US2492404A - Construction of ultra high frequency broad-band antennas - Google Patents

Description

A. STREIB ETAL 2,492,404

BROAD-BAND ANTENNAS 2 Sheets-Sheet 1 INVENI'ORS. ALBkm/r Sreae. 59.9460. THOR/v5. Boat-2T6. WEI/IVER,

ATTORNEY Dec. 27, 1949 CONSTRUCTION OF ULTRA HIGH FREQUENCY Filed Nov. 10, 1945 Dec. 27, 1949 BROAD-BAND ANTENNAS 2 Sheets-Sheet 2 Filed Nov. 10, 1945 NN v w m m 3% M wmfw g LE8 45mm QQkQbQQQQ $835 k0 LQQQNQWY 90 I I I- l mNNNNMRNNNNWNNNNN K WNW ///////w///1'HM M 7//fl///// -m Patented Dec- 27, 1949 CONSTRUCTION OF ULTRA HIGH m- QUENCY BROAD-BAND ANTENNAS I Albrecht Streib, Port Jefferson, Earle Dayton Thorne, East Patohog'ne, and Robert 8. Wehner, Port Jeiferson, N. Y., asslgnors to Radio Corporation of America, a corporation of Delaware Application November 10, 1945, Serial No.- 027,951,

The present invention relates to broad-band U. H. F. antennas and, more particularly, to a method of constructing such antennas on a production basis.

An object of the present invention is the manufacture of antennas having electrical characteristics, approaching 'those of accurately machined model-shop antennas, which are easyto attain and whichinvolve few operations which cannot be satisfactorily performed by inexperienced workmen. 1

A further object of the present invention is the provision of an antenna and matching net workhaving absolutely no physical discontinuities in the interior of the matching section.

The present invention depends upon the fact that the flexible coaxial cable is at present available in a wide variety of sizes and characteristic impedances. We have found that by selecting sizes of these commercially available cables and assembling predetermined lengths of the dielectric portions of the cables together with an antenna rod in a sleeve cut from commercially availof the coaxial transmission line TL. It has been found that the input impedance oi this antenna may be satisfactorily matched to the transmission line TL having a characteristic impedance of the order of fifty ohms over a very broad frequency band with less than two to one standing wave ratio with a transformer section having an impedance of from eighty five to ninety ohms.

.The electrical length of the transformer section is, as will be noted from the inspection of Figure 1, appreciably less than one quarter wave-length at the center of the pass-band. Such a matching section may therefore be built directly into the lower section of the antenna proper by simply appropriately proportioning the relative diameters of the conductors inside the antenna for an appropriate distance. 7

While the antenna shown in Figure 2 has very satisfactory impedance characteristics over a go broad band, when used on aircraft, it also has two major defects.

First, the air-dielectric in the open-ended matching section l2-I6 provides no support for able thin wall brass tubing, an antenna may be the inner conductor and theprotruding radiator II. This results in a system mechanically poor satisfactorily assembled by inexperienced work men. No difflcult machining and molding operations such as are called for by previously known methods are employed.

The present invention will be more. fully understood by reference to the following detailed description inwhich:

Figure 1 illustrates in diagrammatic form a previous known type of antenna of which the present antenna is an improvement:

Figure 2 illustrates a further development of the antenna of Figure 1, while Figure 3 is a flow chart illustrating the method of manufacture of the antenna of the present invention.

Referring now to Figure 1, there is shown a broad-band antenna having an overall length of a quarter wave-length and including an extending inner conductor In and a sleeve radiator portion ii. The antenna is energized from the transmission line TL, having outer shell I4 and an inner conductor Iii. The outer shell I4 is an electrical continuation of sleeve l2. The inner conductor I5 is connected to a radiator portion It by an intermediate conductor section it. The ratio of the inner diameter of sleeve I! to the diameter of inner conductor II is so determined that the intermediate series section formed thereby within sleeve l2 acts as an impedance matching network, matching the antenna input imfor use on aircraft where it is exposed to wind forces and to extreme vibration.

