Ambient load waveguide switch

These and other objects of the present invention are provided by an ambient load waveguide switch mounted between a feed horn and a low noise amplifier in a ground based antenna. The ambient load waveguide switch includes a housing assembly having a through path. A shuttle assembly is positioned within the housing assembly and moves linearly within the housing assembly on a guide assembly between a first position and a second position. In the first position, the shuttle assembly has a waveguide alignable with the through path of the housing assembly. In a second position, the shuttle assembly has an ambient load element alignable with the through path of the housing assembly. A motor assembly moves the shuttle assembly in a first direction and compresses a first spring to bias the shuttle assembly into the first position. The motor assembly moves the shuttle assembly in a second direction and compresses a second spring to bias the shuttle assembly into the second position. There are horizontal and vertical alignment mechanisms for accurately and repeatedly positioning the waveguide and the ambient load cell in the first and second position, respectively. Advantageously, the guide assembly is relatively flexible resulting in long life and infrequent calibration. The waveguide provides a straight line path and is under two inches in length.

FIELD OF THE INVENTION 
The present invention relates generally to ground based antennas, and more 
specifically, to an ambient load waveguide switch used for calibrating a 
low noise amplifier in a deep space ground based antenna. 
BACKGROUND OF THE INVENTION 
There is a system of three ground based antennas mounted on the surface of 
the earth which receive signals from deep space. These signals are 
transmitted by spacecraft orbiting bodies such as the Saturn or Mars. NASA 
is always trying to improve the performance of the system of ground based 
surface antennas. Presently, signals are transmitted from the earth to the 
spacecraft using an x-band uplink and signals are transmitted from the 
spacecraft to the ground based surface antennas on earth using an x-band 
down link. The x-band is at microwave frequency. 
The x-band down link signal is focused by the antenna into a feed horn. The 
feed horn further focuses the signal into a rotating waveguide ambient 
load switch. The rotating waveguide ambient load switch has two positions. 
In the first position, the feed horn focuses the signal into a choke (also 
called a waveguide). The signal then passes through the choke into a low 
noise amplifier. The low noise amplifier is cryogenically cooled and 
includes an oscillating crystal for tuning to the frequency of the signal 
transmitted by the spacecraft. In the second position, a load element is 
used to calibrate the low noise amplifier. 
As the signal passes from space through the earth's atmosphere, background 
noise is added to the signal received by the system of antennas because 
the atmosphere has a background amount of radiation. The background noise 
can be approximated and removed or filtered from the received signal by 
tuning the receiver in the ground based equipment to account for the 
background noise. The amount of background noise is variable and is based 
primarily on atmospheric conditions such as temperature. By filtering out 
background noise, data rates can be increased. 
A load element simulates the amount of background noise used for 
calibrating the low noise amplifier. The load element has a known 
calibrated radiation temperature; at a certain temperature the load 
element has a predetermined amount of radiation. By knowing the 
temperature of the atmosphere or sky, the temperature of the load element 
can be monitored to calibrate the noise generated by the atmosphere in 
order to filter or effectively remove the background noise generated by 
the atmosphere by auxiliary instrumentation. The low noise amplifier may 
need to be calibrated three times per day to receive signals from 
different spacecraft. 
Thus, the ambient load waveguide switch can be moved to either of two 
positions. In the first position, microwave signals are allowed to pass 
through a waveguide port in the switch from the feed horn to the low noise 
amplifier when the antenna is configured to receive signals from 
spacecraft. In the second position, a load element is inserted into the 
path before the low noise amplifier in order to calibrate the amplifier. 
Unwanted additional noise is added by prior art rotating waveguide ambient 
load switches if the choke is not accurately and repeatedly positioned. 
Improper calibration results as well if the load element is not accurately 
and repeatedly positioned. Prior art ambient load waveguide switches 
disadvantageously added noise because the prior art switches had a long 
and tortuous signal path. Prior art ambient load waveguide switches 
typically have through paths of approximately ten inches. Further 
disadvantages of prior art ambient load switches are that the choke and 
load element cannot be accurately and repeatedly positioned to enable the 
ambient load element switches to handle the high data rates now required. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an ambient 
load element waveguide switch in which a waveguide mounted in the ambient 
load element waveguide switch has a minimum length. 
It is another object of the present invention to provide an ambient load 
waveguide switch in which a waveguide mounted in the ambient load 
waveguide switch has a straight line path. 
Another object of the present invention is to accurately and repeatedly 
position a waveguide mounted in the ambient load waveguide switch. 
It is yet another object of the present invention to provide an ambient 
load waveguide switch which can receive higher data rate signals. 
