US2236102A - High frequency wave transmission system - Google Patents

Description

March 25, 1941.- F. 'A.KOLSTER 2,236,102

HIGH FREQUENCY WAVE TRANSMISSION SYSTEM Filed April 28, 1938 2 Sheets-Sheet 1 I sou/Fa: 0F 047/?4-5/40 sou/m? 0F ULTRA- WAVES A/VQ Dl/FL-CT 2 SHORT WAVES I CU/F/PEWT con/M04 VOLT/1658 RECEIVING APPARATUS INVENTO R FEEDER/CK A. K0LSTR ATTORNEY 194-1 F. A. KOLSTER 2 HIGH FREQUENCY WAVE TRANSMISSION SYSTEM Filed April 28, 1938 2 Sheets-Sheet 2 ATTORNEY Patented Mar. 25, 1941 UNITED ST PATENT OFFICE HIGH FREQUENCY w VE TRANSMISSION A STEM Application April 28, 1938, Serial No. 204,735

8 Claims.

The present invention relates to wave blocking devices and to transmission line arrangements incorporating such devices and adapted. for feeding waves to a restricted portion of a conductive structure while preventing the waves from passing out over the remainder of the conductive structure. Reciprocally the invention likewise relates to transmission line arrangements incorporating such devices and adapted for receiving waves collected by a restricted portion of a conductive structure while suppressing the reception of waves collected by other portions of the conductive structure. More particularly the invention relates to dipole antenna structures and means for feeding the same, or for leading energy away from the same, Without distorting the radiation pattern so as to render it unsymmetrical.

It is an object of the invention to provide a wave suppressor unit which can be conveniently applied to a transmission line and conveniently adjusted independently in its effective position and in its eifective frequency of blocking. Especially it is an object to provide such a wave blocking arrangement which can be applied to a continuous conductor at any intermediate point without interrupting the conductor, and which can be adjusted longitudinally along the conductor and can independently be tuned to various 1 frequencies.

It is a further object of the invention to provide a dipole antenna structure together with a lead-in line for the same which shall produce a perfectly symmetric radiation pattern undis- 35 torted by the lead-in arrangement used for feeding energy to the dipole antenna-or leading energy away from the dipole antenna in the case of reception. More particularly it is an object to provide such a dipole and lead-in structure whose radiation pattern shall be symmetric with respect to the axis of the dipole, being defined by a surface of revolution whose axis coincides with the axis of the dipole.

' It is a further object to provide such a dipole and lead-in" arrangement for producing such a pattern symmetric about the dipole axis and accurately adjustable with respect to the shape and orientation of a section taken in a plane passing through the dipole axis. In particular it is an object to provide a vertical dipole and lead-in arrangement whose horizontal patterns of radiant action, 1. e. the sections in all horizontal planes, shall all be undistorted circles, and whose vertical patterns, 1. e. the sections in all vertical planes through the axis, shall all be alike and shall be adjustable in shape and inclination. Especially it is an object to provide such an arrangement in which the shape and inclination of such vertical patterns may be readily adjusted by adjusting the eifective length and height of the dipole.

It is a further object of my invention to provide such an arrangement in which one end of the dipole is defined by means of a wave blocking device applied to an otherwise continuous conducting structure, and in which the position of said wave blocking device may be adjusted to vary the length and effective position of the dipole, and in which the wave blocking device may be independently tuned to anti-resonance at a desired frequency.

Vertically disposed dipoles of suitable length and elevation above the earth are commonly employed for producing radiation patterns symmetric about the dipole axis. Such patterns are required in certain applications, particularly in course beacons for guiding airplanes along a desired course or route. It has been suggested that in order to obviate the distorting influence of the lead-in line which feeds such a dipole antenna, the two lead-in wires should be shielded and should be carried up through the lower half of the dipole antenna itself, which is made hollow for this purpose. In such structures for the purpose of preventing the energy of the dipole from passing capacitatively into the lead-in line or its shield, and thus distorting the radiation pattern, it has been suggested to interpose specially designed bi-filar balanced chokes between the dipole and the lead-in wires, but such arrangements are somewhat complex and are attended with certain difficulties especially at ultra-high frequencies.

