Coaxial Transmission Line With Reflection Compensation

It is well known that a coaxial transmission line with a movable probe may be used as a component of an overall apparatus for measuring the impedance of a load. The present invention relates to this form of coaxial transmission line.

A coaxial transmission line must, of course, have suitable supporting means for supporting the inner conductor. The supporting means may take the form of spaced supports such as beads, or dielectric pins. At points close to the generator end of the transmission line reflection from the supporting means is not a serious problem but at points further along the line toward the other end, the supportng means becomes of primarily importance because reflections from said supports are directly additive, in vectorial fashion, to the waves that are intended to be measured. Groups of radially disposed pins are often used as the supporting means, in which case all of the pins of a given group are located in a single plane perpendicular to the inner conductor, the groups being spaced apart in order to support the inner conductor along its entire length. To avoid reflections from a dielectric pin, various prior art techniques have been used. For example, holes may be drilled in the inner conductor, longitudinally forward of and/or rearward to the pin, while maintaining a certain distance from the pin, to provide inductive reactance opposing the capacitive reactance of the pin (see U.S. Pat. No. 2,796,589). Alternatively, one or more circumferential grooves spaced longitudinally ahead of and/or behind the pin have been used, the grooves however being spaced from the pin. It has also been proposed to have cavities entirely enclosed within the inner conductor which are longitudinally ahead of, as well as behind, a group of pins, to provide the necessary compensation (see Bondon U.S. Pat. No. 3,151,925 entitled "COAXIAL TRANSMISSION LINE UTILIZING REACTANCE COMPENSATED, PAIRED PIN-TYPE INSULATOR SPACING ASSEMBLY", dated Oct. 6, 1964; and Bondon U.S. Pat. No. 2,589,328 entitled "COAXIAL TRANSMISSION LINE SPACING ASSEMBLY", dated March 18, l952).

All of the above arrangements for compensating for the capacitive effect of the dielectric pin have their drawbacks, including manufacturing problems, and this is especially true as the coaxial transmission lines are made in small sizes so as to be suitable for use at frequencies up to as high as 40 GHz.

It is, therefore, an object of this invention to provide improved compensation for the capacitive effect of a dielectric pin used to support the inner conductor of a transmission line.

It is a further object of the invention to provide a coaxial transmission line suitable for use at frequencies up to 40 GHz.

An additional object is to provide a coaxial transmission line which may be easily assembled.

It is also an object of the invention to provide a coaxial transmission line that may be manufactured at low cost.

It is still another object of the invention to provide a slotted transmission line with a probe, suitable for measuring the impedance of a load, which may efficiently operate at frequencies up to as high as 40 GHz and yet be capable of manufacture at low cost.

This invention relates to a coaxial transmission line with a probe movable along the line to enable measurement of the impedance of a load, particularly at frequencies up to 40 GHz, although in its broader aspects the invention is not limited to any particular frequency range or to a coaxial line with a probe for measuring impedance of a load.

According to the present invention, the problem is solved by reducing the distance between the pin and the compensating hole to zero. This can be accomplished by counterboring the surface of the inner conductor so as to leave a shallow depression around the pin location. This depression is dimensioned to provide the desired compensating effect, and in practice it has been found possible to greatly reduce the SWR, in some cases to less than one-third of the best previously obtainable value. Furthermore, it is much easier to do, since the bores, which must in any case be provided to receive the pin ends, can now be used to accurately and easily center the counterbore drill, and as this is a larger drill, it has less tendency to break. Also, only one hole is bored at each pin location. Furthermore, instead of having the three (more or less) supporting pins in the same transverse plane, they are spaced from each other longitudinally.

The specific nature of the invention, as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing, in which:

FIG. 1 is a longitudinal sectional view of the invention as applied to a typical slotted line, the view being taken on line 1-- 1 of FIG. 2;

FIG. 2 is a sectional view taken on line 2-- 2 of FIG. 1;

FIG. 3 is an enlarged view of the center section of FIG. 1 showing the pin support in more detail;

FIG. 4 is a view similar to FIG. 3, showing an alternative form of the invention;

FIG. 5 is a sectional view taken on line 5-- 5 of the center conductor shown in FIG. 4; and

FIG. 6 is a view of a center conductor showing a modified form of the invention;

FIGS. 7 and 8 are schematic diagrams used in explaining the principle of the invention.