Yet a further object of the present invention is to accurately and 
repeatedly position the ambient load element mounted in the ambient load 
waveguide switch. 
Yet a further object of the present invention is to provide an ambient load 
waveguide switch which requires infrequent adjustment. 
Yet another object of the present invention is to minimize noise added to 
the received signal by the ambient load waveguide switch. 
Yet a further object of the present invention is to be able to accurately 
position the waveguide and ambient load element in the X, Y and Z 
directions in the ambient load waveguide switch. 
These and other objects of the present invention are provided by an ambient 
load waveguide switch mounted between a feed horn and a low noise 
amplifier in a ground based antenna. The ambient load waveguide switch 
includes a housing assembly having a through path. A shuttle assembly is 
positioned within the housing assembly and moves linearly within the 
housing assembly on a guide assembly between a first position and a second 
position. In the first position, the shuttle assembly has a waveguide 
alignable with the through path of the housing assembly. In a second 
position, the shuttle assembly has an ambient load element alignable with 
the through path of the housing assembly. A motor assembly moves the 
shuttle assembly in a first direction and compresses a first spring to 
bias the shuttle assembly into the first position. The motor assembly 
moves the shuttle assembly in a second direction and compresses a second 
spring to bias the shuttle assembly into the second position. There are 
horizontal and vertical alignment mechanisms for accurately and repeatedly 
positioning the waveguide and the ambient load cell in the first and 
second position, respectively. Advantageously, the guide assembly is 
relatively flexible resulting in long life and infrequent calibration. The 
shuttle assembly includes bushings with a clearance between the shuttle 
assembly and each of the bushings. O-rings are mounted in each of the 
clearances. The use of o-rings and relatively flexible guides allows the 
shuttle assembly to be accurately and repeatedly positioned. The waveguide 
provides a straight line path and is under two inches in length. 
The foregoing objects are also achieved by a switch assembly which includes 
a housing assembly having a through path. A shuttle assembly is positioned 
in the housing assembly. The shuttle assembly has a waveguide and a load 
element. The shuttle assembly is movable between a first position where 
the waveguide of the shuttle assembly is aligned with the through path of 
the housing assembly and a second position where the ambient load element 
is aligned with the through path of the housing assembly. A motor assembly 
is operatively connected to the shuttle assembly by a linear movement 
device for moving the shuttle assembly between the first position and the 
second position. 
The foregoing objects are also achieved by a method of positioning a switch 
in a first position and a second position. The method includes compressing 
a first spring to bias a shuttle assembly into the first position against 
a first vertical stop and a first horizontal stop. A second spring is 
compressed to bias the shuttle assembly into the second position against a 
second vertical stop and a second horizontal stop. 
Still other objects and advantages of the present invention will become 
readily apparent to those skilled in the art from following detailed 
description, wherein the preferred embodiments of the invention are shown 
and described, simply by way of illustration of the best mode contemplated 
of carrying out the invention. As will be realized, the invention is 
capable of other and different embodiments, and its several details are 
capable of modifications in various obvious respects, all without 
departing from the invention. Accordingly, the drawings and description 
thereof are to be regarded as illustrative in nature, and not as 
restrictive.

BEST MODE FOR CARRYING OUT THE INVENTION 
Refer now to FIG. 1 where an antenna 20 using an ambient load waveguide 
switch 26 according to the present invention is depicted. The antenna 20 
reflects the signal sent from a spacecraft into a feed horn 22 in a known 
manner. The feed horn 22 is a cone about fifteen inches in diameter at the 
widest end and about three inches in diameter at the throat 24. The feed 
horn focuses the radio frequency (RF) energy into a low noise amplifier 28 
through the ambient load waveguide switch 26 as explained in detail below. 
The antenna 20 includes uplink and downlink feed cones for transmitting 
and receiving signals. The present invention is only concerned with the 
downlink for receiving signals transmitted by spacecraft. One side of the 
ambient load switch 26 according to the present invention is mounted 
adjacent the throat 24 of the feed horn 22. The low noise amplifier 28 is 
mounted on the opposite side of the ambient load switch 26. 