It has also been suggested heretofore to feed the dipole by means of a coaxial line which extends up through a tubular lower limb of the dipole and to space the outer conductor of this coaxial line accurately with respect to the tubular lower limb of the dipole so that said outer conductor is out of contact and coaxial with the lower limb of the dipole. In such a system the outer conductor of the coaxial line is connected to the tubular lower limb of the dipole only at the central point of the dipole, and the lower limb of the dipole is designed to be of exactly such length that it will resonate with the corresponding portion of the outer sheath of the leadin line, as a quarter-wave line so as to block the transmission of the high voltage and high current waves of the dipole over the outer conductor of the coaxial lead-in line. Such an arrangement, however, not only necessitates the accurate design of the tubular lower limb of the dipole so as to constitute exactly a quarter-wave line with precise corrections for the unusual end efiects, but also is inherently limited in its wave blocking action by the reactance-to-resistance ratio or Q factor of the line constituted by the tubular lower limb of the dipole and the corresponding section of the outer sheath of the coaxial feed line. Furthermore, such a system inherently restricts the freedom of adjustment of the length of the dipole so that the length of the dipole cannot be freely varied to produce the desired inclination or distortion, in respect of the shape of the axial sections, of the toroidal radiation pattern. With such a system neither limb of the dipole can be varied-to produce the desired adjustments of the radiation pattern since the length of the lower limb is inherently fixed by the requirement that this lower limb itself must resonate as a quarter-wave line to suppress the passage of the oscillatory energy of the dipole into the lead line, while the length of the upper limb is fixed by the requirement of balance between the two dipole limbs.

In accordance with the present invention, the disadvantages of both the above mentioned systems are avoided and, at the same time, a very simple and structurally convenient design is at 'tained. In accordance with my invention, these advantages are attained by providing a special wave blocking device so designed that it can be attached to a unitary structure, such as a conducting tube which may form the outer conductor of the lead-in line and may also constitute the lower arm of the dipole antenna, without the need of cutting or otherwise interrupting this continuous conducting structure. Because of the fact that this wave blocking device may be adjustably moved along the continuous conductor which forms both the outer conductor of the lead-in line and the lower arm of the dipole antenna, the length of the lower arm of the dipole antenna may readily be adjusted without providing any telescopic members or other cumbersome and complex arrangements for the purpose. The wave blocking device of my invention is also independently adjustable to tune it to exact antiresonance at the desired frequency, so that it is not necessary to pre-design the apparatus exactly for a given frequency, but rather the apparatus can be approximately designed and then adjusted.

The wave blocking device of my invention is also useful in other combinations and in itself,

' antenna conductors to produce a two-course beacon for guiding aircraft along a desired route;

Fig. .3 represents a lead-in arrangement for separately feeding a plurality of dipoles in an array;

Fig. 4 is an elevation partly in section of the wave blocking device which is incorporated in the systems of Figs. 1 and 2;

Figs. 5, 6, '7 and 8 are partially sectioned elevations of further forms of wave blocking devices which may be used in the systems of Figs. 1 and 2.

Fig. 9 illustrates a form of wave blocking deduced by other antennae or by parasitic reflectors. The dipole l is supplied with ultra-short waves from the source 2 over the coaxial line 3, the sheath of this coaxial line forming the lower limb of the dipole and the central conductor of the coaxial line being extended to form the upper limb of the dipole. A telescopic extension 4 is provided at the end of the upper limb of the dipole for adjusting the length of this limb. The effective electrical length of the lower limb of the dipole is determined by the position of a wave blocking device 5 which is slipped over the sheath of the coaxial line 3 and clamped thereto, and 1 which serves as hereinafter explained, to block the passage of Waves traveling down over the sheath of the coaxial line. As more fully explained hereafter, the cap, or upper portion 6 of the wave blocking device 5 is electrically a part of the lower limb of the dipole so that this lower limb may be regarded as extending from the center of the dipole to the lower edge of the cap 6, thus having the length B as shown in the figure. The lower portion 1 of the blocking device 5 is electrically at the same potential as the lower half of the coaxial line 3, which is preferably at ground potential. The upper limb of the dipole has a length A as shown in Fig. 1. Because of the fact that the lower limb of the dipole is larger in diameter than the upper limb, and has at its end the extended capacity surface of the cap 6, the lengths A and B should be slightly different in order to provide the same equivalent electrical length for the two halves of the dipole I. If desired, however, the upper limb of the dipole may be increased in size so as to be similar to the lower limb, and in case exact symmetry is essential the upper extremity of the dipole may be provided with a cap similar the coaxial line 3 are in push-pull relationship or phase opposition, and since the potentials at corresponding points of the inner and outer conductors are equal and opposite, the waves traveling upward through the coaxial line 3 will have substantially no external effects and will pass unblocked throughthe device 5 which is located entirely outside of the coaxial line 3, as previously mentioned. On arrival at the center of the dipole I, the waves will spread upwardly along the upper half of the dipole and downwardly over the lower limb of the dipole, just as in the case of waves supplied to an ordinary dipole from a transmission line connected to the dipole from one side. The upward waves along the upper limb of the dipole will be partly radiated and partly re flected from the open termination of the dipole. Similarly the downwardly traveling waves on the lower limb of the dipole will be partly radiated and partly reflected, the reflection in' this case occurring at the lower edge of the cap 6 which effectively acts as an open termination aslater explained.

