WO2001024308A1

WO2001024308A1 - Broadband coaxial transmission line

Info

Publication number: WO2001024308A1
Application number: PCT/US2000/026934
Authority: WO
Inventors: Ronald Wilensky
Original assignee: Tci International, Inc.
Priority date: 1999-09-29
Filing date: 2000-09-28
Publication date: 2001-04-05
Also published as: AU7740000A

Abstract

A transmission line (100) includes a plurality of N groups of lines (102a, 102b, 102c, 102d, 102e) wherein each group of lines includes M sections of line (104) and a plurality of junctions between the sections of line (104). Each sections of M lines within each group of N lines has the same length and the length varies from group to group. The VSWR of the transmission line (100) at a given frequency resulting from reflections from the plurality of junctions is decreased.

Description

BROADBAND COAXIAL TRANSMISSION LINE

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to United States Patent Application No 60/156,816, (Attorney Docket No TCI1P006+) entitled "Broadband Coaxial Transmission Line" filed September 29, 1999 which is herein incorporated by reference

FIELD OF THE INVENTION

The present invention l elates generally to transmission lines More specifically, a broadband coaxial transmission line manufactured from multiple sections is disclosed

BACKGROUND OF THE INVENTION

The conversion of the U S television system from the present analog transmission standaid to the new digital standard mandated by the Federal Communications Commission gives each U S television station a second channel on which it can bioadcast digital TV There is a severe shortage of tower space m the U S so that it ma\ be necessary or advantageous for broadcasters to install broad bandwidth antennas capable of handling both their existing analog and new digital transmissions In addition, it may be necessary for several broadcasters to group their transmissions on a common antenna Similar situations are arising and will continue to arise in markets outside the TJ S as countries convert from analog to digital TV Since existing analog antennas are generally operable in only one channel, the transmission lines that feed the antennas are generally also operable in only one channel. In most cases these transmission lines are not designed to provide the required low voltage standing wave ratio (VSWR) over a wide range of channels. As broad bandwidth antennas come into use, it will be necessary to provide transmission lines capable of operating over a wide range of frequencies for the purpose of feeding such antennas .

One common transmission line configuration is a coaxial transmission line that consists of two coaxial conductors that are generally cylindrical in shape. These lines operate at frequencies from DC up to a high frequency cutoff that depends on the diameter of the conductors. Coaxial lines are available commercially either in the form of semi-flexible lines made from corrugated inner and outer conductors, or rigid lines which are generally made from non-corrugated conductors. The semi-flexible lines are available in long continuous lengths but often are not available commercially in the large diameters and impedance levels needed to carry two or more high power digital TV transmissions from the transmitter to the antenna. Therefore, rigid lines are often required for their higher power handling capability since rigid lines are generally available in a wider variety of impedance levels and power handling capacities.

The difficulty with rigid lines is that because of the way the lines are constructed, there are frequencies at which the VSWR exhibits sharp resonances, making the line inoperable over a wide bandwidth. The reason for these resonances is that a run of rigid transmission line is fabricated out of a large number of smaller sections, each typically 10 ft. to 20 ft. in length. The smaller sections are joined together mechanically and electrically using flanges on the outer conductors and sliding RF connectors, often called "bullets", that interconnect the inner conductors. When a wave is fed into one end of the rigid line, each of the junctions reflects a small portion of the wave, causing a small increase in VSWR. When the flanges are spaced at an integer multiple of half- wavelengths at a given frequency, the small reflections add up causing a much larger reflection and hence a higher VSWR at that frequency

A scheme for avoiding flange spacing at an integer multiple of half- wavelengths for a certain frequency is disclosed in US Patent No. 5,455,548 (the "548 patent") The 548 patent describes a broadband coaxial transmission line that is designed primarily for UHF television and provides low insertion VSWR over the 470 to 806 MHz frequency range used by US UHF broadcasters. The '548 Patent reduces the reflections from the junctions between line sections by using a progression of line section lengths rather than equal line section lengths. The '548 patent determines line section lengths using the following formula:

L_n = L + λ(n-l)/2N

where

L_n is the n^th section of line, where n=2, 3, 4, ...., N

L is the length of the first section of line

λ !S the wavelength at a design frequency (the 548 patent uses 775 MHz,

which corresponds to λ = 15 53 inches) N is the number of sections of line

When this scheme is followed, each section length differs by a very small amount from the adjacent sections. For example, TV antennas are generally mounted on very tall towers, some 2000-ft tall. If the total length of rigid line is 2000 ft and the typical section length is about 20 ft, there will be 100 sections, i.e. N=100. Thus the difference between adjacent section lengths L_n+ι and L_n is

Ln₊i-L_n = λ/200 = 15.53 inches /200 = 0.0778 inches.

Such small differences present manufacturing difficulties. But more importantly, the difference is similar in magnitude to the length change that occurs because of thermal expansion and contraction. The temperature of the line changes as a result of internal RF heating, changes in the ambient temperature or by direct solar heating. The amount of thermal expansion or contraction may be randomly distributed over the length of line in a manner that destroys the perfectly linear length progression that the 548 patent teaches.

What is needed is a better distribution of section lengths that is more robust under realistic thermal environmental conditions and that does not require strict manufacturing tolerances among the line sections. A better scheme for varying section lengths that results in a smaller VSWR over a wide range of thermal conditions is required.

SUMMARY OF THE INVENTION Accordingly, an improved length distribution scheme is disclosed that provides low VSWR over a wide frequency range in a robust manner. In one embodiment, the total length of line is divided into a group of sub-sections. In each sub-section, the line lengths are equal. However, there is an increase in length from one group to the next. For long lines, the length increment is much larger, often by a factor often or more, than that described in the '548 Patent. This distribution of lengths described herein is therefore much easier to fabricate than the distribution taught in the '548 Patent. Significantly, a transmission line manufactured from such sections suffers much less from the detrimental effects of thermal expansion and contraction as temperature changes.

It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Several inventive embodiments of the present invention are described below.

In one embodiment, a transmission line characterized by a VSWR over an operable frequency range includes a first group of transmission line sections that includes a first plurality of transmission line sections having a first length and a second group of transmission line sections that includes a second plurality of transmission line sections having a second length that is different than the first length. The first plurality of transmission line sections and the second plurality of transmission line sections are joined together at transmission line section junctions. The difference between the first length and the second length reduces the VSWR of the transmission line resulting from additive reflections from the transmission line section junctions.

In one embodiment, a transmission line includes a plurality of N groups of lines wherein each group of lines includes M sections of line and a plurality of junctions between the sections of line. Each of the M sections of line within each group of lines is the same length and the length varies from one group to the next. The VSWR of the transmission line at a given frequency resulting from reflections at the plurality of junctions is decreased.

In one embodiment, a method of manufacturing a reduced VSWR transmission line from a plurality of sections includes dividing the sections of transmission line into groups wherein the length of the sections of transmission line in each group is different from the length of the sections of transmission line each other group. The reduced VSWR transmission line is assembled from the sections.

These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: Figure 1 is a diagram illustrating a transmission line constructed according to one embodiment of the present invention.

DETAILED DESCRIPTION

A detailed description of a preferred embodiment of the invention is provided below. While the invention is described in conjunction with that preferred embodiment, it should be understood that the invention is not limited to any one embodiment. On the contrary, the scope of the invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications and equivalents. For the purpose of example, numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured.

