Radio-frequency and microwave load comprising a carbon-bonded carbon fiber composite

A billet of low-density carbon-bonded carbon fiber (CBCF) composite is machined into a desired attenuator or load element shape (usually tapering). The CBCF composite is used as a free-standing load element or, preferably, brazed to the copper, brass or aluminum components of coaxial transmission lines or microwave waveguides. A novel braze method was developed for the brazing step. The resulting attenuator and/or load devices are robust, relatively inexpensive, more easily fabricated, and have improved performance over conventional graded-coating loads.

FIELD OF THE INVENTION 
The present invention relates to the field of attenuators and load 
elements. More specifically, it relates to improved microwave and/or 
radio-frequency (RF) attenuators having higher power absorption 
capability. 
BACKGROUND OF THE INVENTION 
In the field of radio-frequency (RF) and microwave circuits, resistive 
devices or loads are used for a variety of purposes including: a resistive 
circuit element per se; an energy-dissipative element, for example, to 
absorb reflected power in conjunction with a circulator or isolator; and 
as a calibrated energy-dissipative element, particularly during testing of 
high-power microwave sources. 
It is well known in the art, and can be shown using traditional 
transmission line theory, that the resistance of a load must ideally be 
graded in some way along its length in order to avoid reflections that 
would tend to propagate back into the circuit. One conventional way of 
grading the resistance is to apply a resistive coating, which may be 
carbon film on BeO, to the central conductor of a coaxial transmission 
line. The thickness of the resistive coating is gradually increased along 
its length, crudely approximating the desired resistance profile. 
The aforementioned method has several disadvantages that limit its 
usefulness, particularly at high power. First, it is difficult to apply 
the resistive coating in a well-controlled and reproducible manner. 
Second, the coating is usually very thin and fragile, and tends to spall 
from thermal shock. Third, all of the power is dissipated in the thin 
coating, and it is difficult to cool the coating because of poor thermal 
coupling, particularly to the outer wall of the load. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an RF load having 
improved capability for absorbing and dissipating power. 
Another object is to provide an RF load or microwave attenuator that is 
robust, simple to manufacture, and easy to cool during operation. 
A third object is to provide an RF load that is reproducible and accurately 
replicates a desired resistance profile. 
Yet another object is to provide an RF load or microwave attenuator in 
which the energy is dissipated in a bulk material rather than in a thin 
coating or film. 
In accordance with one aspect of the present invention, the foregoing and 
other objects are achieved by an RF attenuator comprising at least a 
coaxial transmission line comprising an inner and an outer conductor; and 
a tapered resistive body comprised of a carbon-bonded carbon fiber 
composite having a bulk density less than 2 g/cc and bulk resistivity 
greater than 0.2 ohm.cm, the body disposed between the inner and the outer 
conductors, the resistive body maintaining thermal contact with at least 
one of the conductors. 
In accordance with a second aspect of this invention, a method of making an 
RF attenuator comprises the steps of making a resistive body; forming the 
resistive body to a desired, generally tapering shape; and disposing the 
tapered resistive body between the inner and outer conductors of a coaxial 
transmission line such that the resistive body maintains thermal contact 
with at least one of the conductors. 
In accordance with a third aspect of this invention, an RF attenuator 
comprises at least a waveguide transmission line comprising an interior 
cavity and an outer conductor; and a tapered resistive body comprised of a 
carbon-bonded carbon fiber composite having a bulk density less than 2 
g/cc and bulk resistivity greater than 0.2 ohm.cm, the resistive body 
disposed within the inner cavity, and maintaining thermal contact with the 
outer conductor. 
In accordance with a fourth aspect of this invention, a method of making an 
RF attenuator comprises the steps of making a resistive body; forming the 
resistive body to a desired, generally tapering shape; and disposing the 
tapered resistive body within the cavity of a waveguide transmission line 
such that the resistive body maintains thermal contact with the conductive 
wall of the waveguide. 
Further and other aspects of the present invention will become apparent 
from the description contained herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The above objects and advantages are accomplished by the present invention 
in which a lossy material is formed into a selected, generally tapered 
geometry, and disposed within either a coaxial or a waveguide transmission 
line. In a preferred embodiment of the invention shown at 10 in FIG. 1, 
the lossy material is a low-density carbon-carbon composite (carbon-bonded 
carbon fibers, or CBCF) machined to desired dimensions, and brazed onto 
selected surfaces of the conductor which is preferably copper. The CBCF 
may be further machined after the brazing operation, if desired. 
The brazing operation is preferably facilitated by the use of the novel 
brazing method described in our U.S. Pat. No. 5,648,180 entitled "Method 
for Joining Carbon-Carbon Composites to Metals" incorporated herein by 
reference in its entirety. The brazing method provides means for sealing 
the low-density CBCF surface prior to brazing to prevent infiltration or 
"wicking" of the braze alloy into the CBCF body. The sealing is preferably 
accomplished by applying a coating of pitch or resin to the CBCF body, and 
carbonizing this coating to yield a completely carbonaceous, dense 
(impermeable) layer on the CBCF. The braze alloy that is used is 
preferably of such composition and melting temperature that it will not be 
adversely affected by subsequent brazing or soldering operations as the 
device is further assembled. 
