US2823381A - Antenna - Google Patents

Description

Feb. 11, 1958 J. F. P. MARTIN ET AL 2,823,381

ANTENNA Filed Jan. 18, 1952 3 Sheets-Sheet l INVENTORS, J F" E M ear: in.

y I...H.KEUD

RTIN ET AL lower Feb. 11, 1958 Feb. 11, 1958 J. F. P. MARTIN ET AL 2,823,381

ANTENNA 3 Sheets-Sheet 3 Filed. Jan. 18, 1952 0 000 m m M a a w fl pmm INVENTORS United States Patent ANTENNA John F. P. Martin, Mendham, and Louis H. Kellogg, Far Hills, N. J., assignors, by inesne assignments, to the United States of America as represented by the Secretary of the Army Application January 18, 1952, Serial No. 267,213

6 Claims. (Cl. 343-797) This invention relates to an electrical apparatus and particularly to an antenna system. More specifically the invention relates to a turnstile antenna for ultra high frequencies whereby a compact and efiicient system is provided.

In accordance with the invention herein the beacon tracking requirements for the ballistic studies of the Nike missile, a surface-to-air guided missile, indicate the need for an antenna which will cover the upper middle range of radar frequencies or S band and which is circularly polarized to permit operation with the corresponding S hand SCR-584 tracking radar, which does not have a nutating feed. The S band referred to is a frequency band of 1550 to 5,200 megacycles with wave lengths 19.35 to 5.77 centimeters respectively. In addition, the antenna which is to be mounted externally to the missile must present low drag at the supersonic speeds to be encountered.

It is accordingly an object of our invention to provide a simple and effective antenna system.

It is a further object of our invention to provide a turnstile antenna adapted for use with a missile wherein space and area are limited.

The above mentioned and further objects and advantages of our invention and the manner of attaining them will be more fully explained in the following description taken in conjunction with the accompanying drawings in which:

Figure 1 is a perspective view of the antenna attached to a missile fin.

Figure 2 is a fragmentary longitudinal section of the antenna.

Figure 3 is a cross section view taken on lines 3--3 of Figure 2 and looking in the direction of the arrows.

Figure 4 is a cross section view taken on lines 4-4 of Figure 2 and looking in the direction of the arrows.

Figure 5 is a view taken on lines 55 of Figure 2 and looking in the direction of the arrows.

Figure 6 isa side fragmental elevation viewed 90 degrees from the position of Figure 2.

Figure 7 is a curve showing the antenna power pattern.

Figure 8 is the susceptance curve of the antenna.

Figure 9 is the conductance curve of the antenna.

Figure 10 is standing wave ratio curve.

Referring to the drawings in which like reference characters indicate like parts the antenna comprises two dipoles crossed and spaced 90 degrees physically. Reference numeral 2 indicates the fin of a missile body having secured thereto by coupling 16, standard 4 which serves as the feed line to the radiator elements 6 and 8 and includes a quarter wave detuning slot 10 to balance the line to the elements. A tapered cap 12 of Plexiglas is cemented to the outer extremity of standard 4 to reduce the drag at the extremely high velocities expected. Within standard 4 which itself serves as a conductor and maintained coaxial therewith by polystyrene spacers 11 is inner conductor 13 which is soldered to outer conductor 4 as at 15. Radiator elements 6 and 8 of unequal length for a purpose to be presently explained are soldered to standard 4 as best seen in Figure 4.

The antenna which is the embodiment of this invention consists of two dipoles crossed and spaced degrees physically and having currents approximately 90 electrical degrees out of phase in the respective elements to obtain the circular or rotating polarization. The method normally employed in turnstile antennas to obtain the phase quadrature currents involves feeding the dipoles in phase quadrature. It is not feasible from a mechanical standpoint to accomplish the result in the antenna of certain research and development missiles of the surfaceto-air type in this fashion because of the small space and area requirements. The antenna was developed on the basis of obtaining the phase quadrature currents by causing the individual dipole elements themselves to have impedances or admittances in phase quadrature, and such that the susceptance components of the admittances approximately cancelled when fed in parallel. In this way the antenna was caused to present a nearly resistive load to the antenna feed.

