US2241582A

US2241582A - Arrangement for matching antennae for wide frequency bands

Info

Publication number: US2241582A
Application number: US311798A
Authority: US
Inventors: Buschbeck Werner; Meinke Hans
Original assignee: Telefunken AG
Current assignee: Telefunken AG
Priority date: 1938-10-07
Filing date: 1939-12-30
Publication date: 1941-05-13
Anticipated expiration: 1958-05-13

Description

W 1941- w. BUSCHBECK EI'AL 2,241,532

ARRANGEMENT FOR MATCHING ANTENNAE FOR WIDE FREQUENCY BANDS FiledDec. 30, 1939 2 Sheets-Sheet 1 400 my! v 2504 Fly. 2

.Q, i 1 200 i F R,X I T a a kl o lk $1 R a; A :5

I 100 i i l I l 1 l l l l l i I 5 I l 0 I 2 3 4 A INVENTORS 1 WERNER BUSCHBECK HA/VS M INKE ATTO R N EY ating wave.

Patented May 13, 1941 ARRANGEMENT FOR MATOHIN G AN TENNAE FOR WIDE FREQUENCY BANDS Werner Buschbeck and Hans Meinke, Berlin, Germany, assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application December 30, 1939, Serial No. 311,798 In Germany October 7, 1938 3 Claims.

The present invention relates to an antenna arrangement such as is employed more particularly for the radiation or reception of very wide frequency bands. Suggestions have already been made, for example, in German application No. T 47,939 VIIIa/ 21a 4, to achieve a matching with high-ohmic energy lines by antennas which are longer than a quarter of the length of the oper- The bottom point resistance of a single wire antenna having a length of exactly M4, when A is the wavelength, is 36 ohms, while the wave resistance of one of the customary antenna supply cables is about 60 ohms. In order to match a N4 antenna with such a cable the antenna is chosen slightly longer so that the effective resistance value at the antenna base point is equal to the wave resistance of the cable. The remaining inductive reactance is then compensated by a capacitive parallel reactance at the base point of the antenna.

For wide bands such an antenna arrangement, however, has the disadvantage that owing to the different variation with respect to frequency of the wattless component of the antenna and of the compensation capacity reactance effectively in parallel therewith, additional reactance components appear in the side bands so that a matching exists only for the carrier frequency while insufficient matching exists for the antenna at the side band frequencies.

When considering the fact that the curve of the variation of the reactance of an antenna with frequency depends substantially on the wave resistance of the antenna, it is seen that by choosing a suitable wave resistance of the antenna, the curve of variation of the antenna reactance can be so adapted that it becomes as much as possible identical with that of the wattless compensating reactance; this is utilized in accordance with the invention for compensating as much as is possible the characteristic of the reactance for the side bands.

The invention can be employed for various antennas. Its application to television antennas in which very wide frequency bands are to be transmitted with constant amplitude offers a particular advantage. The application of the invention ofiers also an advantage in cases where several messages which are modulated on different carriers are to bev transmitted, or where a transmitter is to be operated in succession, or simultaneously with different frequencies, since the invention renders superfluous the use of several antennas or the use of special tuning means at the antennas.

sponding parallel reactance.

A more complete understanding of the invention may be had by reference to the following detailed description which is accompanied by drawings in which Figures 1 and 2 are curves showing variation of the resistance and impedance of an antenna with variation of length; Figures 3 and 6 illustrate embodiments of the invention and Figures 4 and 5 are curves explanatory of the principle of the invention, as illustrated in Figures 3 and 6. v

Figure 1 reveals the theoretically determined effective resistances and reactances transformed by calculation to parallel resistance values, of an antenna erected above ground with a wave resistance of 250 ohms; RAp designates the parallel resistance of the antenna and XAp is the corre- The effective resistance and the reactance are plotted in this figure as functions of the value wherein 1 is the antenna length and. A is the wavelength; thus the value oz= corresponds to a M4 antenna. From these curves the capacitive compensating reactance can be found which pertains respectively to any desired value of the effective resistance. As already pointed out, an exact matching can be realized through such an arrangement generally only for a single wave such as, for instance, the carrier wave whereas even with slight frequency deviations such as are encountered on account of the side bands, reactive components immediately appear again in View of the principally different frequency characteristics of the parallel compensation capacity and of the antenna.

