US2490957A - Antenna system - Google Patents

Description

Dec. 13, 1949 T. M. GLUYAS, JR

ANTENNA SYSTEM Filed June `'50, 1945 y INVENTOR.

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Patented Dec. 13,F 1949 ANTENNA SYSTEM Thomas M. Gluyas, r., Westmont, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application June 30, 1945, Serial No. 602,444

(Cl. Z50- 33) 4 Claims. 1

This invention relates to antennas, and more particularly to systems operable selectively over more than one band of frequencies. Although it is possible to adjust the tuning of an antenna system to resonance at any desired frequency within practical limits, such adjustment is timeconsuming and may require a high degree of engineering skill and the use of expensive instruments. Accordingly it has become usual practice to employ pre-tuned antenna assemblies, designed to operate at one pre-determined frequency or band, and requiring only ordinary mechanical skill for installation.

Such antennas will operate emciently only at the frequency for which they aredesigned. If operation is required at some other frequency, a different antenna must be substituted.

It is the principal object of the instant invention to provide an improved type of antenna system which has the simplicity of installation of a pre-tuned system and yet may be arranged readily to operate with high efficiency at either of two pre-determined frequencies.

Another object of the invention is to provide an antenna system including non-adjustable tuning means, reversible in position to produce resonance at either of two frequencies.

The invention will be described with reference to the accompanying drawing, wherein:

Figure 1 is a schematic diagram of an antenna system embodying the present invention,

Figure 2 is a detail view of one of the elements of the system of Figure 1,

Figure 3 illustrates a modication of the element shown in Figure 2,

Figure 4 is a graph illustrating the characteristics of the system of Figure 1 with two different connections of the tuning means, and

Figure 5 is a sectional view of a modified impedance transformer which may be used in the practice of the invention.

In accordance with the present invention, the radiator system is designed to resonate by itself at a frequency substantially midway between the two frequencies at which the overall antenna system is to be operable. Referring to Figure 1, the illustrated radiator system comprises a dipole including the radiator elements I and 3. It is to be understood that a m'ore complex radiator system, such as a plurality of spaced interconnected dipoles, could be used instead of the simple dipole of Figure 1 without altering in any way the practice of the present invention.

The terminals of the radiator system are secured, for example by bolt and nut connections E and l, to an impedance transformer 9. The transformer 9 is an open transmission line, substantially one-quarter wavelength long at the resonant frequency of the radiator system, and consists of two sections of approximately equal length but of different characteristic impedances.

Referring to Figure 2, each conductor of the transformer 9 comprises two pieces II and I3 of conductive tubing, secured together at the point l5 by welding, brazing or the like, and flattened at their outer ends Il and I9 respectively to form terminals adapted for cooperation with the bolts and nuts 5 and 'i on the radiator terminals. The diameters of the members I l and I3 are predetermined in accordance with the desired resonance characteristics of the overall antenna system, as will appear more clearly hereinafter.

The impedance transformer 9 is connected to a feed system including conventional coupling and transmission line means. In the system of Figure l, a coaxial line 2l extends to a radio utilization device, not shown, and is provided at its upper end with terminals 23 and '25 connected to its inner and outer conductors respectively. The terminals 23 and 25 include nut and bolt means similar to those on the radiators I and 3, for connection with the ends I'l or I9 of the conductors of the transformer 9.

The final quarter wavelength section 2l of the coaxial line 2l is surrounded by a conductive sleeve or cage 29, connected at its lower end t0 the outer conductor of the line 2l by a conductive disk 3 I. This arrangement constitutes a so-called bazooka, or line balance convertor, for coupling the unsymmetrical or unbalanced-to-ground line 2l to the balanced-to-ground circuit of the dipole I, 3 and the transformer 9.

Insofar as the operation of the present invention is concerned, it may be assumed that a balanced line or other balanced-to-ground circuit is connected to the terminals 23 and 25. The impedance presented by this circuit across the terminals 23 and 25 is substantially resistive, and is of a magnitude denoted herein as R1.. When the impedance presented by the radiator system l, 3 through the transformer 9 to the terminals 23 and 25 is also resistive, and of the value RL, energy is transferred with the maximum eiciency between the utilization circuit and the radiator system.

In accordance with prior art practice, the

radiators i and 3 would @be connected directly to the terminals 23 and 25, and would be designed to have an impedance RL at resonance. The operation would be entirely satisfactory at the resonant frequency. But to obtain reasonably eiliciency at any other frequency, it would be necessary to redesign the radiator to exhibit a resistive impedance RL at that frequency. This would usually involve more than merely retuning theradiator system, because the resonant impedance ofV a given radiator' tunedV to one frequency may be higher or lower than that of the same radiator tuned to a diiferent frequency.

