Plane wave rectangular waveguide high impedance wall...

Details Plane wave rectangular waveguide high impedance wall... Plane wave rectangular waveguide high impedance wall...

1999-09-29

2003-08-05

Lee, Benny (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including distributed parameter-type coupling

C333S248000

Reexamination Certificate

active

06603357

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to plane wave rectangular waveguides with high impedance walls.
2. Description of the Related Art
New generations of communications, surveillance and radar equipment require substantial power from solid state amplifiers at frequencies above 30 gigahertz (GHz). Higher frequency signals can carry more information (bandwidth), allow for smaller antennas with very high gain and provide radar with improved resolution. However, amplifying signals with frequencies above 30 GHz using conventional methods does not provide optimal results.
At lower frequencies, available signal power can be increased by adding the output power of two or more amplifiers in a power combining network. For solid state amplifiers, as the frequency of the signal increases the size of the transistors within the amplifier devices decrease. This results in a corresponding reduction in the amplifier power output so that more amplifier devices are required to achieve the necessary power level. For instance, at millimeter wave frequencies the power per amplifier device for a set 10 dB gain ranges from 100 milliwatts (mW) at 30 GHz to 10 mW at 100 GHz. To attain power of more than a watt, at the higher frequencies, hundreds of amplifiers must be combined. This cannot be done by conventional power combining networks because of the insertion loss of the network transmission lines. As the number of amplifiers increases, a point will be reached at which the loss experienced by the transmission lines will exceed the gain produced by the amplifiers.
One method of amplifying high frequency signals is to combine the power output of many small amplifiers in an quasi-optic amplifier array. The amplifiers of the array are oriented in space such that the array can amplify a beam of energy rather than amplifying a signal guided by a transmission line. The amplifier array is referred to as quasi-optic because the dimensions of the array become more than one or two wavelengths. The beam of energy can be guided to the array by some form of a waveguide or the beam can be a Gaussian beam aimed at the array. {C. M. Liu et al,
Monolithic
40
Ghz
670
mW HBT Grid Amplifier
, (1996)
IEEE MTT
-
S
, p. 1123}.
Amplifier arrays can be produced as monolithic microwave integrated circuits (MMIC). In MMICs all interconnections and components, both active and passive, are fabricated simultaneously on a semiconductor substrate using conventional deposition and etching processes, thereby eliminating discrete components and wire bond interconnections. Quasi-optical amplifier arrays can combine the output power of hundreds of solid state amplifiers formed in a two-dimensional monolithic array on the plane normal to the input signal.
The primary method for guiding high frequency signals to an array amplifier uses a rectangular waveguide with conductive sidewalls.
FIG. 1
shows a conventional metal waveguide
10
having four interior walls
11
a
,
11
b
,
11
c
,
11
d
. A signal source at one end
12
transmits a signal down the waveguide to a quasi-optical amplifier array mounted at the opposite end
13
, normal to the waveguide. The numerous small amplifiers of the array amplify the signal and the combination of the amplifiers results in significant amplification of the signal. The E field orientation from the output of the amplifier will be orthogonal to the input E field orientation to reduce oscillatory instability. An output waveguide can be included to guide the output signal to a useful load. Using this method, results have been published showing an ability to reach substantial power at frequencies from 35 to 44 Ghz,{J. A. Higgins,
Development of a Quasi-Optic Power Amplifier for Q Band
, A Contract Final Report. Contract F30602-93-C-0188, USAF Rome Laboratory, 26 Electronic Parkway, Griffis AFB NY 13441.}
However, a rectangular waveguide with conductive sidewalls does not provide an optimal signal to drive an amplifier array. As shown in
FIG. 2
, a vertically polarized signal
21
has a vertical electric field component(E)
22
, a perpendicular magnetic field component(H)
23
, and a propagation axis (P). Because the sidewalls
11
a
and
11
c
of the metal waveguide of
FIG. 1
are conductive, they present a short circuit to the E field. The E field cannot exist near the conductive sidewall and the power densities of both the E field
24
and the H field
26
drop off closer to the sidewall as shown in FIG.
2
. As a result, the power density of the transmission signal
21
varies from a maximum at the middle of the waveguide to zero at the sidewalls
11
a
and
11
c
. If the waveguide cross-section were shaped to support a horizontally oriented signal, the same problem would exist only the signal would drop off near the top wall
11
d
and bottom wall
11
b.
For an amplifier array to operate efficiently, each individual amplifier in the array must be driven by the same power level, i.e. the power density must be uniform across the array. When amplifying the type of signal provided by the metal waveguide, the amplifiers at the center of the array will be overdriven before the edge amplifiers can be adequately driven. In addition, individual amplifiers in the array will see different source and load impedance depending upon their location in the array. The reduced power amplitude along with impedance mismatches at the input and output make most of the edge amplifiers ineffective. The net result is a significant reduction in the potential output power.
As an example of the power loss in conductive sidewall rectangular waveguide applications, measurements of a 1.2 cm by 1.2 cm array of 112 small amplifiers have provided an output power of 3.0 W at 38 Ghz. If a signal with uniform power density were applied to the same amplifier array the output power would be in excess of 10 W.
A high impedance surface will appear as an open circuit and the E field will not experience the drop-off associated with a conductive surface. A photonic crystal surface structure has been developed which exhibits a high wave impedance over a limited bandwidth. {D. Sievenpiper,
High Impededance Electromagnetic Surfaces
, (1999) PhD Thesis, University of California, Los Angeles}. The surface structure comprises “thumbtacks” of conductive material mounted in a sheet of dielectric material, with the pins of the thumbtacks forming conductive vias through the dielectric material to a continuous conductive layer on the opposite side of the dielectric material. This surface presents a high impedance to an incident EM wave but it has the characteristic of not allowing surface current flow in any direction. The gaps between the thumbtacks present an open circuit to any surface conduction.
Dielectric-loaded waveguides, so called hard-wall horns, have been shown to improve the uniformity of signal power density. {M. A. Ali, et.al.,
Analysis and Measurement of Hard Horn Feeds for the Excitation of quasi-Optical Amplifiers
, (1998)
IEEE MTT
-
S
, pp. 1913-19211}. While an improvement in uniformity, this approach still does not provide optimal performance of an amplifier array in which input and output fields of a signal are cross polarized.
SUMMARY OF THE INVENTION
The present invention provides an improved high impedance surface structure used in waveguides which allows for the transmission of high frequency signals with a near uniform power density across the waveguide cross-section. The new sidewall surface provides a high impedance termination for the E field component of the signal flowing in the waveguide and also allows conduction down the other two walls to support the H field component of the signal. The power wave assumes the characteristics of a plane wave with a transverse electric and magnetic (TEM) instead of a transverse electric (TE) or transverse magnetic (TM) propagation. This transformation of the energy flow in the waveguide provides a wave similar to that of a free-space wave propagation having near uniform power density.
The new wall

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