NASA CASE NO.

PRINT FIG. 11

ND_TJCE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 08/269,268

06/28/94 LaRC

(NASA-Case-LAR-15088-1) SPIRAL

MICROSTRIP ANTENNA WITH RESISTANCE

Patent Application (NASA. Langley

Research Center) 20 p

N95-14139

Unc]as

G3/32 0029851
LAR 15088-1 AWARDS ABS T 

SPIRAL MICROSTRIP ANTENNA WITH RESISTANCE 

The present invention relates to microstrip antennas, and 

more particularly to wide bandwidth spiral antennas with 

resistive loading. 

A spiral microstrip antenna having resistor elements 

embedded in each of the spiral arms is provided. The antenna is 

constructed using a conductive back plane as a base. The back 

plane supports a dielectric slab having a thickness between onesixteenth and one-quarter of an inch. A square spiral, having 

either two or four arms, is attached to the dielectric slab. 

Each arm of the spiral has resistor elements thereby dissipating 

an excess energy not already emitted through radiation. The 

entire configuration provides a thin, flat, high gain, wide 

bandwidth antenna which requires no underlying cavity. The 

configuration allows the antenna to be mounted conformably on an 

aircraft surface. 

i Inventor: David G. Shively 

1 Home address: 13 Cantamar Court 

Hampton, Virginia 23664 

Employer: NASA Langley Research Center 

Hampton, Virginia 23681 

Initial Evaluator: Dr. Marion C, Bailey 

Serial No. : 08/,6 82~ 

Filed: June 28, 1994 
LAR 15088-1 -i- PATENT APPLICATION

SPIRAL MICROSTRIP ANTENNAWITH RESISTANCE

Origin of the Invention

The invention described herein was made by an employee

of the United States Government and may be manufactured and

used by or for the Government for any governmental purpose

without payment of any royalties thereon or therefor.

i0 BackQround of the Invention

v

The invention relates to microstrip antennas, and more

particularly to wide bandwidth spiral antennas with

resistive loading.

15 The advent of spread spectrum, frequency-hopping

transmitters and receivers, used in jam resistant voice and

data transmissions and in various radar applications, has

created a requirement for wideband, high gain antennas.

Additionally, as many of these antennas are mounted on

20 aircraft, it is desirable to have a relatively thin, flat

antenna which can be mounted in a conformal patch to a

fuselage or wing section. Among the prior art devices are

several types of spiral antennas.

Spiral antennas are inexpensive, low profile, wideband

25 radiators. Most commercially available spiral antennas

cover a I0:i frequency range and are backed by a cavity

which is filled with electrically lossy material. For

spirals operating at microwave frequencies, the cavity is

typically one or two inches in depth. The fields in the

30 cavity are dissipated in order to minimize the effects of

the cavity on the radiated fields. Although the lossy

cavity enhances the wideband performance of the antenna, it

also lowers the gain of the antenna. Circular Archimedean

and circular equiangular spirals are most common, but

35 square spirals and zig-zag spirals also offer wideband
LAR 15088-1 2 PATENT APPLICATION

performance. Please see H. Nakano, Helical and Spiral

Antennas - A Numerical Approach, John Wiley and Sons, Inc.,

New York, 1987.

Recent scientific investigations indicate that the

5 lossy cavity behind the spiral can be eliminated. For

example, an Archimedean spiral above a perfectly conducting

ground plane has been analyzed and shown to provide limited

bandwidth. See H. Nakano, et al., "A Spiral Antenna Backed

by a Perfectly Conducting Plane Reflector," IEEE Trans. on

i0 Ant. and Prop., Vol. 34, No. 6, pp. 791-796, June

1986.

