US2543085A - Wide frequency band antenna - Google Patents

Description

Feb. 27, 1951 o, wlLLOUGHBY 1 I 2,543,085 V WIDE FREQUENCY BAND ANTENNA Filed April 15, 1945 2 sheets-sheet 1 Feb. 27, 1951 E. o. WILLOUGHBY Q 2,543,085

WIDE FREQUENCY BAND ANTENNA Filed April 13, 1945 v 2 Sheets-Sheet 2 Patented Feb. 27, 1951 UNITED STAT-ES PATENT OFFICE WIDE nastiness-11m ANTENNA Eric Osborne Willoughby, London, England, as-

signor, by mesne assignments, to International Standard Electric'Corporation, New York, N. Y., a corporation: of Delaware Application al.151945 Serial No.: 588,269 In. Great. Britain April. 21', 1944,

6 Claims. 1

The present invention relates to radio antennas, and concerns particularly antennas of small size adapted for stream lining for use on aeroplanes.

It is frequently found that antennas used-on aeroplanes are liable to be broken off' by the air resistance when driving at very high speed. It is therefore most desirable that the antenna should be as small as possible so that the minimum air resistance be offered.

The present specification describes a new type of dipole antenna which can be. made. very short compared with the wave-length, (for example about one fifth).

According to the invention, there is provideda radio antenna comprising a. hollow metallic body enclosing two electromagnetically coupled resonating systems. both tuned to the operating frequency; and a transmission line feeder for conveying currents to or from the antenna coupled to one of the said systems.

According to the invention also. there is pr.0. vided' a radio antenna comprising a. hollow resonator, a resonating circuit inside the said resonator and coupled electromagnetically thereto, and a transmission line coupled to thesaid circuit,

the said resonator and circuit. being each tuned to the operating frequency.

The invention also provides; a radio antennav comprising a hollow coaxial resonator, aresonating input transmission line inside the said resonator and coupled thereto,, and a, transmission line feeder for conveying currents. to or from the. antenna coupled to. thesaid input line, the said.

input line and resonator being. tuned to the-operating. frequency.

The invention willfbe described with referencev to the accompanying drawings, in which:

Fig. 1 shows diagrammatically the features of an antenna according to the. invention;

Figs. 2, 3 and 4 show equivalent. circuits :em

played in explaining the action of the antenna;

Figs. 5 and 6 show respectively a longitudinal sectional View, and an. end view (with the cap removed), of an antenna. constructed. according to the invention; and

Figs. 7 and; 8. show diagrams corresponding to part. of Fig. 1 to show the manner in which a rectifier may be connected inside the antenna.

Fig. .1 shows diagrammatically an. antenna according to the invention. It comprises-arodor wire. conductor I loaded. at the ends with rela tively large metal masses land 3.. Surrounding the. conductor I. is. a hollow cylindrical-s bodys 4 forming therewith, a hollow resonator. consisting.

of.l an. inner, coaxial transmission line open at both ends, which ends are terminated by the capacities between, the body 4 and the masses 2 and 3. These terminating capacities may be supplemented by adjustable tuning capacities 5 and 6.

Currents are led toor'from the'antennaby a coaxial transmis'sion line feeder I entering the body 4 at the median plane l2, and the central conductor 8 is connected to a point. on a conductor 9 placed inside the body 4 parallel to the rod I. The conductor 9 is connected at H)" to the wall of the body 4 (either directly or. through an appropriate blocking condenser, not shown) and the other end is terminated by a tuning condenser H.

The conductor 9' forms with the body 4 a tuned input transmission line whose electromagnetic field is coupled with that of the resonator formed" by the conductor I and the body- 4. The system is equivalent totwo coupled resonant circuits both of which should be tuned to thefrequency of the waves to be radiated or received. According to the usual practice with coupled tuned circuits, the Q value (ratio of reactance to resistance) of the two circuits should be the same and they should be sufiic'iently' over coupled (that is, the

coupling factor should be a little greater than- The red I'- with the two masses 2 and 3 forms a dipole antenna, but its potential variations are transferred to the outer surface of the enclosing body 4 which thus acts as the real radiator. The physical length of the rod 1 and body 4 can be small compared with the wave length ('for example about one fifth of the wave length). The inner transmission line will have a voltage node at the median plane l2 and a current antinode, andthe current atany instant will be nearly the same at all points of the body owing to its short length.