The exterior of the ambient load waveguide switch 26 of the present 
invention is depicted in detail in FIGS. 2-4. The ambient load waveguide 
switch 26 includes a housing assembly generally indicated as housing 
assembly 50. Housing assembly 50 includes a rectangular housing 102 and a 
raised flange 100. A bottom raised flange 52 is one side of the housing 
102 and is bolted to the low noise amplifier 28 (see FIG. 1). Flange 52 
includes a circular opening 54. Opening 54 is aligned with an opening in 
low noise amplifier 26 (not shown). A motor assembly 56 is bolted to 
housing 102 on one end thereof. A plurality of electrical connectors 60 
are bolted to the housing 102 and extend into the interior of the housing 
102. Electrical cables extend from motor assembly 56 and have connectors 
62 connected to connectors 60. A motor shaft 66 extends from motor 
assembly 56 into one side of a hub assembly 68. A ball screw 70 has one 
end thereof extending into the opposite side of the hub assembly 68. A 
raised flange 72 is on the housing assembly 50. The ball screw 70 extends 
through the flange assembly 72 into housing 102 as will be described in 
greater detail below. 
In FIG. 5, the ambient load waveguide switch 26 is depicted with a top 
cover 100 bolted to a housing 102. A raised flange 104 is on top cover 
100. Similarly, the bottom raised flange is on the housing 102. A circular 
opening 106 extends through the top access plate 104 and is aligned with 
the opening 54 in bottom flange 52. Together, openings 54 and 106 define a 
through path through the housing 102. A plurality of bolts 108 are used 
around the periphery of the top cover 100 to bolt the top cover 100 to the 
housing 102. 
Although not shown in the Figures, the ambient load waveguide switch 26, 
when assembled, forms a sealed assembly. An o-ring seals the cover 100 to 
the housing assembly 50. There is also a shaft seal on the ball screw 70 
where it penetrates the housing 102. There are also gaskets between each 
of the electrical connectors 62 and the housing 102. All screw holes in 
the housing 102 are blind tapped holes from the inside of the body. The 
purpose of these seals is to allow the inside of the switch 26, including 
the waveguide path, as well as the connecting external waveguide, to be 
pressurized with dry nitrogen. The purpose of this pressurization is to 
keep external air containing moisture from entering the waveguide. The 
moisture can, over time, oxidize the internal copper and aluminum surfaces 
of the waveguide, leading to a loss of performance. 
The ambient load waveguide switch 26, according to the present invention, 
is depicted in FIG. 6 with the top cover 100 removed. Housing 102 has a 
first longitudinally extending outer surface I 10, a second transversely 
extending outer surface 112, a third longitudinally extending outer 
surface 114, a fourth transversely extending outer surface 116, a first 
longitudinally extending inner surface 118, a second transversely 
extending inner surface 120, a third longitudinally extending inner 
surface 122, a fourth transversely extending inner surface 124, a first 
shoulder 126, a second shoulder 128, a longitudinally extending through 
bore 130 extending from the second outer surface 112 through to an inner 
recess defined by the housing 102. The through bore 130 is located between 
the first and second shoulders 126 and 128, respectively. A bottom surface 
134 of housing 102 is defined between surfaces 118, 120, 122, 124. The 
inner recess forms a closed chamber defined by the housing 102 and the top 
cover 100. 
A shuttle assembly 160 is positioned within the recess defined by the 
housing 102. The shuttle assembly 160 is positioned on a shuttle support 
assembly 162 and is movable thereon into two positions where a waveguide 
and a load cell can be positioned in the through path defined by housing 
assembly 50 as depicted in FIG. 7. 
Returning to FIG. 6, a cylindrical mount ball nut assembly 164 surrounds a 
portion of ball screw 70 and is connected at one end thereof to shuttle 
assembly 160. Two vertical locators 166a and 166b are bolted to the first 
inner surface 118 and two vertical locators 167a and 167b are bolted to 
the third inner surface 122, respectively. A left horizontal stop 168a and 
a right horizontal stop 168b are bolted to the bottom surface 134 of 
housing 102. For convenience, terms such as "left", "right", "above" and 
"below" are used, although these terms are to be construed in the relative 
sense. A limit switch 170 is bolted to the bottom surface 134 of housing 
102 to limit travel in the right direction as depicted in Figure 6. A 
limit switch 172 is bolted to the bottom surface 134 to limit travel in 
the left direction as depicted in FIG. 6. Limit switch actuators 174, 176 
are attached to mount ball nut assembly 164 to turn off motor assembly 56 
and limit the travel of shuttle assembly 160 in the lateral direction. 
Shuttle support assembly 162 includes a pair of elongated cylindrical guide 
rails 200, 202. Guide rail 200 extends parallel to first inner surface 118 
and is constrained at opposite ends thereof by mounting blocks 204a, 204b. 