Wang and Tripp have developed a microstrip type of

spiral antenna covering a 6:1 frequency band and producing

a significantly higher gain than the conventional cavity15 backed spiral. The Wang and Tripp spirals include both

Archimedean and equiangular spirals placed on dielectric

slabs and backed by a ground plane. The dielectric

thickness required for these spirals is i/i0 to 1/4 of an

inch. Lossy material is still required around the

20 circumference of the spirals however, in order to absorb

unwanted energy in higher order modes. This requirement

for lossy materials arises because of the operation of the

spiral. During the typical operation, radiation from the

spiral will occur at a particular location on the spiral

25 along a length of the arms depending on the frequency. At

higher frequencies, the radiation occurs nearer the center

of the spiral (equivalent to a shorter antenna length). At

lower frequencies, the radiation occurs further out on the

spiral, thereby acting as a longer antenna length. The

30 problem occurs when energy reaches the end of the spiral

arms and is reflected back through the spiral towards the

center. The reflected energy degrades the transmission and

causes a substantial reduction in performance. This

characteristic means the energy in the antenna must be

35 either radiated or dissipated prior to the end of the
LAR 15088-1 3 PATENT APPLICATION

spiral regardless of frequency. The prior art method of

accomplishing this result is to use lossy material behind

the spiral to absorb part of the antenna energy as

previously discussed. Without the use of lossy material

5 the antenna is limited to higher frequencies where the

energy is completely radiated away. At lower frequencies,

the interference degrades the radiation, effectively

limiting the bandwidth. If lossy material is used,

bandwidth increases, but the gain of the antenna is reduced

i0 mostly at lower frequencies. Additionally, the use of

lossy materials requires a thickness in the antenna

assembly which precludes conformal mounting.

15

Summary of the Invention

Accordingly, it is an object of the present invention

to provide a spiral antenna having a wide bandwidth and a

high gain.

It is another object of the invention to provide a

20 spiral antenna having a thin, flat structure suitable for

conformal mounting.

It is yet another object of the invention to provide

a spiral antenna which has an increasing dissipation of

excess energy as the position along the spiral arms is

25 increased.

It is a further object of the invention to provide a

wideband, high gain antenna without the use of electrically

lossy material behind the spiral antenna.

The invention is a square spiral antenna placed on a

30 dielectric slab backed by a ground plane. The spiral

comprises either two or four or more paired arms winding

outward from the spiral center. Each arm has resistor

elements located so as to reduce resistive losses at the

higher frequencies, but to preclude reflections at the end

35 of the spiral arms. The resistive effect is achieved by
5

LAR 15088-1 -4- PATENTAPPLICATION

either a tapered increasing resistance or, alternately, by

step increases in resistance, the steps increasing in

magnitude, as the energy travels from the center to the

outer edges of the spiral.

Brief Description of the Drawings

For a more complete understanding of the present

invention and for further objects and advantages thereof,

i0 reference is now made to the following description taken in

conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a prior art

conventional cavity-backed spiral antenna;

FIG. 2 is a partial cutaway view of a prior art spiral

15 antenna using lossy material with a backplane;

FIG. 3 is a top view of the square spiral of the

present invention;

FIG. 4 is a cross sectional side view of the square

spiral;

20 FIG. 5 is a view of a section of a square spiral arm

with a graphical representation of resistance;

FIG. 6 is a graphical representation of current

distribution along the square spiral arms;

FIG. 7 is a graphical representation of input

25 impedance over a selected frequency range;

FIG. 8 is a graphical representation of gain and axial

ratio along the z axis over a selected frequency range;

FIG. 9 is a graphical representation of current

distribution and phase along a square spiral arm;

30 FIG. i0 is a graphical depiction of gain, axial ratio

and input impedance for the square spiral antenna; and

FIG. Ii is an alternate embodiment of the antenna with

four spiral arms.

35
LAR 15088-1 -5-

Detailed Description of the Invention

PATENT APPLICATION

A conventional prior art spiral antenna ii is shown in

FIG. i. The spiral 12 is located above a metallic cavity

5 15 which is filled with electrically lossy material 13, and

is usually 1-2 inches deep. The spiral radiates in both

directions perpendicular to the plane of the spiral.

Radiation in the downward direction is absorbed in the

lossy material 13. Because the lossy material inside the

i0 cavity reduces the antenna current to zero by the end 17 of

the spiral and eliminates any reflection in the cavity, a

wide bandwidth radiation is achieved, covering for example

2 to 20 GHz, but with poor gain characteristics. In this

type of antenna, 1/2 of the input power is lost through

15 absorbed back plane radiation.

