Symmetrizing network

The invention is in the field of radio frequency technology. In particular, it relates to a symmetrizer which is provided on the one hand with a housing with internal space and on the other hand with a first conductor piece having first and second partial lengths inside the housing, the first conductor piece being coaxial with a coaxial inner conductor as a coaxial conduit through the housing, forming a coaxial inlet there and, on the other hand, freely terminating inside the housing, on the other hand, inside the housing with a second conductor piece of first and second partial lengths, wherein the second conductor piece is arranged parallel to and spaced from the first conductor piece, is connected to the housing on the coaxial inlet side and freely terminates inside the housing on the other hand, and through an opening provided in the first conductor piece at the fixed length the coaxial inner conductor extends at right angles from the first conductor piece to which the second conductor piece is connected, and on the other hand by two symmetrical output lines, which are connected on the second partial length to one of the pieces of conductor, which are led out of the box and there produce a symmetrical output.

Such a symmetrizer is known, for example, from the book of H. Meinke and FW Gundlach, Taschenbuch der Hochfrequenztechnik - Handbook on HF Technology, 3rd Edition, Springer-Verlag (1968), page 394, Fig. 18.11.

In radio transmitter technology, so-called Balancing Units with one coaxial input and one symmetrical output are used in order to be able, for example, to supply grounded symmetrical antennas with a coaxial power line.

A possible implementation of such a symmetrizer is formed by two parallel, coaxial lines, which are connected together as shown. To accommodate different frequencies in the desired frequency range from about 4 MHz to 26 MHz, a variable capacitor is provided at the input of the second line and a shorting bridge is provided at its output end.

The main disadvantage of this type of symmetrizer is that with an unbalanced load of only a few pF, a large asymmetric voltage, ie resonance, arises. This sensitivity to asymmetry cannot be removed by simple adjustment.

Very often, a resistance transformation is also associated with the asymmetry in the symmetrizer. In the solution shown in Fig. 1, an unbalanced input impedance of 50 ohms is increased in a ratio of 1: 4 to a symmetrical value of 200 ohms.

In order to achieve greater transformation ratios, for example 1: 6, i.e. from 50 ohms to 300 ohms, it is necessary to connect an additional conductor type transformer of about 12 m long, which is difficult to manufacture and has, especially at lower frequencies. , relatively large impedance fluctuations.

The type of symmetrizer mentioned in the introduction would also be able to be adapted to different frequencies so that it would be usable for a larger frequency range, since otherwise it would not be adaptable to different frequencies. As can be seen from FIG. 2, such an adjustment could be made by means of a shunt shorting on the inlet side and a variable capacitor on the outlet side.

The main disadvantage of such a symmetrizer would be that two parameters would have to be adjusted when changing the frequency. In addition, the slide valves with their contacts cause various problems such as wear, kinking and bending of the contacts and their tanning.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a symmetrizer which, at least in the frequency range of 4 MHz to 26 MHz, allows a transformation ratio greater than 1: 4 to be easily tuned and of robust construction.

The object is solved by a symmetrizer according to the invention, characterized in that at least one variable capacitor is arranged on the second partial length between the first conductor piece and the second conductor piece and the first partial length is selected so that the symmetrizer can be tuned to different frequencies exclusively with at least one capacitor.

The present invention is based on the discovery that, in the symmetrizer shown in FIG. 2, parameters can be selected, i.e. line impedance and line length, such that one variable capacitor is sufficient for tuning to different frequencies.

In this way it is possible to realize a robust and compact symmetrizer, which at the same time can be easily tuned.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained in more detail below with reference to the accompanying drawing, in which: Figure 1 shows a schematic circuit diagram of a tunable symmetrizer according to the prior art; Figure 2 shows a possible construction of another tunable symmetrizer; . 8 FIGS. 9 to 9B show further embodiments of a symmetrizer according to the invention in which the connection points of the capacitors are displaced; and FIGS. 10C are exemplary embodiments of a symmetrizer according to the invention in which multiple capacitors or additional inductance are used.

FIG. 1 shows a known symmetrizer used in shortwave transmitters, and in a similar embodiment is shown, for example, in the Brown Boveri Mitteilungen print, Brown Boveri Communication, Workbook 2/3, 1972, pp. 97-103.

This known symmetrizer has a coaxial input 1 and a symmetrical output 2, between which two coaxial lines 18 and 19 are connected in parallel. The symmetrizer is tuned at the input by a modified variable capacitor C and a shorting bridge 20 which allows the effective length of the second line 19 to be changed.

The main disadvantage of this arrangement is that the symmetrizer in this embodiment is limited to a 1: 4 transformation ratio. In addition, when changing the frequency, two parameters need to be changed, i.e. capacitance C and the effective length of the second conduit 19.

A solution allowing greater transformation ratios, based on the type of symmetrizer shown in the book by Meinke and Grundlach, is shown in Figure 2. In the interior 6 of the box 5, two equal lengths 14 and 17 of conductor are arranged in parallel so that at one end conductively connected to the housing 5 and on the other hand open freely in the interior 6.

The first conductor piece 4 is formed on the first partial length 21 as a coaxial guide with a coaxial inner conductor 3. This coaxial guide terminates on one side in the coaxial inlet 1. On the other hand, the coaxial inner conductor 3 extends through a hole 9 at right angles from the first piece. 4 of the conductor, extends to the opposite second conductor piece 17 and is conductively connected thereto. On the second partial length 12, the two conductor pieces 4, 17 are identical.

A symmetrical output line 8 or 7 is connected to the free ends of the conductor pieces 4 and 17, respectively. The two symmetrical output lines 7, 8 form a symmetrical output 2.

