(Ham Radio) Schantz - Uwb Magnetic Antennas (Ieee Aps 2003).pdf - Amatorskie anteny - marek33x

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http://www.uwbantenna.com

UWB Magnetic Antennas

Hans Gregory Schantz

Q-Track Corporation

315 North Main Street

Tennille, GA 31089

h.schantz@q-track.com

Introduction:

A wide variety of electric ultra-wideband (UWB) antennas have seen commercial

use. A few examples of these include diamond dipoles [1], and elliptical dipoles [2].

These electric antennas tend to have relatively large electric near-fields that are prone to

undesired coupling with near-by objects.

Many commercial applications, however, call for UWB antennas that are less

prone to near-field coupling. Magnetic antennas are well suited for these applications,

because the relatively large magnetic fields tend not to couple as strongly with near-by

objects. The purpose of this article is to provide a brief overview of some UWB

magnetic antennas. In particular, this paper will discuss large current radiators,

monoloop antennas, and magnetic slot antennas.

Large Current Radiators:

A “large current radiator” is ideally a current sheet whose return currents are

isolated by a ground plane (see Figure 1). Harmuth pioneered this basic design [3,4], and

variations of the large current radiator concept have been developed by various

investigators [5].

Ground

Plane

Farr et al proposed an

interesting variation on this basic

architecture [6]. The “balanced-

dipole” antenna (shown in Figure

2) has the interesting property

that it can be fed from either end,

while the opposite end is

terminated with an appropriate

impedance. Thus, the pattern of

this antenna may be dynamically

switched.

Differential Feed

Large

Current

Radiator

Figure 1: Harmuth’s large current radiator

The principal dis-

advantage of large current

radiators is that they tend to be

lossy antennas. A current sheet

will necessarily radiate from

both sides. This energy is

trapped between the large current

radiator and the ground plane

yielding undesirable resonances.

Ground

Plane

Feed

Resistive

Termination

Figure 2: Balanced-dipole antenna of Farr et al

Reprint: 2003 IEEE APS Conference

Accordingly, a ferrite or other absorptive coating is typically used to dissipate these

undesired emissions. Thus, large current radiators are generally not very efficient.

Monoloop Antennas:

The main disadvantage of

large current radiators follows

from the fact that these antennas

trap energy between their

radiating elements and their

ground planes. This suggests

that it might be fruitful to

consider radiating elements

oriented in a plane perpendicular

to the ground plane.

One early antenna with

this architecture was Turner et

al’s scimitar antenna shown in

Figure 4 [7]. This antenna is

characterized by an operating

bandwidth in excess of 1:10.

Although well matched, the

scimitar antenna exhibits some

variation in pattern as a function

of frequency. Ideally, a UWB antenna should have a stable and consistent pattern as a

function of frequency.

Feed

Figure 3: Scimitar antenna of Turner et al.

Ground

Plane

Feed

Figure 4: The author’s monoloop antenna.

The author devised a similar antenna, dubbed a “monoloop” [8]. The “monoloop”

name follows from the fact that antennas with this architecture are essentially half a loop

driven against a ground plane, just as a “monopole” is half a dipole antenna driven

against a ground plane.

The author’s monoloop antenna differs from the scimitar antenna in the feed

region. This antenna has a round, bulbous end that offers an excellent match to 50 ohms.

Like the scimitar antenna, the author’s monoloop antenna suffers from some variation in

pattern as a function of frequency. Also, the monoloop pattern is not uniform in the plane

of the monoloop element. The reason for this behavior may be understood by

considering the current flow in the monoloop element and the resulting radiation.

Assume each infinitesimal current element along the monoloop is the source of

radiation along the radius of curvature at that point. If the monoloop element is

considered in cross-section, this means that each infinitesimal current element generates a

direct ray of radiation radially outward, and a radially inward ray directed toward the

ground plane. This radially inward ray ends up reflected.