An alternative prior art device aimed at reducing back

plane losses is shown in FIG. 2. In this device, the

spiral antenna 12 is mounted over a conductive ground plane

21 with electrically lossy material 13 formed underneath

20 the outer circumference of the spiral 12. This lossy

material has the same effect as the lossy material in the

cavity in the case of the previous spiral. This spiral

(FIG. 2) is much thinner, however, typically only 1/4 inch.

This type of antenna provides somewhat greater gain that

25 the spiral in FIG. I, while still providing a relatively

wide bandwidth. Typically, the cavity backed spiral has a

I0:i bandwidth whereas the spiral with a backplane and
LAR 15088-1 -6- PATENT APPLICATION

circumferential lossy material has a 6:1 bandwidth. In

developing a better solution to the gain (bandwidth)

problem, a method of moments analysis computer program has

been used to model a new spiral antenna.

5 The analysis involves the use of spectral domain

Green's functions to predict the electric field produced by

an electric current element located on a dielectric slab

which is backed by a perfectly conducting ground plane. In

this way, the currents and fields of a spiral are

i0 rigorously modeled. The method also models any resistance

which is located on the arms of the spiral. A system of

simultaneous equations results, with the unknown being the

coefficients of the electrical current on the spiral arms.

The system of equations is solved on a computer using

15 standard matrix solvers. The system of equations is as

follows:

inc (x,y)÷ _cat (x,y)-R(x,y) js(x,y) E t E_

0T ;o ] qIIzXpXq mn z pqmnlxmn[l 0 s Pqmnlxmn |

jL yx YY I Y JL YY .I. Y J
LAR 15088-1 -7- PATENT APPLICATION

x/Ax yq

vPq "x f f EinC (x' Y) JxPq (x' y) dydxx

Xp-AX yq-Ay

xp+Ax yq

RPqmn" xx f f R(x'Y) Jmn(x'Y)Pq x J_ (x,y) dydx

xp-Ax yq-Ay

oo oo

x

-(3o -co

2II Co

x

0 0

21i CO

X

0 0

21i 00

x

0 0
LAR 15088-i PATENT APPLICATION

2ii

x

0 0

= Free Space Impedance where _a-(er÷l)/2

Referring to FIG. 3, the spiral antenna of the present

invention 30 is shown in top view. The spiral shown is

5 formed by two arms (31 and 32) but in general can have any

number of arms. Each individual element (33, 34, 35) forms

a square spiral having a spacing between adjacent arms

approximately equal to the width of the spiral arm. As

shown in FIG. 4, the spiral 30 is supported by a thin

i0 dielectric 42 material with parameters er and _r, the

relative permitivity and relative permeability,

respectively. Typically, the thickness 41 of the

dielectric material is 1/16 to 1/4 inches. The dielectric

slab, fabricated from a dielectric in the preferred

15 embodiment, is attached to a perfectly conducting ground

plane 44. An aluminum ground plane is used in the preferred

embodiment. There is no electrically lossy material behind

the spiral, nor is there any cavity.

Referring to FIG. 5, electrical current elements 33,

20 34, and 35 in the spiral are shown unshaded representing

antenna elements having no added resistance. Antenna

element 50 and the following shaded elements are shaded to

represent elements having added resistance. Electrical
LAR 15088-1 -9- PATENT APPLICATION

resistance can be placed on a portion of the spiral arms

and can be varied as a function of position on the arms.

Various types of resistance profiles are shown in FIG. 5.

A incremental step increase in resistance is shown in line

5 51, a linear increase is shown in line 52 and an

exponential increase is shown in line 53. At a resistance

of 0_ (ohms) the spiral arm is perfectly conducting.

Typical resistance tapers for a spiral would start at

0_(ohms) and increase, in steps or in a taper, to a few

i0 hundred ohms. Calculation of the antenna performance

parameters using a standard Archimedean spiral is shown in

FIGs. 6, 7, and 8. Fig. 6 shows the calculated values of

current flow compared to antenna arm length using a

frequency range in the 5.0-6.5 GHz range. FIG. 7 shows the

15 real and imaginary components of spiral input impedance for

same spiral and FIG. 8 shows the expected gain and axial

ratios in the direction perpendicular to plane of the

antenna spiral.