It will thus be seen that inspite of the fact that the lower limb of the dipole is integral with the sheath of the coaxial lead-in line, thewave distribution on the dipole is generally similar to the wave distribution on an ordinary dipole fed by a lead-in line Which joins the dipole from one side. Thus the design of the apparatus with respect to impedance matching and with respect to the standing wave pattern may under most ordinary conditions be made in accordance with the accepted theory of dipoles fed by lead-in lines separately connected from the side, while at the same time the distortions of the radiation pattern which would be introduced by the presence of a lead-in wire asymmetrically positioned with respect to the dipole, are completely eliminated. At the same time the dimensional values and impedance relationship required for impedance matching or for producing a given standing wave distribution may be in general determined in accordance with the known theory of separately fed dipoles.

Under one particular condition, however, the arrangement of Fig. 1 will differ in respect to the standing wave distribution and impedance relationship from an equivalent dipole separately fed by a transmission line connected to the dipole from one side. Such a condition arises either when an unbalanced wave containing a longitudinal or cophasal component is supplied to line 3 for feeding the dipole or when the two arms of the dipole are for some reason made of unequal electrical lengths so that the reflected waves returning to the center of the dipole do not arrive at the center of the dipole in phase opposition. In the latter case even if the wave supplied from the source were balanced an unbalanced wave would be produced by reflection, and if the lead-in arrangement were of the conventional type the unbalanced or longitudinal component would tend to return cophasally over the lead-in line. In the arrangement of Fig. l, on the other hand, the passage of such an unbalanced wave over the lead-in line is blocked by the wave suppressor which, as later described, has practically the same eifect as an open circuit with respect to waves traveling cophasally over the lead-in line 3. Under the unbalanced condition, therefore, the system of Fig. 1 will behave differently from an ordinary dipole since the unbalanced waves refiected from the dipole will be prevented from entering the lead-in line. Thus the detrimental effect of a slight unbalance of the wave supply source or of the two halves of the dipole will be eliminated.

Expressed from another viewpoint, it may be said that the dipole will behave in a similar manner to an ordinary dipole with respect to waves traveling in push-pull fashion up or down the transmission line, so that with respect to such balanced or push-pull waves the feeding of the dipole and the matching of impedances may be effected in accordance with the well-known theory. With respect to any unbalances, however, which tend to create a cophasal excitation of both halves of the dipole, or in other words, which tend to cause the dipole to oscillate as a single capacity antenna instead of as a dipole, the arrangement of the present invention eliminates any possibility of such excitation of the dipole. This is in many cases a very valuable feature, since a slight cophasal excitation of both halves of the dipole so as to radiate as a single capacity antenna tends to superpose upon the desired radiation pattern an undesired radiation pattern which sharply distorts the radiation.

Thus it is evident that a dipole fed in accordance with my invention not only is free from the distortions due to a lead-in line disposed alongside of the dipole so as to distort the field of the latter, but also is inherently free from longitudinal waves or waves which are not in true phase opposition, even if the source or the two halves of the dipole are slightly unbalanced.

Fig. 4 is an elevation partially in section which more clearly shows the exact construction of the wave blocking device 5. Referring more particularly to Fig. 4, it will be seen that the wave blocking device 5 consists essentially of the cap 6 and the body portion 1 as generally indicated in Fig. 1. Both the cap and the body portion are provided with screws 8 which serve to clamp these portions to the sheath of the coaxial line 3, and the two portions 6 and l are shaped so that together they define with the sheath of the line 3 an annular or roughly toroidal cavity 9. The body por tion I is provided with a flange l0 which is normally closely adjacent to the cap 6 so as to form a capacity therewith. By adjusting the position of the body portion 1 along the line 3 the value of this capacity may be adjusted so that .this capacity together with the inductance of the toroidal cavity 9 forms an anti-resonant wave suppressor circuit tuned to the frequency of the waves to be blocked. In case it is desired to employ the wave blocking device 5 in the open, the cap 6 is preferably provided with a down turned rim to prevent the entrance of rain or snow into the cavity 9. Furthermore, suitable draining means may be provided at the bottom of the body portion I.