In order that the possibly identical characteristics of the antenna resistance and compensating reactance can be easily recognized graphically, it is advisable instead of plotting the curves for RAp and XAp in function of u as is done in Figure 1, to draw them in regard to the wavelength A proper or in regard to the value M2, as above in Figure 2, since in this case the curve of the reactance of the compensation capacity X0 is represented by a straight line passing through the zero point of the coordinate system. As is known, there is:

480% 480-1 2 v T T) 1 and furthermore:

X17 tan 1/0 whereby 5 represents the angle of the straight line, representing the variation of the reactance, with respect to the abscissa. When introducing C in centimeters and 7\ in meters there is:

Now, if in accordance with the invention C is so chosen that the straight X line and the XAp curve just touch one another or slightly cut each other, rather wide frequency ranges without a large reactive component will be obtained as seen in the figure. The insufficient matching resulting from the varying effective resistance is numerically given directly by the ratio of the resistance values and hence is small enough to be negligible. If, also, this insufficient matching due to the pure resistance variation is to beovercome, Various possibilities exist for compensating it. When considering the straight compensation line to be a tangent to XAp, then for shorter waves, beginning with the point of touching, an increase of the ohmic component occurs. But at the same time there appears a rapidly increasing capacitive parallel conductivity (Figure 4) so that the resistance value Rs recalculated for the series connection must decrease again after reaching a maximum (Figure It is seen from the curves of Figure 5 that the conditions for exact tangent and two cases of somewhat lower compensation capacity show that the capacitive reactance curve XS recalculated for series connection is practically linear in the vicinity of the maximum of the effective resistance curve Rs, so that it can be compensated by a series combination of L and C, or by a corresponding line inserted in series. When designating the shorter and longer limit wave of the range employed by the indices a and b then there is Ab=m, is and the series reactances of the series compensating circuit will be found from the following equations:

Then the inductive series compensating reactance is:

For the case of the tangent in Figure 5 this reactance is shown too and reveals the favorable way in which the compensation of the reactance can be provided for the entire range considered. If the parallel reactance is so chosen that slight intersections occur, so that C has lower values than would correspond with the tangential point, the points of the maximum resistance move to shorter waves at a simultaneous increase of the maximum value of the resistance. This pro vides a means, at a wave resistance once given and which can be varied only with great difiiculties at a given construction, to vary within certain limits the value of the maximum resistance which will actually correspond to the matching with the carrier.

The advantage of such an arrangement lies essentially in the great simplicity in that the capacity of the base point of the mast can be utilized directly for the resistance matching for wide bands. Furthermore, by a suitable choice of the wave resistance of the antenna a matching with any desired ohmic resistances is possible in the simplest manner. The wave resistance of the antenna can be varied as is known, in wide limits as may be desired, for instance between 60 ohms for a cone standing on its apex up to about 600 ohms for a simple wire. Therefore, with the methods proposed all cables and line resistances in question can be taken care of without additional resistance transformation networks.

A further example of application of the invention is shown in Figure 6. This figure indicates a conical antenna K which is to be matched with a feed cable S. The antenna is operated between the M4 and the M2 resonance because in this case the variation of the reactance is the smallest. The compensation of the remaining reactance is carried out in the present case by a capacity placed in series just as in the case of the parallel capacity in the aforesaid examples of construction. This capacity is inserted at C between the cone extension and the inner conductor of the feed cable. Now there appears in back of this capacity a pure effective resistance which varies greatly with the frequency, while the variation of the reactance can be neglected. Now, in order to compensate the variations of the effective resistance there is provided a transformation line L1 having the length M8 and which transforms the variation of the effective resistance with frequency into a variation of reactance. The M8 transformation line whose wave resistance is equal to the mean radiation resistance, in fact generally transforms a variable ohmic resistance without reactance into a constant ohmic resistance with frequency variable reactance. The frequency characteristic of the resistance can be compensated in a manner known as such by a M2 transformation line L2 of suitable wave resistance or by another suitable method. In place of the M8 line in certain cases a line having the length %A may be of advantage. An antenna so constructed has only the half of the compensation faults as exist in the known antennas for wide bands.

It is obvious that in the various examples of construction also combinations of the individual measures are possible. Thus, series impedances can be substituted always by correspondingly recalculated parallel impedances and vice versa; also all reactances can be replaced in a known manner by suitable line sections.

It is claimed:

1. A wide band antenna system comprising an antenna having a small variation of reactance over a wide frequency range, a transmission line and circuit elements connected in series therebetween, said antenna being of such length that the wave resistance of said antenna is equal to the impedance of said transmission line, said circuit elements comprising a capacity connected to said antenna so proportioned that the variation in reactance of said antenna is transformed into a variation in effective resistance, and means for compensating for said variation in resistance.

2. A wide band antenna system comprising an antenna having a small variation of reactance over a wide frequency range, a transmission line connected transmission line sections, said transmission Iine sections compensating for said variation and resistance.

3. An arrangement according to claim 2, characterized in that said last means comprises a line for transforming the remaining variation of the eifective resistance into a reactance variation and a