The radiation in any given direction is the sum of a direct ray from one part of the

monoloop and a reflected ray from a different part of the monoloop element. A quick

calculation of the path lengths involved demonstrates an asymmetry of the direct and

reflected paths: the relative path lengths varies as a function of angle. This variation is

the root cause of the non-uniform pattern as a “direct” impulse waveform combines with

a “reflected” impulse waveform with a relative delay that varies as a function of look

angle. This behavior is illustrated in Figure 5.

Ground

Plane

Reprint: 2003 IEEE APS Conference

Figure 5: Current and radiation from a

monoloop element.

One way to rectify this undesired

asymmetry is to feed the monoloop in a

symmetric manner. This was the

fundamental idea behind the author’s

center-fed monoloop (see Figure 6). By

moving the feed to the center top of the

monoloop antenna, symmetry is restored.

The direct and reflected paths are now the

same length as the look angle is varied.

Thus the pattern of a center-fed monoloop

is more uniform as a function of look

angle. Figure 7 shows characteristic

current and radiation rays for the center-

fed monoloop.

Direct Ray

Reflected

Ray

Ground

Plane

Feed

Current

Feed Line

Ground Plane

Figure 6: A center-fed monoloop.

Figure 7: Current and radiation from a center fed monoloop.

Magnetic Slot Antennas:

Magnetic slot antennas are a final UWB magnetic antenna architecture. One

example is Barnes’s UWB magnetic slot antenna shown in Figure 8 [9,10]. If an

appropriate taper is chosen

for the slot line of this

antenna, excellent matching

can be obtained. A

magnetic slot antenna like

the one shown in Figure 8

exhibits a quadrupole type

radiation pattern with a

pattern lobe lying in the

normal direction on either

side of the plane. When

driven against a reflecting

back plane, this antenna

exhibited good perform-

ance in the Time Domain

Corporation’s RV1k

through-wall UWB radar.

Figure 8: Barnes’s UWB magnetic slot antenna.

Reprint: 2003 IEEE APS Conference

Conclusion:

In conclusion, there are a wide variety of magnetic type UWB antennas available

for commercial applications. This paper has surveyed three general kinds of magnetic

UWB antennas: large current radiators, monoloops, and slot antennas. Although far from

being exhaustive, this paper provided examples of each type to illustrate the variety of

antennas possible within each of these magnetic UWB antenna architectures.

Acknowledgement:

The author’s monoloop and center-fed monoloop antennas described in reference

8 were originally invented while the author was employed as an antenna engineer by the

Time Domain Corporation.

References:

[1] H. Schantz & L. Fullerton, “The Diamond Dipole: A Gaussian Impulse Antenna,”

IEEE APS2001 (July 2001).

[2] H. Schantz, “Planar Elliptical Element UWB Dipole Antennas,” IEEE APS 2002

(June 2002).

[3] H. Harmuth & S. Ding-rong, “Large-Current, Short-Length Radiator for

Nonsinusoidal Waves,” IEEE International Symposium on Electromagnetic

Compatibility, 1983 pp. 453-456.

[4] H. Harmuth, “Frequency Independent Shielded Loop Antenna,” U.S. Patent

4,506,267.

[5] G. Pochanin, “Large Current Radiator for the Short Electromagnetic Pulses

Radiation,” Ultra-Wideband, Short-Pulse Electromagnetics 4, New York: Kluwer

Academic/Plenum Publishers, 1999), pp. 149-155.

[6] E. Farr, C. Baum, W. Prather, and T. Tran “A Two-Channel Balanced-Dipole

Antenna With Reversible Antenna Pattern Operating at 50 Ohms,” Sensor and

Simulation Note #441 (December 1999).

[7] E. Turner, W. Turner, “Scimitar Antenna,” U.S. Patent 3,015,101.

[8] H. Schantz, “Single Element Antenna Apparatus,” U.S. Patent 6,437,756.

[9] M. Barnes, “Ultra-Wideband Magnetic Antenna,” U.S. Patent 6,091,374.

[10] H. Schantz, M. Barnes, “The COTAB UWB Magnetic Slot Antenna,” IEEE APS

2001 (July 2001).

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