Under these conditions the short inner coaxial transmission, line. formed by. the rod 1 and. th body 4' as seen from the open ends .is substantially equivalent. to. the network of lumped reactances shown in Fig. 2. Let 2d be the total length of. the. body 4 and let 0=2.1rd/ where A- isthe wave-1ength, then it-is easil-y'shown that the reactanceofeach. of the equivalent oon-- densers K is. iZ1 eat 0, and the reactance of the inductance 2! is 721 sin 20,, in which Z1 is the characteristic impedance of. the; inner transmission line. line being open at both ends,. (assuming that the excitation by the conductor 9 is balanced),- there will. be? a voltage node -at theeentne on: the medianeplane 1.2,..andsoas seen:

from either end, the network of Fig. 2 is in effect short circuited in the centre as indicated by the dotted line. The inner transmission line therefore presents an impedance at either end which may be represented by the network of Fig. 3.

The outer surface of the body 4 can be regarded as forming with ground an outer transmission line open at both ends, there being a voltage node at the median plane I2. If Z2 is the characteristic impedance of this outer transmission line, then the reactance presented by this line at each open end will be 722 tan 0. V

It is now possible to construct an'approximate equivalent circuit for the antenna of Fig. 1. This is shown in Fig. 4. The small squares 2 and 3 represent the end masses 2 and 3 of Fig. 1. These are shown connected to earth by approximately equal capacities C2 and C3. The mass 2 is also connected to ground through the two open end impedances of the inner and outer transmission lines, which are in series. These are represented respectively by the network 12, K2, and by the inductance L2 in series with a resistance T2 representing the radiation resistance of half the antenna. Similarly the mass 3 is connected to earth through corresponding impedances 13, K3, and. L3, T3, where where w is 21r times the frequency.

The condensers S2 and S3 are the tuning condensers 5 and 6 shunted across the open ends of the inner transmission line.

The exciting conductor 9 is represented in Fig. 4 by two series connected windings coupled respectively with Z2 and la. The excitation is such that the potentials of 2 and 3 are equal and opposite, so "that the two halves of Fig. 4 are effectively in series. S2 and S3 will be adjusted so that the series circuit resonates at the oper ating frequency. As all the elements of the cir cuit are known the effective Q value can be determined. For example, if the antenna be supposed to be cut inhalf on the median plane I2, then a resonance curve relating to the impedance looking into the cut end to the frequency can be determined for half the antenna, from which the Q value can be found.

By suitable choice ofthe impedance of the transmission line 7 (Fig. l) and the'manner in which the conductor 8 is connected to the conductor 9, the Q of the input circuit may be made to have the same value.

In order that the antenna may be efiicient it is necessary that the masses 2 and 3 should be made relatively large, so that the reactances of C2 and C3 are reasonably small, otherwise the series circuit of Fig. 4 will have such sharp tuning that only a small band width can be handled.

by the antenna.

Figs. 5 and 6 show one form in which an antenna according to Fig. 1 may be made up. The body 4 has the flattened segmental section indicated in Fig. 6. The conductor I of Fig. 1 is represented by two parallel rods IA and iB, the ends of which are seen in Fig. 6. In Fig. 5 only the rod IA is visible.

The ends of the body are closed by insulating plates I3 and I 4. Outside these plates are fixed similarly shaped metal plates I5 and IS. The plates I3, I4, I5 and I6 have clearance slots HA and. "B for therods IA and IB, and the parts 4 are clamped together by means of the nuts I8 which screw on the ends of the rods at both ends. The slots permit the spacing of the rods to be adjusted.

The conductor 9 of the input line comprises a flat strip arranged between the rods IA and IB. The strip 9 is bent round at one end and clamped by screws to the body 4 at II]. The conductor 8 of the transmission line 1 passes through an insulating disc I9 closing the end of the tube and is secured to a metal disc 29 which rests on the disc I9. Another insulating disc 2i covers the metal disc 20, and the strip conductor 9 rests on the top of the disc 2i, being held down by the screw 22 provided with an insulating sleeve and washer, as shown, to prevent the strip 9 from being short circuited.

