cells increases with the decreasing size, the (rt-l-l) cell will be the next smaller cell adjacent to the n" cell. The scaling factor (T) is then defined as and will be less than or equal to unity. Unity 1- antennas are suitable where the bandwidth requirements are not extreme. In general, for efficient antenna operation over a .broad frequency band, the scaling factor should be less than or equal to 0.95. The cell Widths from one zig-zag section of the antenna to the next then vary in accordance with the scaling factor which is preferably maintained independent essentially of the cell index. Considering the radiation pattern as particularly depicted from the antenna of FIGURES 1 and 2, it will be seen from the showing of FIGURE 6 that this is at a selected angle with respect to the ground plane where the coaxial feed is as above explained.

From what has been stated it will be seen that the impedance of the log-periodic antenna is changed very little as the frequency is varied. Further, the cells vary in their parameters with respect to each other in accordance with the selected scaling factor. The antenna components 24a, 24b, etc. are essentially parallel to each other, as are the elements 23a, 23b, etc. The antenna zig-zag sections gradually change in accordance with the multiplier or scaling factor, so that as the feed point 13 is approached relative to the end 27 of the antenna, the change in cell parameters at substantially the selected scaling factor is evident. The impedance of the log-periodic zig-zag antenna changes very little as the frequency is varied. In typical operation curves, it can be shown that the response is generally uniform even with wide frequency variations and changes.

Considering now particularly the arrangement of FIG- URES 3 and 4, double-legged free-space antenna structures have been illustrated where the two zig-zag conducting members or elements lie in quite different planes from each other at a separation angle 7. In the showing of FIG- URE 3 a generally balanced construction has been shown, as compared to the unbalanced antenna of FIGURES l and 2. In the balanced structure of FIGURE 4 a two-wire balanced line composed of conductors 33 and 35 respectively, is used to feed the antenna elements 31 and 29. These antenna elements also have the backfire radiation pattern of the antennas of FIGURES 1 and 2, that is, radiation substantially in the direction from the element toward the feed point. For purposes of identification it is preferable to look upon the balanced structure as being one where the backfire radiation occupies a pattern generally like that shown by 36 in FIGURE 7 when the antennas, such as antennas 31 and 29, are set to aid each other. In this configuration, a highly directive radiation pattern is obtained. If however, one of the zig-zag elements is rotated about its axis through an angle of 180 so that the radiation patterns of the zigzag conductors are not in adding relationship, then a radiation pattern in the form of split lobes, such as the lobes 37a and 37b, is realized. The general pattern of radiation then corresponds to what is shown and depicted and half of this split lobe pattern corresponds to that obtained with a single unbalanced zig-zag element over ground, as previously described.

From this showing it will be appreciated that where the value of T is less than unity and preferably the values of a and d are such that the input impedance assumes a desired value, the antenna structure can be designed to operate over any desired frequency range with operating characteristics which do not change appreciably with frequency.

As an indication of the manner in which the antenna parameters of the log periodic zig-zag antenna, such as shown in FIGURES 3, 4, and 7, of this invention can be varied to provided the desired value of input impedance, the variation of a, the taper angle; [3, the angle of increase in conductor width; and '7', the scaling factor between adjacent cells has the following effect on antenna input impedance:

(1) Increasing or (maintaining p and aconstant) increases the impedance;

(2) Increasing ,8 (oz and 1- constant) decreases the impedance, and

(3) Increasing r (a and ,8 constant) increases the impedance.

Insofar as the radiation pattern is concerned, the directivity in the E-plane can be adjusted by changing the element parameters a and B, whereas the directivity in the H-plane is adjusted by varying the separation angle When :2, the beamwidths in the E- and H-plane are approximately equal.

In particular, where an input impedance of 300 ohms is desired, such as for the reception of signals in the television frequency band, the following range of parameters will provide a substantially unidirectional, high directive gain antenna with a matching level of input impedance between 260 and 320 ohms:

(1) a between 10 and 15;

(2) ,8 between 0 and 5,

(3) 1 between 0.9 and 0.95, and (4) 'y substantially equal to 20c The scaling factor can actually range between 0.85 and 0.95 and the angle a between 5 and 15 to provide a suitable matching impedance with respect to 300 ohms by increasing the angle 7 beyond the value 20:. Thus, the antenna is very simple in configuration and can be readily constructed to provide proper operation without critical adjustment of parameters.