Second, the open mouth of sleeve l2 permits free entry of moisture and other extraneous mat-. ter likely to impair the electrical characteristics of the antenna. Attempts to remedy these defects by means of disk of low-loss solid dielectric, thick enough to provide the desired mechanical support and sealed in to prevent entry of foreign matter at the open end of sleeve l2, result in failure since the discontinuity thus introduced into the matching section greatly reduces the band width of the antenna, the said insulating disk behaving much like a lumped reactance placed in shunt with the antenna. Therefore, it appears necessary to fill the entire matching section: that is, the entire region inside sleeve it with solid dielectric. This provides a sealed an, tenna having adequate mechanical support-for the central conductor and a matching section free from discontinuities.

Such a form of construction is shown in 1'18- ure '2 where the major elements of the antenna itself bear the same reference numerals as those of Figure l and will not therefore again, be described in detail. The solid dielectric is indicated by reference numeral 20L Dielectric II entirely fills the matching section, thus supporting extending radiator l0 and sealing, the matchpedanee at the top of sleeve I! to the impedance in: section l2-l6. Due to the higher dielectric constant of insulator 20, the physical dimensions of the matching section are so altered that the electrical characteristics of the matching section are preserved.

At the high frequencies for which these antennas are intended, it is quite essential that the losses in the solid dielectric 20 be as low as possible. In order that the diameter of the inner conductor IE not be too small to provide adequate support for the external radiator [0, it is also essential that the dielectric constant of the solid dielectric material 20 be low. For these reasons, polyethylene is chosen as a preferable solid dielectrical material.

In Figure 2 the transmission line TL is shown as terminated by a coaxial transmission line connector TLC having a threaded outer shell 2| and inner jack member 22. Connector TLC is only for the purpose of facilitating connection of the antenna to the conventional and widely used solid dielectric coaxial transmission lines.

The antenna constructed according to Figure 2 using polyethylene in the interior of the sleeve met all the expected electrical, mechanical, aerodynamical and temperature cycle tests administered in the Laboratories of the War Department. However, trouble was encountered in manufacturing the antenna of Figure 2 in large quantities, primarily in the manufacture of the polyethylene filled matching section. Ordinarily polyethylene is commercially available in only two bulk forms: a granulated powder for molding and in solid 4 cylinders of various diameters for machining. It

was discovered that the molding procedure though more suitable for mass production purposes was not satisfactory due to the great difiiculty in so molding a dielectric that it was free of air bubbles and at the same time hold the inner conductor centered in the sleeve. Many production samples of the molded sleeve antennas, it was discovered, were far inferior to the first model-shop samples. The machining process, it was discovered, was more successful but, while it gave satisfactory results, it involved either drilling a long accurately centered hole in a cylinder of polyethylene dielectric or the drilling of central holes in a series of I short cylindrical plugs of polyethylene which plugs were, after machining, strung on the inner conductor l6 and cemented together.

These diiiiculties were so extreme that it was impossible to find a manufacturer willing to undertake production of antennas on a large scale. However, it was discovered that flexible coaxial cable is available in a wide variety of sizes and characteristic impedance values. Among the standard cables obtainable is a 50-ohm solid-polyethylene-dielectric cable known as RG-l4/U, having an inner conductor 102 mils in diameter, also a larger 50-ohm cable which is likewise polyethylene-filled, known as the RG-l9/U, which has a dielectric outer diameter of 910 mils. The significance of the above-mentioned dimensions is as follows:

If a length of RG-l9/U cable as shown at a in Figure 3 is taken, the inner conductor 3| removed, the axial hole in the polyethylene cylinder enlarged to take the outer diameter of the dielectric of the smaller RG-14/U cable, the smaller cable with the outer braid removed may be inserted in the axial hole, there is obtained a section of polyethylene dielectric line of diameter ratio of 910/102 which corresponds to a characteristic impedance of 87 ohms, the proper value for the characteristic impedance of the matching section of the sleeve antenna. -In this way the interior of the sleeve antenna is constructed with a single machining operation; namely, the enlarging of the pre-exlsting hole in the RG-lQ/U cable, an operation which is relatively so simple since it is much easier to enlarge a long pre-existing hole than it is to drill a long accurately centered hole in a solid cylinder of polyethylene.