Mounting block 204a is bolted to the fourth inner surface 124 and the 
mounting block 204b is bolted to first shoulder 126. Guide rail 202 
extends parallel to the guide rail 200 and parallel to third inner surface 
122 and is constrained at opposite ends thereof by mounting blocks 206a 
and 206b. Mounting block 206a is bolted into the fourth inner surface 124 
and mounting block 206b is bolted into the second shoulder 128. Guide 
rails 200, 202 extend parallel to each other and are relatively flexible 
and can bend during movement and alignment of shuttle assembly 160. 
Opening 106 is aligned with the throat 24 in the feed horn 22. 
As depicted in FIGS. 6 and 6A, shuttle body 250 is made of aluminum and has 
two bores 252, 254. Guide rails 200, 202 extend through bores 252, 254, 
respectively (FIG. 10). Within each bushing groove is a bushing 256. There 
are two bushing grooves 253a, 253b in bore 252 at opposite ends thereof 
and two bushing grooves 255a, 255b in bore 254 at opposite ends thereof. 
There is a 0.020 inch radial clearance between an inner diameter of each 
of the bushing grooves 255a, 255b and the outer diameter of the bushings 
256 (depicted in dashed lines in FIG. 6A). The radial clearances are 
filled in by resilient o-rings 257. The position of the shuttle assembly 
160 can be accurately aligned and does not rely on the guide rails 200, 
202 for accurate positioning and alignment. Bushings 256 are preferably 
Oilite bushings. The two o-rings 257 are positioned on each guide rail 
200, 202. Vertical locators 270a, 270b are bolted to an upper surface of 
shuttle assembly 160 and each have two transverse faces 268a, 268b and 
269a, 269b, respectively. As depicted in FIG. 6A, on each face of the 
vertical locators 270a, 270b are horizontal v-slots 271a, 271b. Horizontal 
locators 272a, 272b are attached to shuttle assembly 160 at opposite ends 
thereof. A cylindrical waveguide 274, preferably made of copper, is part 
of shuttle assembly 160. The choke rings that form the waveguide through 
the shuttle assembly 160 are sized to attenuate signals in the x-band 
range. The waveguide 274 has a receive waveguide path. This feature allows 
simultaneous uplink and downlink operation of the antenna's 20 microwave 
systems. The RF energy present in the feed horn 22 when the transmitter is 
on must not be allowed to enter the receive waveguide path, because it 
would saturate the low noise amplifier 26 and block all downlink signals. 
The RF energy can penetrate the ambient load switch housing assembly 50, 
but the choke rings keep the RF energy out of the downlink path. The 
distance between the inside of the housing 50 and the shuttle assembly 160 
is critical because this distance forms one of the dimensions of the choke 
ring and must be controlled within 0.001. Waveguide 274 has a through hole 
275 through which the focused signal passes through. The waveguide 274 is 
centrally mounted in the transverse direction of housing 102 and on one 
side of the shuttle assembly 160 in the longitudinal direction. 
As depicted in FIG. 7, an ambient load 278 is mounted within a recess 276 
of the shuttle assembly 160 and connected thereto. A ball nut 300 is 
mounted within the housing 102 and is aligned with the through bore 130 
and serves to guide the ball screw 70. Mounted within the ball nut 
assembly 164 are two balanced opposed compression springs 302, 304. A 
circular flange 306 is mounted between springs 302, 304. The top cover 100 
plays no role in the alignment of the stops and locators and the ambient 
load switch assembly 26 can be assembled and calibrated with the top cover 
100 removed. 
FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 6 and 
depicts slots in faces 268b and 269b, respectively. FIG. 9 is a 
cross-sectional view taken along line 9--9 of FIG. 6 which depicts a flat 
310 on ball nut retainer 300 to clear limit switch actuator 174. 
Referring to FIGS. 6 and 10, the vertical locators 166a and 166b each have 
a ball end 310a, 310b, 311a, 311b. Vertical locators 166a, 166b must be 
carefully positioned when the housing assembly 50 is assembled and 
adjustment is provided. Horizontal stops 168a and 168b each have ball ends 
320a and 320b, respectively. Horizontal locators 272a, 272b have vertical 
v-grooves 330a, 330b, respectively. All of the locators 166a, 166b, 270a; 
167a , 167b, 270b; 168a, 168b, 272a, must be carefully aligned and 
positioned to ensure that the shuttle assembly 160 will be properly 
positioned in the x, y, z directions when the waveguide 274 is aligned 
with the housing 50 through path and when the load element 278 is aligned 
with the opening 54. Each of the ball ends 310a, 310b, 311a, 311b, 320a, 
320b are movable in a lateral direction during assembly. 