As an example of the teachings of this invention, a zig-zag antenna was constructed for operation in the UHF television band (4704390 mc.). A balanced configuration was utilized, as shown in FIGURES 3, 4 and 7 to provide a unidirectional radiation pattern, such as 36, with a relatively constant level of approximately 300 ohms input impedance over the desired band of operation. The antenna was constructed with a=7.5, 13:2.5 1-=0.9. The balanced configuration was approximately 31 inches in height (separation between sections), 16 inches wide and 43 inches long, from the narrow end to the wide end of the zigzag. This provided an increased directive gain of about 11-12 db above that for a dipole antenna.

Due to the relatively compact configuration, an array containing four zig-zag sections operating in the balanced mode was also constructed. This structure is approximately 31 inches in height, 29 inches wide, and 43 inches long, and provides an increased directive gain of about l4l4.75 db above a dipole. In both instances, the individual zig-zag sections were mounted on an insulating member formed of plastic material for support. Thus the antenna of this invention can be readily constructed and for its relatively small size provides a rugged antenna structure having a high degree of gain and a desired input impedance which are substantially frequency independent over a broad operating band.

Still another form of antenna is contemplated within this showing, as exemplified by FIGURE 5 where again the coaxial cable is provided as the central component of the device. In this arrangement, one zig-zag antenna element (as depicted by FIGURE- 1) is formed by bending a coaxial cable element back and forth upon itself in general Zig-zag or Z formation, taking care to see that the limiting conditions are not being avoided. Under these circumstances the zig-zag antenna formed from the coaxial conductor may have its outer shell 17 grounded in any desired manner behind the last largest cell. The central conductor is continued outside of the shell 17" and bent back and forth to form the second zig-zag element of the balanced antenna configuration. The antenna is fed in any desired manner from any suitable form of generator of which a conventional representation is made at 18.

Under some conditions, the antenna of generally zigzag formation may be fed and activated from a wave guide with the antenna unbalanced over ground. This is a form which is usually not recommended for frequency independent performance to the extent of the coaxial feeder above explained in connection with FIGURE 1. In either case, the directive gains are substantial. The particular component lengths are not critical except to remain within the general limits herein described and set forth. The directive gain with respect to frequency is substantially uniform within the range hereinabove proposed.

Various further modification of the invention may be utilized where desired and where falling completely and clearly within the scope of the invention as herein set forth.

What is claimed is:

1. A log periodic zig-zag antenna operating in the backfire mode and having a substantially unidirectional radiation pattern, said antenna comprising:

a pair of conducting members having respective feed points, said conducting members adjacently disposed at said feed points and extending outwardly at a selected angle to each other;

each of said members including a conducting element continuously extending outwardly from the respective feed point in substantially a zig-zag manner wherein the zig-zag pattern is formed from a plurality of generally reversing V'shaped sections electrically coupled solely in an end to end manner, said conducting elements being turned relative to each other to provide a single lobe backfire radiation pattern from the feed point;

each of said V-shaped sections having sides disposed and electrically coupled solely in an end to end manner to define a series of open V-shaped sections with substantially parallel respective sides; and

wherein each of said conducting elements includes a taper angle a between 5 and 15, an angle of increase in conductor width, 5 between 0 and 5, and a scale factor, 1- relating to the lengths of each adjacent V-shaped section, between 0.85 and 0.95, such that the input impedance of said antenna is suitable matched to 300 ohms and is substantially independent of frequency over a broad operating band.

2. A log periodic zig-zag antenna according to claim 1, wherein the selected angle between said pair of conducting members is equal to or greater than twice the taper angle at.