In accordance with the instant invention, the radiator I, 3 is designed toresonateat a frequency fn, substantially midway between two Yfrequencies fi and f2 at which efficient operation is desired. Its impedance at the frequency ,fc

may be approximately Rr.. At the lower frequency f1, the impedance of the radiator is .R1-iXi, where R1 is somewhat less than RL, and X1 is the capacitivereactance of theradiator at the frequency fr. At the higher frequency f2, the radiator impedance is Rz-l-jXz. R2 is somewhat greater than Rr., and X2 is the inductive reactance. The magnitudes of X1 and X2 are approximately equal-to each other.

The transformer 9 may be regarded as two cascade-connected lines, each substantially one eighth wavelength long and having different characteristic impedances, Z1 for the part-formed by the larger conductors I3, and `Zz for the part formed by the smaller conductors Il.

The impedance looking into one end of a transmission line is Zoutjzctanp in ZffI-jzout tafnp c where Zout is the impedance connected to the other end of the line, ZC is the characteristic impedanceA of the line, and p is the electrical length of the line. In the case ofan eighth wavelength line,

and tan p=1. Thus In the case of two eighth wavelength lines connected in cascade, Zin for the rst line is Zout for the second. With the lines connected as shown in Figure 1, the impedance at the terminals 23 and 25 is where ZP. is the impedance of the radiator system. At the lower frequency fi, Zs is .R1-5X1. By proper choice of the impedances Zi and Z2, Zin'may be made substantially equal to RL, at the frequency f1.

Referring to Figure 4, the impedance match at the terminals 23 and 25 under theconditions of Figure 1 is shown by the curve designatedA fr of the standing wave ratio on the line2l. This is unity at the frequency fr, showing that no reection and hence optimum transfer of energy occurs at that frequency.

Now let the transformer 9 be reversed, with the high impedance section connectedi tothe 4 radiators. The impedance at the points 23 and At the frequency f2, the radiator impedance Za is Rz-i-y'Xz. With the characteristic impedances Z1 andV Z2 the same as before, it is found that Z-in is substantially equal tc- RL, at the frequency f2. The corresponding curve of standing wave ratio vs. frequency (Figure 4) shows optimum transfer at the frequency f2.

Thus the described antenna system provides equal and maximum efficiencies of operation at either of two predetermined frequencies, by simply interchanging the connections of a line impedance transformer between the radiators and the feed system. Although the invention has .been-described with reference to the use of a transformer of the stepped-line type, it will be apparent without further detailed explanation that a smoothly tapered line may be used instead; The word tapered is used in the appended claims to mean tapered by steps, as well as smoothly tapered. Referring to Figure 3, tapered conductors, each comprising a conductor continuously varying in diameter throughout its length, maybe substituted for the stepped conductors in the transformer 9 of Figure 1.

Moreover, the practice of the invention is not limited to the use of open-wire line transformers, but may be embodied equally well in a system using unbalanced line sections for impedance transformation. Figure 5 illustrates a transformer of this type, comprising an outer conductor of uniform diameter and an inner conductor including two sections 3l and 39 of different diameters.. This structure may be used exactly; like the transformer 9 of Figure 1 to provide an impedance match at either of two frequencies between an antenna and its feed system.

I claim as my invention:

1. An antenna system including naturally selfresonant radiator means, transmission means lto be coupled to a utilization circuit, impedance transformer means comprising a transmission linefsection substantially one-quarter wavelength long at the. resonant frequency of said radiator means, the conductors of said line section having one ratio of spacing to diameter at one endand a: different ratio of spacingto diameter at the other end, whereby said line section exhibits a relatively high characteristic impedance at one end and a relatively low characteristic impedance at the other end, and means for connecting. said low impedanceY end to said radiator means and said high impedance end to said transmission means for operation of said system at one frequency and for connecting said low impedance end to said transmission means and said high impedance end to said radiator means for operation of said system at another frequency.

2. An antenna system for selective operation at either of two different frequencies, comprising. radiator means naturally self-resonant at a frequency substantially midway betweensaid two frequencies, whereby said radiator means exhibits a' capacitive. reactance at one of said two frequencies and aA substantially equal inductive reactance at theother of said two frequencies,

means,` for coupling said antennal system to a utilization circuit, impedance transformer means comprising a quarter wavelength tapered transmission line section, and means for connecting said line section with its taper in one direction for operation at one of said frequencies and with its taper in the other direction for operation at the other of said frequencies.

3. A multi-band antenna system including radiator means naturally resonant at a frequency substantially midway between the center frequencies of two frequency bands through which the system is to operate, an impedance transformer comprising a quarter wavelength line consisting of at least two cascaded sections having diierent characteristic impedances, means for connecting a utilization device to said radiator means in one direction through said quarter wave line section for operation in one of said bands, and means for connecting a utilization device in the other direction through said quarter Wave line for operation in the other of said bands.

4. In combination with a circuit element selfresonant at a frequency .fo and a utilization circuit to be coupled to said element, a reversible transformer section connected between said element and said utilization circuit and comprising a quarter wavelength tapered transmission line section and means for connecting said line section with its taper in one direction for operation at one frequency greater than fu and means for connecting said line section with its taper in the other direction for operation at a diierent frequency less than fo.

THOMAS M. GLUYAS, Jn.

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS

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