As a comparison, the results of the computer program

20 for the square spiral are shown in FIG. 9. A plot of the

current distribution as a function of arm length is plotted

against arm length referenced as domain numbers,(percentage

of the length). The input impedance, gain, and axial ratio

are also evaluated by the computer program. An example is

25 shown in FIG. I0. By examining the distribution of current

on the spiral arms, the magnitude of the current is caused

to decrease as the current flows out of the arm of the
LAR 15088-1 -i0- PATENT APPLICATION

spiral. If the magnitude of the current does not go to

zero at the end of the spiral arm, the current will reflect

back towards the center of the spiral and produce unwanted

radiation. To ensure a smooth decrease in current on the

5 spiral arms, resistance can be placed on the arms of the

spiral as in FIG. 5. To achieve a smooth current

distribution it is best to have a smooth increase in

resistance beginning at 0 ohms. The resistance should be

placed so as to absorb any current that remains on the

i0 spiral arms after the area of maximum radiation, (a region

on the spiral where the circumference of the arms is equal

to one free space wavelength). Because the resistance on

the arms of the spiral also serves to decrease the gain

(especially at the lower frequencies of operation), it is

15 best to use as little resistance as possible while still

maintaining an acceptable current distribution on the

spiral arms. The resistive portion of the spiral arms

could be made of carbon loaded paint or other resistive

materials. Smoothly increasing the resistance on the arms

20 is best (so as to avoid reflections of current back along

the spiral arms) but a staircased approximation to a smooth

curve can be used. Rather than using materials behind the

spiral (as in the previous art) to absorb unwanted energy,

the new method of analysis and design allows for direct

25 control of the current distribution on the arms of the

spiral by using resistance on the arms themselves. The

resistance can be placed only on the portion of the arms
LAR 15088-1 -ii- PATENT APPLICATION

where the current should be decreased to improve the

antenna performance, thereby providing much greater gain.

An alternate embodiment is shown is FIG. ii where a square

spiral with four arms iii, 112, 113, 114 is shown. As in

5 the prior figures, the shaded area represents antenna

elements 115 having added resistance.

The novel features and advantages of the antenna are

numerous. The antenna provides both high gain and wideband

performance. Losses due to an underlying cavity are

i0 eliminated. The thin flat structure is suitable for

conformed mounting on an aircraft surface. The performance

is available due to the resistive elements added to the

antenna arms and the use of a ground plane to avoid signal

losses.

15 Although the invention has been described relative to

a specific embodiment thereof, there are numerous

variations and modifications that will be readily apparent

to those skilled in the art in the light of the above

teachings. It is therefore to be understood that, within

20 the scope of the appended claims, the invention may be

practiced other than as specifically described.

What is claimed as new and desired to be secured by

Letters Patent of the United States is:
LAR 15088-1 PATENT APPLICATION

SPIRAL MICROSTRIP ANTENNAWITH RESISTANCE

5

i0

15

Abstract of the Inventione

A spiral microstrip antenna having resistor elements

embedded in each of the spiral arms is provided. The

antenna is constructed using a conductive back plane as a

base. The back plane supports a dielectric slab having a

thickness between one-sixteenth and one-quarter of an inch.

A square spiral, having either two or four arms, is

attached to the dielectric slab. Each arm of the spiral

has resistor elements thereby dissipating an excess energy

not already emitted through radiation. The entire

configuration provides a thin, flat, high gain, wide

bandwidth antenna which requires no underlying cavity. The

configuration allows the antenna to be mounted conformably

on an aircraft surface.
I

17 12

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15

PRIOR ART

FIG. 1

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PRIOR ART

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(METERS)

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Subdomain Number

Phase

(degrees)

FIG. 9

Power (db)

10.0 - - 300

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5.5 6.0 6.5 7.0

Frequency (GHz)

FIG. 10
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