When the wave blocking device 5 is properly tuned to anti-resonance with respect to any given frequency, it operates as a very efiicient wave suppressor and almost completely blocks passage of longitudinal or cophasal waves over the line 3. With wave lengths of the order of two and onehalf meters a suppression of approximately twenty decibels may be provided by one single wave suppressor of convenient dimensions constructed as shown in Fig. 4. The dimensions of the wave suppressor may be very small in comparison with the wave length to be blocked. The axial length, which is preferably the largest dimension, should be substantially less than onequarter wave length and preferably of the order of one-tenth wave length or less. At the same time it is evident that since the wave blocking device is entirely external to the line 3, this device will not in any manner impede the passage of push-pull or balanced waves which travel in phase opposition over the two conductors of the coaxial line 3. Thus so far as the normal pushpull waves transmitted over the line are concerned, the wave blocking device will have no effect and the waves can be transmitted to and from the antenna in the usual manner.

In case it is desirable to adjust the position of the wave blocking device without disturbing the adjustment of its tuning, the cap portion 6 and the body portion 1 may be clamped to a further tube which fits closely over the outer sheath of the coaxial line. Either the body portion 1 or the cap portion 6 may be fixed with respect to this tube, leaving the other portion adjustable, but preferably that portion of the wave blocking device which lies toward the waves tobe suppressed should be fixed with respect to the auxiliary tube so that adjustment of the other portion will not vary the apparent position of the wave blocking device even slightly. The auxiliary tube to which both the cap 6 and body portion 1 are attached may then be adjustably clamped to the sheath of the coaxial line 3 so that the wave blocking device as a whole may be adjusted to any desired position.

The device shown in Fig. 4 and above described, is especially adapted for use in the position shown with the cap 6 at the top of the device so as to exclude moisture. In cases where the exclusion of the weather is not important, the device of Fig. 4 may be mounted in any position, and in any case it may be used. to suppress longitudinal or unbalanced waves traveling along the line 3 in either direction. In. some cases it is preferable to arrange the device with the cap 6 lying toward the waves to be suppressed since this cap ordinarily has less surface area than. the body portion 7 and therefore the waves to be suppressed will extend over a smaller area. In case it is desired for this reason or for any other reason to mount the device with the cap downward, the construction shown in. Fig. 5 may be advantageously used. The structure of Fig. 5 is generally similar to that of Fig. 4, but a downwardly depending rim is provided on the body portion instead of on the cap in order to exclude the weather when the device is mounted with the body portion above the cap.

The structure shown in Fig. 6 is also similar in principle to the structure of Figs. 4 and 5, but in this structure both portions have approximately equal surface areas, and the capacity surfaces are positioned approximately at the center of the toroidal cavity 9, as clearly shown in Fig. 6.

The structure shown in Fig. 7 may be preferred in cases where it is desired to reduce the diameter of the wave blocking device. This structure has no radially extending flanges but rather the cap and body portions overlap one another axially so as to provide a capacity between two concentric surfaces. In. case it is desired to arrange a dipole such as shown in Fig. 1 with the upper limb of the dipole structurally exactly like the lower limb, a wave blocking device of the type shown in Fig. '7 may advantageously be used and then each limb of the dipole may be built out or enlarged to the same diameter as the wave blocking device so that there will be no discontinuity in either limb of the dipole.

In some cases where a very high degree of suppression of the waves traveling along the coaxial line 3 is required, it may be advantageous to provide two or more wave blocking devices such as 5 of Fig. l, in cascade along the line. Such an arrangement not only provides a greater attenuation of the wave to be blocked, but also provides further degrees of freedom so that the termination characteristics of the blocking arrangement may be adjusted independently of the degree of blocking and the position of the devices. For example, by employing two wave blocldng devices immediately adjacent one another in the system of Fig. 1, the upper one of these two devices may be adjusted to provide a desirable termination characteristic or reflection coefiicient for the lower limb of the dipole, while the second wave blocking device located immediately below the first one may be adjusted to more completely attenuate the downwardly traveling Waves so as to keep them out of the lower portion of the coaxial line 3. In case it is desired to employ two wave blocking devices immediately adjacent one another, the arrangement shown in Fig. 8 may be advantageous. In this arrangement two wave blocking devices are constructed as a single unit having a common cap portion 6 and two body portions '1 and 1'. All three portions of the wave blocking device of Fig. 8 may be directly clamped to the coaxial line 3 by screws 8 or may be'clamped to an auxiliary tube which is then clamped over the coaxial line as described with respect to the structure of Fig. 4.