The first step in this arrangement was to provide one dipole less than a half wave length long and the other greater than a half wave length. Since broad band operation was not seen to be necessary in this particular application the elements of the dipoles were chosen as small in diameter (.050 in.) as was consistent with mechanical design. The rest of the antenna was developed on the basis of a 50 ohm inch solid coaxial line feed with the beforementioned quarter wave de-tuning slot. The theoretical calculations of the antenna lengths were based on the self-impedance calculations of a symmetrical antenna made by King and Blake and published in the Proceedings of the IRE for July 1942. The curves published had to be extrapolated to cover the large ratio of radiator element diameter to wavelength necessitated at these frequencies (a/)\ approximately equal to 10*). The admittance Y, conductance G, and susceptance B curves were calculated from impedance curves and the final susceptance and conductance curves used are shown in Figures 8 and 9. The input conductance and susceptance of a symmetrical radiator are shown in Figures 8 and 9 plotted as a function of a factor F where H =21rh/ A and h is the physical half length of the radiator.

It was decided to assume a nominal impedance value of the two parallel dipoles of approximately 35 ohms and construct a coaxial feed line with this terminal impedance to match into standard 50 ohm flexible coaxial cable. A tapered matching section was placed in the inner conductor of the inch coaxial feed for this purpose.

The susceptance and conductance curves were used to select pairs of values of F such that the corresponding G and B (Y=G+iB) were approximately equal but with the Bs in the two pairs having opposite sign. The final overall dimensions of the longer dipole h; was 2.480 inches and the shorter dipole h was 1.930 inches. The field pattern in terms of circularity of polarization is shown in Figure 7 and the standing wave ratio SWR (voltage) of the antenna over the frequencies of interest is shown in Figure 10.

A mathematical analysis of the antenna was compared with the actual experimental performance. Using the radiator element lengths from tip to tip as 2.480 in. and 1.930 in. and the diameter a as .050 in. and assuming that only the .050 in. elements to be acting as radiators, the /2 element lengths are then:

i 1.053 in,

where h, and h are the physical half lengths of the shorter and longer radiators respectively.

For an approximate mid-frequency of operation of 2890 megacycles (the desired range of operation is approximately 2870 to 2910 megacycles),

A=10.38 cm.=4.09 in.

where F, and F are the above described factors relating to the shorter and longer radiators respectively.

From the curves of Figures 8 and 9 the following admittances are indicated:

Thus the angle between the two admittances is 66.2+42.6 or 108.8. 'The terminal admittance 2 of the two dipoles in parallel is then and the terminal impedance F =c/r sin 0 cos (tub-Br) H ==c/r cos 6 cos (wtfir+I where c is Idlp, I is the maximum value of the current flowing in each of the dipoles through an elemental length dl of each of the two dipoles, p is 21r/)., 0 is the polarization angle referred to F and w is the angular frequency, d is current phase angle.

T o find the combined field due to I-I and fig, consider the situation in the plane of the crossed dipoles. Here, the magnetic vectors are in space phase and differ in time phase by q Thus, we can combine the two vectors in a less complex mathematical operation. The combined 1 1 component parallel to H or the in-phase component, is fi =clr (sin 6+cos 0 cos I cos (wk-fir) The perpendicular or quadrature component is E =c/r cos 0 sin t sin (wt-Br) =c/r cos 0 sin 1* cos (wtBr+1r/2) The resultant combined field vector then is where Therefore, 5:

c/n/(sin 6+cos 0 cos 10 (cos 6 sin I /cos (wt-Br-l-X) where cos 0 sin 1 This may be reduced to I7= /1+cos D sin 29/eos (wt-B7-|-X) It can be seen that when P is other than the amplitude function /l+cos I sin 20 becomes a minimum or a maximum at 0:1-45", when l is other than 1-90. If I =it90,

=c/r cos [wt-fir+tan (cot 0)] It can be seen that the amplitude is independent of 0, the requirement for perfect circular polarization.