A small metal disc 23 carried on a screwed shank passing through the wall 4 is arranged below the strip 9 and forms. therewith an adjustable condenser corresponding to II of Fig. 1.

The plates I5 and I6 are provided with metal tongues 25 and 26 soldered or otherwise electrically secured thereto. through corresponding slots in the plates I3 and I4. Discs 2! and 28 with screwed shanks similar to 23 pass through the walls of the body 4 and leading masses 2 and 3 of Fig. 1.

The transmission line I is provided with a foot or flange 32 by which the antenna may be fixed to the underside of a wing of an aeroplane, for example. The transmission line I should be about a quarter wavelength long.

. movements.

.may be stream-lined by means of a suitably The use of two parallel rods to form the conductor I enables the characteristic impedance of the inner transmission line to be given a suitable value, and the coupling factor between the inner trnasmission line and the input line may be adj usted by adjusting the spacing of the rods in the slots HA and NB. It is, of course, not essential to use two rods. Gne of them could be omitted, or more than two could be used.

The section of the body 4 of the antenna is only approximately correctly stream-lined. The sharp forward edge sets up some eddies which assist in the prevention of ice formation. The end caps may be suitably stream-lined for end-on The co-axial transmission line I By'suitable choice of the capacitie of these condensers, the impedance of the transmission line I may be made to load the input line so as to produce the desired value of Q. If a direct connection were made between the conductors 8 and 9, this connection would probably need to be inconveniently near one end of conductor 9.

When the antenna is usedas a receiver, a suitable rectifier may very conveniently b housed inside the body 4. If a low impedance rectifier "These tongues pass (such as a copper oxide or selenium rectifier) is used, it may be connected as shown diagrammatically in Fig. 7. A plate 35 insulated from the body 4 forms a by-pass condenser. The rectifier 33 is connected between th conductor 9 and the plate 35 near the end 10 of the conductor 9. The conductor 8 is in this case connected to the plate 35 and not to the conductor 9, and carries the rectified current, the radio frequency current being by-passed. When a high impedance rectifier, such as a diode, i used, it may be connected as shown in Fig. 8. The plate 36 is insulated from the body 4 to form a by-pass condenser, and the conductor 9 is connected to this plate at N. The conductor 8 is'also connected to the plate 36. The diode 34 is connected at the other end of the conductor 9 near the condenser l I. the rectified current flows to th conductor 8, and the radio frequency current is bypassed at l0.

What is claimed is:

1. A radio antenna of the dipole type with means for rendering said antenna short compared to the operating frequency comprising a coaxial line type section open at both ends, a

capacitive load coupled across each end of said coaxial section, said coaxial section being dimensioned to resonat with said capacitive loads at the operating frequency, a circuit resonant at the operating frequency and coupled to said coaxial section at the center thereof, and a translating device coupled to said resonant circuit.

2. An antenna according to claim 1 in which said resonant circuit comprises a conductor mounted parallel to and spaced from the inner conductor of said coaxial section and means connecting said conductor of said resonant circuit to the outer conductor or said coaxial section.

Again 3. An antenna according to claim 1 in which said capacitive loads comprise metallic masses substantially closing the respective ends of said coaxial section and providing relatively large capacities to earth.

4. An antenna according to claim 1 in which said resonant circuit comprises a conductor inside said coaxial section and parallel to the axis thereof, said conductor being connected at one of its ends to the outer conductor of said coaxial section and a tuning condenser connecting said conductor of said resonant circuit at the other of its nds to the outer conductor of said coaxial section.

5. An antenna according to claim 1 further comprising a capacity potentiometer connecting said translating device to said resonant circuit.

6. A radio antenna according to claim 1 further comprising a rectifier connected across said resonant circuit.

ERIC OSBORNE WILLOUGHBY.

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

UNITED STATES PATENTS Number Name Date 2,005,779 Gothe June 25, 1935 2,159,648 Alford May 23, 1939 2,284,405 McArthur May 26, 1942 2,287,220 Alford June 23, 1942 2,287,845 Varian June 30, 1942 2,304,377 Roberts Dec. 8, 1942 2,344,171 Rote Mar. 14, 1944 2,382,693 Dallenback et a1. Aug. 14, 1945 2,424,089 Gethmann July 15, 1947

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