Fig. 3 schematically represents an arrangement for feeding two separate dipole antennae over two separate coaxial lines. Such an arrangement may be desirable in directive arrays or may even be used for separately transmitting two different signals on two separate antennae with the same carrier frequency. As clearly shown in Fig. 3 two dipole antennae I and I are each supplied with ultra-short waves from the transmitting apparatus 2 over two separate coaxial lines 3 and 3'. Adjacent each antenna a wave blocking device 5 or 5' is provided to suppress the passage of waves down the coaxial line. In order to make up the continuous conducting sheaths 3 and 3' so as to avoid distortions of their radiant patterns by the presence of this long continuous conductor, an additional wave blocking device H is provided at an intermediate point between the antennae and the transmitting apparatus 2. Such auxiliary wave blocking device I! may be arranged as shown to block waves along both coaxial lines simultaneously. The details of construction of the wave blocking device may be more clearly seen in Fig, 9. As shown in Fig. 9 the wave blocking device I is essentially similar to the wave blocking device 5 as shown in the other figures, and especially is similar to the device shown in Fig. 4. The device H, however, is arranged to surround two separate coaxial lines 3 and 3' and provides instead of a simple toroidal cavity 9, a figure 8 shaped cavity 9. If the two coaxial lines 3 and 3 are arranged in very close proximity, however, the cavity 9 may constitute substantially a toroid instead of a figure 8. In such a case waves traveling in push-pull fashion over the two inner conductors of the two lines would not be blocked.

Fig. 2 represents a transmitting beacon for providing course indications for aircraft. As shown in this figure, the dipole is fed over a coaxial line 3 from the wave source 2 and is terminated by the telescopic end section 4 and the wave blocking device 5 just as described in connection with Fig. l. The rotationally symmetric radiation pattern of the dipole I must be modified to produce a useful course indicating pattern, and for this purpose the two reflecting dipoles 2| and 3| are provided at equal distances on each side of the principal dipole Each of the dipoles 2| and 3| is provided with adjustable telescopic extensions at each end similar to extension 4 of dipole I, in order that the reflecting dipoles may be tuned so as to produce. the desired radiation pattern. The two reflecting dipoles 2| and 3| are provided with relay 22 and 32 for rendering the two reflecting dipoles alternately efiective according to the desired keying code. For this purpose the relay 22 is arranged so that its contacts are normally closed, thus connecting together the two halves of dipole 2| to render this dipole effective, while on the other hand, the relay 32 is arranged so that its contacts are normally open, thus rendering the dipole 3| ineffective. Lines 23 and 33 connect the two relays to the inner and outer conductors of the coaxial line 3 at the center of dipole I as shown, choke being provided to prevent the fiow of high frequency over the lines 23 and 33. These chokes may, if desired, be omitted since the lines are essentially located at right angles to the electric field component of dipole l, but it is preferred to provide such chokes to completely avoid the flow of high frequency. Further similar chokes may also be provided inside the housings of the relay devices 22 and 32. The wave source 2 is arranged so as to supply not only ultra-short waves but also suitable coded direct current control voltages over the line 3 for actuating the relays 22 and 32. Preferably the relays are so arranged that in response to a simultaneous direct current voltage applied to both relays over line 3 the relay 22 will open its contacts to disable dipole 2| slightly before the relay 32 closes its contacts to render reflector dipole 3| efiective. Similarly upon the cessation of the direct current control voltage over coaxial line 3 relay 32 -should release so as to disable the reflector dipole 3| before relay 22 operates to render reflector dipole 2| effective.

The wave blocking device 5 operates in the same manner as the wave blocking device 5 shown in Fig. 1, and serves in exactly the same way to terminate the lower limb of the dipole I. The ultra-short waves for energizing the dipole 1 from the source 2 pass upward in push-pull or phase opposition relationship over the line 3 so that these are not blocked by the device 5. Also the direct current control voltages are not in any way affected by the presence of the wave blocking device 5.