Returning to the case of d =i90, at 0::45", the amplitude becomes:

the and signs corresponding to maximum and minimum or vice versa depending on whether I 90 or 90".

If the ratio of extremals be called k, then i oimazll fiimin and I6 %gg (for 90) or cos For the value h -108.8 found above, k calculates to be 1.95; the measured figure was about 1.65.

It can be seen that we have provided an antenna well suited for its intended use and that will provide beacon radar tracking over the entire hemisphere.

Glossary of symbols Definition diameter oi radiator element.

tree-space Wavelength.

physical halt length of dipole.

conductance.

susceptauce.

admittance.

length of each shorter radiator 8.

length oi each longer radiator 6.

admittance oi shorter radiators 8, 8. admittance of longer radiators 6, i.

. terminal admittance at two dipoles in parallel. phase angle between currents in two dipoles.

vector magnetic field strength from a first dipole of a pair of dipoles crossed at right angles.

vector magnetic field strength from a second dipole oi a pair of dipoles crossed at right angles.

distance from intersection oi crossed dipoles perpendicular to plane of dipoles.

Idle; I is the maximum current flowing in an element of dipole d1 (delta 1) long and fl Zr/A.

polarization angle referred to H1.

angular frequency.

projected vector component parallel to it.

time in seconds referred to some reierencc tlmg projected vector component perpendicular to HI- resultant field vector.

extii-emeomagnitudes (minimum and maximum) of H ior g ven ratio oi extremals. terminal impedance of two dipoles in parallel.

While we have described a particular embodiment of our invention for the purposes of illustration it will be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

We claim:

1. A turnstile antenna comprising two pairs of radiating elements, the elements of each pair disposed in line, the elements of the first of said pair having a length less than /2 wavelength and the elements of the said second pair having a length greater than /5 wavelength at the frequency at which it is desired to operate to efiect sub stantially phase quadrature currents in the said antenna at the said frequency.

2. An antenna comprising two pairs of radiating elements, the elements of each pair disposed in line, the first of said pairs disposed in a plane perpendicular to the plane of the second of said pairs to form a turnstile like structure, the elements of the said first pair having a length less than wavelength and the elements of the said second pair having a length greater than 95 wavelength at the frequency at which it is desired to operate to effect approximately phase quadrature currents at the said frequency.

3. A turnstile type of antenna comprising a support, four arms extending from said support with said arms lying in a common plane and having an angle of ninety degrees between adjacent arms, each of said arms comprising a conducting member, each pair of oppositely disposed arms constituting a dipole, the arms of one of said pairs having a length greater than wavelength and the arms of the other of said pairs having a length less than wavelength at the frequency at which it is desired to operate whereby a substantially 90 degree phase shift is effected between the currents in said dipoles.

4. The invention as set forth in claim 3 wherein said support comprises a coaxial feed means.

5. A turnstile antenna to be carried by a high velocity missile and for use at high frequency, said antenna comprising four half dipole radiator elements extending outwardly at degree intervals from a common center structure, each pair of oppositely disposed radiator elements comprising a dipole element, the elements of one of said pairs having a length less than 56 wavelength at the said frequency and the elements of the other of said pairs having a length greater than A wavelength at the said frequency whereby the said elements have impedances in phase quadrature and a substantially 90 degree phase shift is effected between the currents in said dipoles.

6. The invention as set forth in claim 5 wherein said common center structure comprises a coaxial feed means.

References Cited in the filc of this patent UNITED STATES PATENTS 1,933,941 Taylor Nov. 7I 1933 2,105,569 White et al. Jan. 18, 1938 2,297,329 Scheldorf Sept. 29, 1942 2,420,967 Moore May 20, 1947 2,480,182 Clapp Aug. 30, 1949 2,512,682 Salinger et al. June 27, 1950

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