Two elements are provided for extruding the symmetrizer of FIG. On the one hand, it is possible to vary the first partial length 11 by means of a shunt 21, and on the other hand, the free ends of the conductor pieces 4, 17 are connected to each other by means of a variable capacitor C1.

Although this symmetrizer permits greater than 1: 4 transform ratios, it is difficult to displace with respect to the two elements, i.e., capacitor C1 and shunt 21, and mechanical shunt 21 is susceptible to failure.

When calculating the various parameters of the symmetrizer according to FIG. 2, depending on the frequency, it has been found that by appropriately selecting these parameters, i.e. the line impedance ,θ, the first partial length 11 and the second partial length 12 can be tuned in the frequency range 4 to 26 MHz. without having to change the first partial length substantially or not at all. This eliminates the necessity of using a short-circuiting short circuit 21. The symmetrizer can only be tuned to the desired frequency by means of a variable capacitor C1.

Accordingly, at least one value of the partial length 11 is formed, over which the partial length 11 is kept constant throughout the frequency range. If variations in the standing wave ratio are allowed, even more values ​​of the first partial length 11 are generated, which remain constant.

The following example assumes an input impedance of 50 ohms. In general, however, any impedance can be selected at the input.

Example input impedance: output impedance 11 + 12

11:

Line impedance:

2,704 m 1.104 m

Ω

300 Ω

111 Ω /

/!

If the parameters are selected as in the example, an almost ideal tuning of the symmetrizer is achieved when the capacitor capacity C1 changes from 1,571 pF at 4 MHz to 25.1 pF at 26 MHz.

In its simplest embodiment, the symmetrizer is then arranged as shown in FIG. 3. It is compact, requires no additional line transformer from 200 to 300 ohms for its transformation, and can be tuned with only one, relatively small capacitor.

However, the simple embodiment of FIG. 3 can be improved for the following reasons.

The 50 ohm standing wave ratio at an ideal 300 ohm load is always greater than 1.35 at the lowest and highest operating frequencies. For this reason, the symmetrizer in this embodiment is only conditionally suitable for use at more than 50 kW.

Furthermore, the symmetry of the voltage at low frequencies, that is at 35% lower than the maximum operating frequency, and at unbalanced load is actually better than the 1: 4 symmetrizer of Figure 1, but the symmetry deteriorates exponentially near the highest operating frequency. The voltage asymmetry can then be up to 1: 10 at the symmetric output depending on the load.

The causes of this problem are the two marginal effects caused by the construction.

The first disturbing factor is the scattering inductance L_Gwhich is generated by the middle branch in the symmetrizer, as schematically shown in Fig. 3. This dispersion inductance L_with it mainly worsens the standing wave ratio and also affects the sensitivity of symmetry.

The second is that the sensitivity of the symmetric to unbalanced load depends essentially on the choice of the minimum adjustable capacity and the associated highest operating frequency.

The first deleterious effect, i.e. the leakage inductance, can be eliminated by further forming the symmetrizer of FIG. 3 as shown in FIG. 4. The central branch is symmetrically overlapped from both sides by two tubular pieces 10 and 11 and is centered according to FIG. The pipe pieces 10 and 11 together with the coaxial inner conductor 3 of the middle tap always form a 50 ohm coaxial line according to an exemplary embodiment or with a different line impedance.

Furthermore, it is also possible to arrange only one tubular piece 13 on one side, which on the other side is separated by a slot 14, as can be seen from FIGS. 5 and 6. The size of the slot 14 is determined by the stress load.

7 and 8. Here, the outer diameter of the tap, i.e. the coaxial inner conductor 2 in the intermediate space between the conductor pieces 4 and 17, is enlarged in the form of a thickened 15 and 16, respectively, and reduced inductance. It must be ensured that the thickenings 15, 16 are not provided within the 50 ohm coaxial line which is inside the first conductor piece 4. This is the reason for the double cone chosen, see fig. 7, or simply a conical shape as shown in FIG. 8.

Another possibility to compensate the L-inductance_with represents the displacement of the capacitor C1 by the third partial length 13, i.e. about 20 cm for the 100 nH dissipation inductance, in the direction of the middle tap as shown in Fig. 9. Also the draw for the symmetrical output 2 via symmetrical output lines 7, 8 then it should be carried out directly through the capacitor ~ Cl. Two exemplary embodiments for such a pickup that can be mechanically easily formed are shown in Figures 9A and 9B.

The second harmful effect, that is sensitivity to unbalanced load, can be reduced by getting for a minimum adjustable capacity ^Cmin of capacitor Cl satisfies the following condition:

^Cmin /^Casym^>4Where C_asvm indicates the maximum output asymmetry.

10A to 10C are designed such that a single capacitor C1 is replaced by a plurality of capacitors C1 ... C8.

In FIG. 10A, in addition to the variable capacitor C1, two capacitors C2, C3 are provided between the conductor pieces 4, 17, which are either variable or not and which are each connected between one conductor piece 4, 17 and between the housing 5.

In Fig. 10B, two variable capacitors C4, C5 are connected in series between the conductor pieces 4, 17 and are grounded in the center. This solution brings various advantages, such as improved resistance to asymmetry on the transmission line.

However, the ground center of FIG. 10B can also be provided via another capacitor C8, as shown in FIG. 10C.

In all cases, it is possible to increase the frequency range of the symmetrizer by adding additional inductance L to the symmetrical output 2 as shown in the embodiment of FIG. 10A. This can extend the compensation to larger frequency ranges for which capacity would need to be C of capacitor C1 smaller