Although the above described antenna systems incorporating my novel wave suppressing device have been described and shown as transmitting antenna arrangements, it will be understood that my invention is also applicable to receiving antenna arrangements. An of the wave suppressing structures shown in Figs. 4, 5, 6, 7, 8 or 9 may be employed Without change for suppressing Waves in a receiving antenna arrangement instead of in a transmitting antenna arrangement. Furthermore, any of the antenna systems of Figs. 1, 2 or 3 may be used as receiving antennae by substituting a receiving equipment for the sources of waves 2 in Figs. 1 and 2. In order to illustrate the applicability of the invention to receiving antenna systems, the element 2 of Fig. 3 has been designated as either a transmitting or receiving apparatus.

It should furthermore be understood that the wave suppressor unit of my invention may also be used not only for blocking waves, but also for partially blocking such waves or generally for controlling the transmission and reflection thereof. The wave blocking unit of my invention may likewise be used for suppressing waves in transmission systems not involving any antenna, as for example, in systems for transmitting waves from point to point over a coaxial line. Also wave blocking units in accordance with my invention may be employed to control the transmission of longitudinal waves not only in coaxial lines but also in shielded lines having two or more conductors within one sheath.

Although I have shown and described certain particular embodiments of my invention for the purposes of illustration, it will be understood that modifications, alterations and adaptations thereof occurring to one skilled in the art may be made without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. A coaxial transmission line having a sheath,

a conductive shell around said sheath at an intermediate position along said sheath, said shell being shaped to enclose an annular cavity around said sheath, and capacity means for tuning said shell to a desired frequency whereby the transmission along said line of waves of said frequency may be controlled.

2. A coaxial transmission line having a sheath, a discontinuity at one position in said line, a conductive shell around said line at a second position on said line and outside said sheath, said shell being so shaped as to enclose an annular cavity around said sheath, means for turning said shell to a frequency to be transmitted over said line, and means for varying the distance between said discontinuity and said conductive shell by independent changing of said first and second positions on said line.

3. A coaxial transmission line having a sheath and an inner conductor, and a resonant device coupled to said sheath for controlling the transmission along said line of waves of a certain frequency, which comprises a hollow shell shaped to enclose an annular cavity around said line, said shell being conductive and forming about said cavity a one-turn toroidal conducting circuit and having a gap interrupting said circuit, and capacity means forming a lumped capacity across said gap whereby said device is tuned to said certain frequency.

4. A multi-conductor transmission line having a sheath and a resonant device coupled. to said sheath for controlling the transmission along-said line of waves of a certain frequency which cedar prises a hollow shell shaped to enclose an annular cavity around said line substantially less than' one-quarter wavelength in axial length at said frequency, said shell being conductive and forming about said cavity a one-turn toroidal conducting circuit and having a gap interrupting said circuit, and capacity means forming a lumped capacity across said gap whereby said device may be tuned to said certain frequency.

5. A coaxial transmission line and a resonant device according to claim 2, said device being slidably positioned on said sheath whereby said device may be adjusted in position along said line.

6. A radiant acting system for operation at a certain frequency comprising a radiant action dipole having a hollow first limb and a second limb, a multi-conductor transmission line ex tending to said dipole, at least one conductor of said line passing through said hollow first limb and connected to said second limb and a wave blocking device below said dipole comprising a shell enclosing an annular cavity around said line substantially less than a quarter wavelength in axial length at said frequency, and means for turning said shell to said frequency whereby waves of said frequency may be blocked from passing over said sheath.

7. An antenna system for operation at a certain frequency which comprises a coaxial transmission line having a sheath and an inner conductor, the inner conductor being extended beyond the end of the sheath to form a first dipole limb, a wave blocking device on said sheath spaced a distance from the extended inner conductor comprising a shell enclosing an annular cavity around said sheath substantially less than a quarter wavelength in axial length at said frequency, and means for tuning said shell to said frequency, said device being so tuned as to substantially block the passage of waves flowing over the outside of said sheath whereby the portionvof the sheath between the said device and the end of the sheath constitutes a second dipole limb.

8. A plurality of coaxial transmission lines each having a sheath, a conductive shell around said lines at an intermediate position along said line, said shell being shaped to enclose an annular cavity around said sheaths, and adjustable capacity means for tuning said shell to a desired frequency whereby the transmission along said lines of waves of said frequency may be controlled.

FREDERICK A. KOLSTER.

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