3. A log periodic zig-Zag antenna according to claim 1, wherein said antenna operates in the UHF band (470- 890 me).

4. A log periodic zig-zag antenna for television reception of signals in the UHF band, 470-890 me, operating in the backfire mode and having a substantially unidirectional radiation pattern, said antenna comprising:

a pair of conducting members having respective feed points, said conducting members adjacently disposed at said feed points and extending outwardly at a selected angle to each other;

each of said members including a conducting element continuously extending outwardly from the respective feed point .in substantially a zig-zag manner wherein the zigzag pattern is formed from a plurality of generally reversing V-shaped sections electrically coupled solely in and end to end manner, said conducting elements being turned relative to each other to provide a single lobe backfire radiation pattern from the feed point;

each of said V-shaped sections having sides disposed and electrically coupled solely in an end to end manner to define a series of open V-shaped sections with substantially parallel respective sides; and

wherein each of said conducting elements includes a taper angle a substantially equal to 7.5 an angle of increase in conductor width, 3 substantially equal to 2.5 and a scale factor, 7' relating to the lengths of each adjacent V-shaped section, substantially equal to 0.9, such that the input impedance of said antenna is suitably matched to 300 ohms and is substantially independent of frequency over said UHF band.

References ited UNITED STATES PATENTS 2,083,260 6/1937 Godiey et al. 343-7925 2,923,007 1/1960 Carpenter 343--792.5 2,977,597 3/1961 Du Hamel et al. 34 379*2.5 3,079,602 2/1963 Du Hamel et al. 343-792.5 3,210,768 10/1965 Hudock et a1 343-792.5 3,213,457 10/1965 Carr 343-792.5 3,221,330 11/1965 Berry 343-7925 ELI LIEBERMAN, Primary Examiner.

Claims (1)

1. 4. A LOG PERIODIC ZIG-ZAG ANTENNA FOR TELEVISION RECEPTION OF SIGNALS IN THE UHF BAND, 470-890 MC., OPERATING IN THE BACKFIRE MODE AND HAVING A SUBSTANTIALLY UNIDIRECTIONAL RADIATION PATTERN, SAID ANTENNA COMPRISING: A PAIR OF CONDUCTING MEMBERS HAVING RESPECTIVE FEED POINTS, SAID CONDUCTING MEMBERS ADJACENTLY DISPOSED AT SAID FEED POINTS AND EXTENDING OUTWARDLY AT A SELECTED ANGLE TO EACH OTHER; EACH OF SAID MEMBERS INCLUDING A CONDUCTING ELEMENT CONTINUOUSLY EXTENDING OUTWARDLY FROM THE RESPECTIVE FEED POINT IN SUBSTANTIALLY A ZIG-ZAG MANNER WHEREIN THE ZIG-ZAG PATTERN IS FORMED FROM A PLURALITY OF GENERALLY REVERSING V-SHAPED SECTIONS ELECTRICALLY COUPLED SOLELY IN AND END TO END MANNER, SAID CONDUCTING ELEMENTS BEING TURNED RELATIVE TO EACH OTHER TO PROVIDE A SINGLE LOBE BACKFIRE RADIATION PATTERN FROM THE FEED POINT; EACH OF SAID V-SHAPED SECTIONS HAVING SIDES DISPOSED AND ELECTRICALLY COUPLED SOLELY IN AN END TO END MANNER TO DEFINE A SERIES OF OPEN V-SHAPED SECTIONS WITH SUBSTANTIALLY PARALLEL RESPECTIVE SIDES; AND WHEREIN EACH OF SAID CONDUCTING ELEMENTS INCLUDES A TAPER ANGLE A SUBSTANTIALLY EQUAL TO 7.5*, AN ANGLE OF INCREASE IN CONDUCTOR WIDTH, B SUBSTANTIALLY EQUAL TO 2.5*, AND A SCALE FACTOR, R RELATING TO THE LENGTHS OF EACH ADJACENT V-SHAPED SECTION, SUBSTANTIALLY EQUAL TO 0.9, SUCH THAT THE INPUT IMPEDANCE OF SAID ANTENNA IS SUITABLY MATCHED TO 300 OHMS AND IS SUBSTANTIALLY INDEPENDENT OF FREQUENCY OVER SAID UHF BAND.

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