The rectenna's microwave power beam absorption limit was theoretically confirmed by two mathematical models descriptive of the microwave absorption process; first one model was based on the current sheet equivalency of a large planar array above a reflector and the second model, which was based on the properties of a waveguide with special imaging characteristics, quantified the electromagnetic modes (field configurations) in the immediate vicinity of a Rectenna element spacing which permit total power beam absorption by preventing unwanted modes from propagating (scattering) were derived using these models. Several factors causing unwanted scattering are discussed

Ott, J. H.

Rice, J. S.

Thorn, D. C.

English

The theoretical operational parameters for the workable satellite power system were examined. The system requirements for efficient transmission and reception of an environmentally benign microwave beam were determined

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A Theoretical Study of Microwave Beam Absorption by a Rectenna

8 2,'
A THEORETICAL STUDY OF MICROWAVE BEAM ABSORPTION BY A RECTENNA
James H. Oft - James S. Rice - Donald C. Thorn
Novar Electronics Corporation - Barberton, Ohio
22737
Power Beam Absorption
The Rectenna's microwave power beam absorption limit is theoretica11y
confirmed to be I00_. Two mathematical models descriptive of the micro-
wave absorption process were derived from MaxweIl's equations. The first
model is based on the current sheet equivalency of a large planar array
above a reflector. The model is characterized by a mathematical expression
for the fraction of an incident plane wave's power that is reflected from
the sheet.
The second model, which is based on the properties of a waveguide
with special imaging characteristics, quantifies the electromagnetic
modes (field configurations) in the immediate vicinity of a Rectenna
element in the Rectenna array. Thls model then gives the limits for
element spacing which permit total power beam absorption by preventing
unwanted modes from propagating (scattering).
Rectenna Design Improvements
Several improvements in the Rectenna design have been indicated by the
mathematical modeling.
I . A significant reduction can be made in the density of the Rectenna
elements needed for total, beam absorption. This would not only
significant|y reduce the cost of the Rectenna but also indicates
greater diode efficiency because of the higher power density
per diode.
. The Rectenna panels can be made to totally absorb at any angle
of incidence by adjusting reflector and element spacing and load
impedance as seen by the dipole elements. This suggests a flat
or terrain conforming Rectenna eliminating the need for the
"billboard" or "Venetian blind" design and essentially conforming
to the terrain.
3. The screen reflector should be able to be replaced by parasitic
reflector dipole elements.
332
https://ntrs.nasa.gov/search.jsp?R=19820014863 2020-03-21T16:26:55+00:00Z
Scattering
Specular scattering of the power beam at the power beam fundamental
frequency is expected to resu|t from most deviations in the Rectenna
parameters. The properties of this scattering, including the modeling-
determined scattering |osses due to variations in several parameters
from design center values required for total absorption at normal
incidence, are shown in Figure 1.
Ray of Scattered
Power Power
.YQ •SLPeoct_le'
Ray of Incident
.._.Edge Vilw of
Rectennc fls_l
SPECULAR REFLECTION
• SPECULAR IS THE PREDOMINANT FORM OF
SCArI"ERING AT THE FUNDAMENTAL FREQUENCY
MW
-lo -s o _ _o
Impedince M isml'tCh ( n/Q )
_%
MW
- SO
i i -2'2 _'3 _, 2s 26 27 28
Dipole/Reflector Seperal_on
MW
l/
,_O.S
.10a .So o_ _o 1_, r"e
Angle of Incidence From Normal
REFLECTEO POWER AS A FUNCTION OF
VARIOUS PARAMETERS (_)6 GW Normal
Incidence)
MATHEMATICAL MOOEUNG PRE01CTION
FIGURE I
PROPERTIES OF SCATTERING FROM RECTENNA
AT FUNDAMENTAL FREQUENCY OF POWER BEAM
333
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• * 2. ° .'._
"ct
. ':.. ,
'/._ '
• ,..
; .-,r :-
_ '_- ,_ .
: ,,,.-a, t o
The Rectenna dipole-filter-diode assembly and power bus are expected
to be the significant sources of harmonic radiation. The harmonic
energy will be concentrated in calculable grating lobes, as shown in
Figure 2.
2nd Hlrmonic
Eiemet_t
/
(ELEVATION EXAMPLE)
[AZIMUTH EXAMPLE)
• DIRECTIONAL CHARACTERISTICS OF HARMONIC SCATTERING FROM A RECTENNA CAN BE
DETERMINED BY GRATING LOBE ANALYSIS. WiTH THE RECTENNA RECEIVING ELEMENTS,
POWER BUS, AND OTHER RECTENNA COMPONENTS AS RADIATORS.
FIGURE 2
EXAMPLES OF DIRECTIONAL CHARACTERISTICS OF SCATTERING
FROM RECTENNA AT HARMONICS OF POWER BEAM FREQUENCY
A large object flying through the power beam over the Rectenna causes
diffraction patterns to be generated at the Rectenna, as depicted in
Figure 3. Therefore, Rectenna diodes should have tolerance to the
resulting overvoltage and thermal transients.
Among the factors causing scattering are microwave beam depolari-
zation and amp}itude fluctuations caused by disturbances in the atmosphere.
Depolarization is not expected to be a significant source of scatter.
Amplitude fluctuations cause scattering by disrupting the uniformity of
the Rectenna illumination. In addition, this disruption of the RF power
level from design values for the diodes causes impedance mismatches
result|rig in further scattering.
334
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f _T/ :,
:: :.. :_"... ,
/IT :::
• z' /
_, '. L; "'
,' :_
L_ _,: ..
_.-_ :
i.:'- ; :,.
-?/.:
;iii i
• RECTENNA DIODES SHOULD HAVE TOLERANCE TO:
• Overvoltage trensJenls //
--Fill aircraft //
• Thermal transients
_Slower objects (e.g. helicopters)
Possible 6-gdB
Signal Increase
(Oepending on size, shape,
end height of obiect)
FIGURE 3
DIFFRACTED SIGNAL ENHANCEMENT AT THE RECTENNA CAN BE
CAUSED BY AN OBJECT FLYING THROUGH THE POWER BEAM.
Although existing earth-space propagation measurements to date have
indicated that amplitude fluctuations would cause insignificant scattering
at a Rectenna, there are two factors which impair the application of thls
data to a Satellite Power System (SPS). In a11 studies found, there is
significant aperture averaging due to the large aperture receiving antennas
used. This Is in contrast to the very smail aperture area of each
"independent" receiving element in the Rectenna. The second factor is
that the signals measured in those studies were wide-band. Most deep
fades are frequency selective. Therefore, observed amplitude fluctuations
would be expected to be less than those of the monochromatic SPS power
beam. Thus, further space-earth transmissions'studies are proposed.
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335


A theoretical study of microwave beam absorption by a rectenna

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