In operation, FIG. 6 depicts shuttle assembly 160 in a first position where 
the waveguide 274 is aligned with through holes 106 and 54 in the housing 
assembly 50. In this position, the ball screw 70 is rotated by the motor 
assembly 56 moving the shuttle assembly and compressing spring 302. Motor 
assembly 56 rotates ball screw 70 until the limit switch actuator 174 
actuates the limit switch 170 and shuts off the motor assembly 56. 
Vertical locators 270a, 270b are brought into engagement with vertical 
stops 166b and 167b, respectively. Simultaneously, the horizontal locator 
272b is brought into position with horizontal stop 168b. The ball ends 
310a, 311a and 320a are brought into engagement with respective v-grooves 
271a, 272a, 330b on each of the locators 270a, 270b, 272a so that 
horizontal and vertical positioning is accurate and repeatable within one 
thousandth of an inch. The ball ends 310a, 311a, 320a are self-aligning 
with the v-grooves 271b, 272b, 330a in the horizontal x and y directions 
as well as the vertical z direction. Advantageously, by having the radial 
gap between the bushing 256 and the guide rails 200, 202 and o-rings 257 
located in the radial gap, the shuttle assembly 160 can be brought into 
exact position. Also advantageously, neither the ball screw 70 or the 
motor assembly 56 needs to be braked because the spring 302 provides a 
biasing force to keep the shuttle assembly 160 properly positioned even if 
the ball screw 70 or the motor assembly 56 were to rotate. Thus, the 
shuttle assembly 160 is not brought into aligned positions by the motor 
assembly 56. Spring 302 is compressed and exerts a biasing force in the 
left direction as depicted in FIG. 7 so that if motor assembly 56 and ball 
screw 70 should rotate, the bias of spring 302 will ensure that the 
shuttle assembly 160 and the waveguide 275 are kept in alignment with the 
through path through housing assembly 50. The retractable cord 150 is 
extended as shown in the extended position in FIG. 6 and one side thereof 
rides in a groove (not shown). 
Shuttle assembly 160 is movable to a second position depicted in FIGS. 10 
and 11 where the load element 278 is aligned with the bottom opening 54. 
Movement of shuttle assembly 160 from the position shown in FIGS. 6, 7 to 
the position shown in FIGS. 10, 11 is performed by rotation of the ball 
screw 70 by the motor assembly 56 in an opposite direction compressing the 
spring 304. Vertical locators 270a, 270b are brought into engagement with 
vertical locators 166b, 167b, respectively. Simultaneously, the horizontal 
locator 272b is brought into engagement with horizontal stop 168b. The 
ball ends 310b, 311b and 320b are brought into engagement with respective 
v-grooves 271b, 272b, 330a on each of the locators so that horizontal and 
vertical positioning is accurate and repeatable within one thousandth of 
an inch. The ball ends 310b, 311b, 320b are self-aligning with v-grooves 
271b, 272b, 330a. Limit actuator 176 is brought into engagement with limit 
switch 172, tripping the limit actuator 172 to stop further rotation of 
motor assembly 56. In the second position, spring 304 is compressed and 
exerts a biasing force to keep the shuttle assembly 160 properly were to 
rotate. Spring 304 will ensure hat the load element 278 and the shuttle 
assembly 160 are kept in alignment with the bottom opening 54. The 
retractable cord 150 is retracted as shown in FIG. 10. 
A temperature sensor 400 (FIGS. 6 and 10) is used to sense the temperature 
of the load element 278. The temperature of the load element 278 can be 
monitored in a known manner to vary the load and calibrate the low noise 
amplifier 28. 
It is important that horizontal locators and vertical locators are properly 
aligned as the assembly is fabricated. As previously mentioned, this is 
done with the top cover 100 removed. Advantageously, the present invention 
is more reliable than prior art constructions because of the flexibility 
provided by relatively flexible guide shafts 200, 202 and o-rings 257 
taking up a radial gap as compared to relatively rigid arrangements of the 
prior art. The greater flexibility of the present invention allows the 
present invention to flex without coming out of alignment whereas the 
prior art, because of the rigidity of the construction, was less reliable. 
Advantageously, the through path through housing assembly 50 is less than 
two inches long and is a straight path thereby adding much less noise to 
the signal than prior art ambient load waveguide switches. 
It will be readily seen by one of ordinary skill in the art that the 
present invention fulfills all of the objects set forth above. After 
reading the foregoing specification, one of ordinary skill will be able to 
affect various changes, substitutions of equivalents and various other 
aspects of the invention as broadly disclosed herein. It is therefore 
intended that the protection granted hereon be limited only by the 
definition contained in the appended claims and equivalents thereof.