In one embodiment, the completely assembled line is divided into N groups of lines. Each of the groups has M sections of line within the group. The length of line in the first group is L. The length of lines in the second group is L + d, where d is much smaller than L. In the third group, the length of lines is L+2d. The distribution of lengths is as follows: First group of M sections Length = L

Second group of M sections Length = L + d

Third group of M sections Length = L + 2d

Fourth group of M sections Length = L + 3d

Fifth group of M sections Length = L + 4d

N^th group of M sections Length = L + (N-l)d

The total length of line Lj is given by the formula:

L_τ = NML + M(N)(N-1) d/2

Given the number of groups N, the number of sections within each group M, and the incremental length change between groups d, the basic length L is found by solving the equation:

L = [L_τ - M (N)(N-1) d/2]/NM

In one embodiment, there are 10 sections of line within each group and the length increment is 1 inch. Thus M=10 and d=l inch. For a 2000-ft line, if the line is divided into 10 groups (N=10), the basic length, i.e., the lengths of the first ten sections, is 19.625 ft. In the next group of ten sections, the lengths are 19.625 ft plus 1 inch, and so on. This line has an insertion VSWR under ideal conditions that is very similar to a 2000-ft line that is ideally constructed according to the '548 Patent scheme requiring that the line lengths be varied at each segment. When likely variations in manufacturing tolerance and thermal expansion is taken into account, the performance of a line constructed according to the scheme disclosed herein is superior.

Figure 1 is a diagram illustrating a transmission line constructed according to one embodiment of the present invention. Transmission line 100 includes 5 groups 102 of line sections 104. Each group includes 4 line sections. The 4 line sections in each group are of equal length. Each of the 5 groups is characterized by a different line section length. In the embodiment shown, the line length progressively increases with each group. It should be noted that in some embodiments, the groups of lines may monotonically decrease in length. Other arrangements for ordering the line lengths may be used as well.

Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing both the process and apparatus of the present invention. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.

Claims

1. A transmission line characterized by a VSWR over an operable frequency range comprising: a first group of transmission line sections that includes a first plurality of transmission line sections having a first length; and a second group of transmission line sections that includes a second plurality of transmission line sections having a second length that is different than the first length; wherein the first plurality of transmission line sections and the second plurality of transmission line sections are joined together at transmission line section junctions; whereby the difference between the first length and the second length reduces the VSWR of the transmission line resulting from additive reflections from the transmission line section junctions.

2. A transmission line as recited in claim 1 wherein the transmission line sections are made of coaxial cable.

3. A transmission line as recited in claim 1 wherein the transmission line sections are made of rigid coaxial cable.

4. A transmission line as recited in claim 1 wherein the transmission line is used for feeding a broad bandwidth antenna.

5. A transmission line as recited in claim 1 further including a third group of transmission line sections having a third length and wherein the increment in length between the second length and the third length is the same as the increment in length between the first length and the second length.

6. A method of manufacturing a reduced VSWR transmission line from a plurality of sections comprising: dividing the sections of transmission line into groups wherein the length of the sections of transmission line in each group is different from the length of the sections of transmission line each other group; and assembling the reduced VSWR transmission line from the sections.

7. A method as recited in claim 6 wherein the groups of lines are ordered so that the length of the sections is incremented by a constant amount from one group to the next.

8. A method as recited in claim 6 wherein the groups of lines are ordered so that the length of the sections is decremented by a constant amount from one group to the next.

9. A transmission line comprising: a plurality of N groups of lines wherein each group of lines includes M sections of line; and a plurality of junctions between the sections of line; wherein each of the M sections of line within each group of lines is the same length and wherein the length varies from one group to the next; whereby the VSWR of the transmission line at a given frequency resulting from reflections at the plurality of junctions is decreased.

10. A transmission line as recited in claim 9 wherein the transmission line sections are made of coaxial cable.

11. A transmission line as recited in claim 9 wherein the transmission line sections are made of rigid coaxial cable.

12. A transmission line as recited in claim 9 wherein the transmission line is used for feeding a broad bandwidth antenna.

13. A transmission line as recited in claim 9 wherein the groups of lines are ordered so that the length of the sections is incremented by a constant amount from one group to the next.

14. A transmission line as recited in claim 9 wherein the groups of lines are ordered so that the length of the sections is decremented by a constant amount from one group to the next.