At b in Figure 3 is shown the resulting cylinder 20 of polyethylene dielectric after the outer braid 30 and inner conductor 3| have been removed and after the hole in the interior has been enlarged to take dielectric of the smaller cable. A groove 34 is preferably cut in one end of the polyethylene cylinder for securing the sections together later on in the process of manufacture. The inner conductor 3| of the matching section may be easily removed if the length of conductor is immersed in hot water having a temperature of around 170 for a few minutes. The steps on fabricating the inner conductor are shown at c and d in Figure 3. At 0 is shown a length of RG-14/U cable. The outer vinyl coating 40 is cut back as shown at d. The braid conductor 4| under the outer coating 40 is likewise cut so that about one centimeter of it protrude beyond the outer vinyl coating 40. The protruding portion should be tinned as should also about one centimeter of the inner conductor 44 protruding beyond the dielectric 43.

The outer conductor i 2 of the matching section may easily be made by cutting off a. desired length of thin wall brass tubing having an outer diameter of one inch and an inner diameter of 910 mils. This is a commercially available size. Preferably the outer edge of one end should be rounded oil! slightly.

Now, the assembly of the inner conductor is completed as shown at e in Figure 3. A spacer is slipped over the outer vinyl covering 40 and a second spacing washer 5| slipped over the protruding end of the tinned braid 4| and soldered in place. Washers 50 and 5! are provided wi h spinning grooves 53 whereby the outer sleeve i2 is finally maintained in position. A five-sixteenths inch diameter cylindrical rod forming the antenna l 0 is slipped over the protruding end of 44 and soldered in position. The soldering operation is facilitated by solder entry hole 55 near the end of rod It). Now the matching sec tion of dielectric from b is slipped over the portion 43 of the inner conductor assembly, preheating if necessary, to insure a snug fit. Then the entire assembly is inserted inouter conductor l2, aligning the outer conductor so that the radiator rod l0 and one inch of the dielectric 20 protrude from the outer tube. The outer tube I2 is spun into groove 53 in washers 50 and 5i and also into groove 34 in dielectric 20, thus locking the entire arrangement into a single rigid structure.

The previous structure shown in Figure 2 utilizes a standard transmission line connector TLC in the hm of the antenna. In the present method a transmission line connector pair TLC consisting of cooperating male and female coupling members may be inserted in the RG-l i/U cable leading out of the antenna base at any convenient point, an operation which may easily be done in the field with no, tools other than a knife and a soldering iron. Further, the connector may be strategically located in the vicinity of a minimum of thevotage standing wave, thus minimizing the adverse efiects that a standard cable connector may have on the band width of a very broad-band antenna. In applications where thedistance from the antenna location to the transmitter is known in advance, it is possible to entirely dispense with connectors at the base of the antenna simply by continuing the 50 ohm RG-14/U line TL fromthe base of the matching section on to the transmitter. Thus, the dielectricsurrounding the inner conductor 44 is continuous over the entire distance from the mouth of the sleeve l2 to the connector TLC wherever located. There is thus no possibility of arc-over inside the antenna due to the presence of air-gaps or to the presence offoreign matter in such air-gaps. Antennas constructed by this method are therefore certain to handle any power that can be carried by the cable itself.

From an inspection of Figure 3 it will be evident that most of the machining procedures required in constructing the antenna of Figure 2 have been entirely eliminated. Preparation of the sections of standard cable involves only very simple operations of the type commonly performed by inexperienced labor using simple, easily constructed jigs. Preparations of'the spacers of washers and El and the drilling out of the hole in the end of rod Ill may be done on the production basis using common semi-automatic machine tools. Aside from the operations of 4 drilling out the hole in the plug of the large diameter cable, rounding off its exposed corners and spinning in a wall of the sleeve there are no operations requiring the use of a lathe or requiring skills that are not quickly picked up. The greater part of the work in making sleeve antennas by this method is simply assemly work. A further advantage of the method is that the large coeiiicient of thermal expansion of polyethylene is an asset rather than a liability as in other methods since it makes possible the easy extraction of the inner conductor of the large cable and the easy application of the modified section to the small diameter transmission line.

