Transmission line conductor for log-periodic dipole array

Details Transmission line conductor for log-periodic dipole array Transmission line conductor for log-periodic dipole array

2001-12-13

2004-01-13

Le, Hoanganh (Department: 2821)

Communications: radio wave antennas

Antennas

Logarithmically periodic

C343S810000, C174S07000A, C174S1020SP

Reexamination Certificate

active

06677912

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to conductive elements for antennas and, more particularly, to a conductive element allowing improved log-periodic dipole array performance.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Log-periodic dipole array (LPDA) antennas are popular broadband antennas for many applications. An LPDA includes an array of electric dipoles having varying length extending outward from a pair of feed conductors. The pairs of elements are arranged from shortest to longest, with both the element length and the spacing between elements varying logarithmically along the antenna. The LPDA is a type of “quasi-frequency-independent” antenna, having relatively constant radiation pattern and input impedance characteristics over a frequency range extending (approximately) from the half-wavelength frequency of the longest dipole to the half-wavelength frequency of the shortest dipole.
The LPDA is typically oriented during use such that the end with the shortest elements is pointed in the desired direction of transmission or reception. Furthermore, the antenna is generally designed to be fed at the end with the short elements. These practices help to avoid pattern distortions by reducing effects such as shadowing, reflections, and excitation of harmonics in the longer elements. The feeding at the front end (the short-element end) of the antenna is typically accomplished by running a coaxial feed line along the interior of one of the conductors to which the antenna elements are connected. In this way, the feed signal can be brought to the front of the antenna, while the connector to the signal source (or receiver) is at the back (the long-element end). In such an arrangement, the inner conductor of the coaxial feed line is kept isolated from the outside of the conductor through which it is fed, and connected to the other conductor, so that the feed voltage is applied across the two conductors. An illustration of this connection at the front of an LPDA is shown in FIG.
1
. In this embodiment, feed line inner conductor
16
is isolated from outer conductor
12
by insulator
18
. Inner conductor
16
is connected to conductor
14
, which is in this case a solid conductor, so that the feed voltage may be dropped between conductor
14
and conductor
12
.
In addition to the mechanical convenience generally realized by having the connector at the back of the antenna, and the reduced possibility of pattern interference from having a connector at the front, the arrangement of
FIG. 1
provides the advantage of creating an intrinsic balancing mechanism for the antenna. Connection of a typical single-ended, or unbalanced, feed voltage directly across the front end of the antenna, on the other hand, would require use of an additional balancing transformer. In the particular configuration of
FIG. 1
, the inner surface of conductor
12
functions as the shield of the coaxial feed line. At the end of the conductor, currents induced in the shield may flow back along the outer surface of conductor
12
, resulting in a balanced line. (The antenna currents described herein are AC currents and exist only within a few skin depths of the surface of a conductor. AC current can therefore flow in one direction on one surface of a conductor, such as the inner wall of a tube, and in the other direction on another surface of the conductor, such as the outer wall of the tube. The tube wall is so many skin depths thick that the current on the inner wall doesn't “see” the current on the outer wall.) In some practical LPDA configurations, as discussed further below, the feed conductor is separate from the coaxial feed line shield. In such cases the feed line shield is connected to the conductor to allow the return current path.
In order for the antenna's radiation to be directed “forward” (out from the short-element end), even though it is being fed “backwards” (feed signal starting at short-element end and traveling along transmission line toward long-element end), the phasing of the feed signals seen by each dipole must be such that the radiation adds constructively in the reverse direction to that of the feed signal travel. In particular, alternating pairs of elements must be fed by signals 180° out of phase. Referring to
FIG. 1
, elements
20
a
and
20
b
constitute the first, and smallest, pair of dipole elements in the LPDA, while elements
22
a
and
22
b
constitute the next pair. Several other element pairs not shown in
FIG. 1
would typically be present, extending from points further along the feed conductors. It can be seen that for the first element pair in
FIG. 1
, the upper element is connected to conductor
14
, and the lower element to conductor
12
. For the second element pair, the opposite is true: the upper element is connected to conductor
12
, and the lower to conductor
14
. The feed voltage applied between the upper and lower halves of the first dipole, therefore, is of opposite polarity to the feed voltage applied between the corresponding halves of the second dipole. The third dipole (not shown) in such an arrangement would be connected in the manner of the first, the fourth in the manner of the second, and so on.
Because each feed conductor in the LPDA of
FIG. 1
has elements extending from it in two directions, the two feed conductors cannot be arranged side-by-side in the plane of the elements. Instead, feed conductors
12
and
14
are spaced apart within a plane perpendicular to that of the elements. This arrangement leads to an offset in position between the halves of the dipole, as represented by distance “D” between the positions of elements
20
a
and
20
b
in FIG.
1
. Ideal dipoles have both of their elements arranged along the same line, and the offset of
FIG. 1
can give rise to cross-polarization and pattern distortion. It would therefore be desirable to minimize this offset by, for example, minimizing the spacing between conductors
12
and
14
.
The spacing between the conductors is constrained by other design considerations, however. In fact, the conductor spacing affects the antenna performance more directly and strongly in other ways than through the cross-polarization distortion described above. As alluded to above and shown in
FIG. 1
, the combination of conductors
12
and
14
, and the currents flowing on their outer surfaces, give rise to an overall transmission line
24
feeding the LPDA. The characteristic impedance of this transmission line should be designed such that the input impedance of the antenna matches that of the entire system (including transmitter or receiver) to the extent possible. Furthermore, proper operation of the antenna itself may require the characteristic impedance to have a particular level (or at least lie within a particular range). The characteristic impedance of a transmission line such as line
24
depends upon factors including the spacing between conductors and the shape of each individual conductor. Some “feel” for this can be obtained by considering an approximate expression for the characteristic impedance Z
0
of an ideal two-wire transmission line:
Z
0
≈120 ln(2
D/d
),
where D is the conductor-to-conductor spacing and d is the diameter of each of the conductors. In order to reduce the spacing between conductors of the two-wire line without changing the characteristic impedance of the line, therefore, the diameters of the conductors must also be reduced.
The above expression generally does not apply directly to the feed transmission line of an LPDA, however. For example, LPDA conductors are typically not cylindrical as shown in
FIG. 1
, because such conductors would require brazing or soldering of the elements to the conductors. For improved manufacturability, it is desirable to have conductors with flat surfaces to allow the use of screws to fasten the elements to the conductors. A typical design uses a rectangular tube for a conductor,

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