EXAMPLE I 
Coaxial Transmission line 
In this embodiment, shown in FIG. 2, the RF load 20 is constructed as a 
section of coaxial transmission line in which a central conductor 21 is 
preferably copper. A bulk resistive material such as carbon-bonded 
carbon-fiber composite (CBCF) 10 is machined to a generally cylindrical 
shape whose outside diameter fits within, the inside diameter of the outer 
conductor 22. The inside diameter of the carbon-carbon composite 10 tapers 
linearly along part of its length 23, thereby achieving a gradation in the 
effective impedance of the coaxial transmission line. 
One procedure for making the CBCF composite is presented in detail in our 
U.S. Pat. No. 5,243,464, which is incorporated herein by reference in its 
entirety. 
A billet of CBCF having a bulk density of about 0.25 g/cc and a bulk 
resistivity of about 1/2 ohm-cm is machined into the tapered shape shown 
in at 11 FIG. 1. In this example, the inside diameter was about 0.125 
inches, and the outer diameter was about 0.300 inches. The tapered length 
was about 3 inches and the untapered length was about 1.5 inches. When 
inserted into a coaxial transmission line as shown in FIG. 2, we 
discovered, surprisingly, that this structure had a loss of about 20 dB. 
Calculations showed that by extending the tapered region to a length of 5 
inches, the insertion loss could be increased to about 30 to 40 dB. 
In many applications, especially at high power, the CBCF 10 must be 
actively cooled to remove the heat generated during dissipation of RF 
energy. One cooling means is shown in FIG. 2, in which the outer conductor 
22 is provided with fins 24 on its surface. To facilitate cooling, it is 
desirable to maintain good thermal coupling between the resistive material 
and the outside wall of the device. One means of doing so is to braze or 
solder the resistive material directly to the outer conductor as shown 
generally at 25. The forced flow of air across the fins will accommodate 
operation at power levels up to a few hundred watts. 
For operation at greater power levels, say a kilowatt or more, a 
water-cooled load 30 is desirable as shown in FIG. 3. In FIG. 3, the fins 
have been replaced by a water jacket 34 though which water or another 
liquid coolant 35 circulates via inlet 36 and outlet 37. 
The load may be provided with end caps and a hermetic seal (not shown) to 
prevent the accumulation of moisture or other contaminants within the body 
of the load. The end caps are preferably of an insulating material such as 
ceramic, glass, or polymer. It will be appreciated by those skilled in the 
art that a variety of end connections may be used that are compatible with 
other standard circuit connectors used within the industry. Typical of the 
art are connectors defined in Military Specification MIL-C-39012 and 
MIL-STD-348. 
EXAMPLE II 
Microwave Waveguide 
Many RF circuits, particularly those operating at microwave frequencies, 
often employ waveguides rather than coaxial transmission lines because 
they generally have lower losses. FIG. 4 shows the lossy low-density CBCF 
composite 10 described hereinabove machined to desired dimensions, and 
brazed onto the inner surface(s) 41 of a typical microwave waveguide 40. 
In this case, the waveguide may be either circular or rectangular in cross 
section; with the rectangular waveguide (FIG. 4a, 4b) being of the 
single-ridged, dual-ridged, or unridged varieties. In the case of a 
circular waveguide, (FIG. 4c, 4d) the inside diameter of the resistive 
material may be tapered linearly to form a generally conical surface, 
whereas in a rectangular waveguide the resistive material may be tapered 
along one or both of the axial planes of the waveguide (FIG. 4). In the 
case of a ridged Waveguide 50, the resistive material 10 is preferably 
applied to the surface 51 of the ridge (FIG. 5). 
In both examples above, the resistive CBCF material was tapered in a 
smooth, generally linear fashion. Skilled artisans will appreciate that 
many types of taper may he used, including linear, sinusoidal, 
logarithmic, and others. For some applications, an acceptable degree of 
grading can be achieved by forming the taper as a series of discrete steps 
indicated at 12 in the modified view shown in FIG. 1. Even in this case, 
the grading can be better controlled and more uniform than is achievable 
by painting or otherwise depositing a thin coating of, say, colloidal 
graphite. Furthermore, skilled artisans will appreciate at once that our 
invention provides a means for dissipating the RF power uniformly 
throughout a volume (or bulk) of resistive CBCF material rather than in a 
thin layer, thereby making attenuators and loads designed according to 
this invention inherently more robust. 
Some other attendant features and advantages of our invention are as 
follows. CBCF is relatively inexpensive, easily machined to close 
tolerances, and can be securely brazed into a copper, brass or aluminum 
waveguide or used as a stand-alone load element. Machined CBCF is more 
reproducible than carbon films or coatings. Brazing gives good thermal and 
electrical contact with the outer wall of the waveguide. Common failure 
modes of conventional devices (solder melting, carbon film spalling) are 
eliminated, giving a much more robust device. CBCF is very lightweight. 
Attenuators and loads made of machined CBCF composite are thermal shock 
resistant. Ferrite materials (which require sintering and grinding) and 
silicon carbide (which must be machined) can be eliminated from the design 
of RF loads. 
While several preferred embodiments of the improved RF load have been shown 
and described, it will be understood that such descriptions are not 
intended to limit the disclosure, but rather it is intended to cover all 
modifications and alternate methods falling within the spirit and scope of 
the invention as defined in the appended claims or their equivalents.