While the foregoing method of structure has been described with reference to one broad-band type of U. H. F. antenna, it should be understood that it may be applied to any ultra high frequency antenna system, balanced or unbalanced, in which it is necessary that sections of transmission line be built into the antenna as an integral part of its structure.

While we have illustrated a particular embodiment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in their arrangement and it is therefore contemplated by the appended claims to cover any such 'modifications as fall within the spirit and scope of the invention. 7

What is claimed is:

1. A method of fabricating an antenna and series connected impedance matching network which includes the steps of stripping the outer conductor from a solid dielectric coaxial line for a distance equal to the length of said matching section, attaching a radiator rod to the extreme end of the inner conductor of said coaxial line, stripping the outer conductor and inner conductor from a section of larger diameter, solid dielectric coaxial line having a length equal to said distance plus an amount suflicient to support said radiator rod, enlarging the hole left by the removal of the inner conductor from said section whereby said section may he slipped over the stripped portion of said first mentioned coaxial line and radiator, and securing a section of conductive tubing over said dielectric section.

2. a same fabricating an antenna and series connected impedance matching network which includes the steps of stripping the outer conductor from a solid dielectric coaxial line for a distance equal to the length of said matching section, attaching a radiator rod to the extreme end of the inner conductor of said coaxial line. connecting a conductive spacing disc to the end of the outer conductor of said coaxial line, stripping the outer conductor and inner conductor I over the stripped portionof said first mentioned coaxial line and radiator with an end in contact with said spacing washer and slippinga section" of conductive tubing over said dielectric section.

3. A method of fabricating an antenna and series connected impedance matching network which includes the steps of stripping the outer conductor from a solid dielectric coaxial line for a distance equal to the length of said matching section, attaching a radiator rod to the extreme end of the inner conductor of said coaxial line, connecting a conductive spacing disc to the end of the outer conductor of said coaxial line, stripping the outer conductor and inner conductor from a section of larger diameter, solid dielectric coaxial line having a length equal to said distance plus an amount sufllcient to support said which includes the steps of stripping the outer conductor from a solid dielectric coaxial line for a distance equal to the length of said matching section, attaching a radiator rod to the extreme end of the inner conductor of said coaxial line, placing a pair of spacing discs over said coaxial line, connecting one of said spacing discs to the end of the outer conductor of said coaxial line, stripping the outer conductor and inner conductor from a section of larger diameter, solid dielectric coaxial line having a length equal to said distance plus an amount suflicient to support said radiator rod, enlarging the hole left by the removal of the inner conductor from said section, slipping said section over the stripped portion of said first mentioned coaxial line and radiator and slipping a section of conductive tubing over said dielectric section.

5. A method of fabricating an antenna and series connected impedance matching network which includes the steps of stripping the outer conductor from a solid dielectric coaxial line for a distance equal to the length of said matching section, attaching a radiator rod to the extreme end of the inner conductor of said coaxial line,

, 8 said distance plus an amount suflicient to support mono (71'!!!) said radiator rod, enlarging the hole .left by the The noun: "an" I: removal of the inner conductor from said section, m of this of rd in the slipping said section over the stripped portion 0! said first mentioned coaxial line and radiator and 5 '7 UNITED STATES PATENTS slipping a section of conductive tubing over said Num "1! dielectric section, and connecting said tubing to 23 Burks! fg s id pacing dis 2,091,839 Tanyeman 'Aug: 31: 1937 ALBRECHT 3112,1513 m 2,205,874 Buschbeck June 25, 1940 W DAYTON THQRNE, 2,311,472 Roocenstein Feb. 16, 1943 ROBERT s. wrzrmm. 1.454 Brown June a, 1943 2,403,252 Wheeler July 2, 1946

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