Referring to FIG. 1, the invention is shown schematically as applied to a typical slotted line section having a probe 2 which can be slid along the length of the slotted line, the probe end terminating close to a center conductor 4 which is retained within the cylindrical bore of a grounded outer conductor 6 having the usual connector ends 8 and 10. The center conductor 4 is retained at the input end by any conventional means such as an insulating disc 12, since the supporting means at this end is not critial as explained above. Along its length, the center conductor is held in place by means of one or more sets of insulating pins 14a, b and c, and 16a, b and c, which are rigidly fastened to the center conductor at one end, as will be explained in more detail below, while at the other end they merely abut the inner surface of the outer conductor. The supporting pins are preferably provided in sets of three as shown at each support location, as best seen in FIG. 2.

In FIG. 3, the center conductor is partly broken away in order to show in detail the manner in which the supporting pin 14a is attached thereto. The center conductor is drilled to provide a small hole which can snugly receive the end of pin 14a with a force fit, and a shallow circular depression 18 is then counterbored concentrically with this hole so as to provide a shallow depression surrounding the pin 14a at the point where it engages the center conductor.

The electrical principle employed as a basis of compensation is explained with reference to FIG. 7 which shows at the left side a section of the coaxial conductor with a pin 14, which may be any pin of the type shown above, while on the right hand side of the Figure, the electrical equivalent of the distributed impedance is schematically shown by conventional symbols. The cross-section of the line to the left of the plane A-- A and to the right of plane B-- B represents the unsupported section of the line, and is such that the ratio of the inductance and capacitance of a piece of length dl is ##EQU1## which is the characteristic impedance of the line. where

dL is the inductance per unit length of the coaxial line

dC is the capacitance per unit length of the coaxial line.

The support portion, that is the section between planes A-- A and B-- B adds capacitance to the line C.sub.p. Therefore the relation of (1) is not obtained in the section between planes A-- A and B-- B, but can be achieved by adding inductance in the right amount so as to obtain the overall ratio ##EQU2##

The distributed inductance of a length of transmission line which is short in comparison to a wavelength is

where

1 = length Z = characteristic impedance c = transmission speed (speed of light in an air filled line). Similarly, the distributed capacitance is

The total capacitance of the section from plane A-- A to plane B-- B is then C.sub.tot = C + C.sub.p where C.sub.p = capacitance of the dielectric pin. Compensation of the pin capacitance is achieved when ##EQU3## where Z = characteristic impedance of the line between A and B

1 = length of the line section A to B. The equivalent circuit of this line section is a low pass filter with the cut-off frequency in terms of C.sub.tot capacitance of the section is given by; ##EQU4## The characteristic impedance of a low pass filter is frequency dependent and is given by ##EQU5## It is therefore necessary to achieve a cut-off frequency much higher than the highest operational frequency in order to obtain only a small tolerable deviation from the required characteristic impedance Z.sub.O. It can be seen from equations (5) and (6) that for a deviation of Z.sub.1 by 0.5% the operational frequency has to be less than 0.10Wg, and from equation (5) that the maximum length has to be less than ##EQU6##

The present construction is designed to provide the minimum possible length of section A-B, since the compensating section and the construction shown in FIGS. 1-3 is made as short as possible by completely surrounding and including the supporting pin.

FIGS. 4 and 5 show an alternative construction in which the compensating depression is formed not by counterboring with a larger size drill, but simply by filing or gouging out small section 19 around the base of pin 14'. The principle in this case is the same, since the longitudinal dimension of the shallow depression is what is important as can be seen from the preceding analysis.

FIG. 6 shows still another form of construction whereby a depression 21 is formed on the opposite side of the center connector 4b from pin 14". The important thing is that the effective compensating will occur in the correct region along the length of the insulating pin 14".