Focal point positioning tool

A tool for indicating the position of a receiving device mounted on a convergent reflector with respect to the focus of that reflector, the tool having a collar and clamp for attaching it to the receiving device and converging, telescoping legs extending toward the reflector to indicate the angular and/or axial displacement of the receiving device from the focus of the reflector.

BACKGROUND 
The present invention relates to the rapid positioning of a receiving or 
transmitting device at the focal point of a convergent reflector. 
High gain antennas for microwaves and higher frequency radiation use 
reflectors having a surface described by the rotation of a conic section 
having a focus. The usual figure described by a reflector surface is a 
parabola, but any reflector shape that has a focal point, a point where 
parallel incoming rays of energy converge, i.e. any convergent reflector, 
will provide a high gain. The highest quality performance is obtained from 
the antenna if the receiving or transmitting device is positioned at the 
focal point of the reflector. For microwaves, this receiving or 
transmitting device is usually a feedhorn. For a solar energy 
concentrator, the receiving device could be a thermopile or other thermal 
energy converter. 
Recently high gain reflector antennas have become popular within consumers 
for receiving television signals relayed to earth by synchronous 
satellites. The antennas are typically assembled at the point of use. 
Locating the receiver feedhorn precisely at the focus of the antenna at 
the time of first installation and from time to time as maintenance is 
performed can be quite difficult. One positioning method involves use of 
an expensive signal strength meter monitored by one person who gives 
instructions to another who adjusts the position of the receiving device 
to maximize the signal strength. This method is too expensive for home 
use. Other methods involve straight sticks that are inserted in the 
reflector or receiving device in order to attempt to align the device and 
the center of the antenna. While these sticks are cheap, they are also 
inaccurate and unreliable. 
SUMMARY OF THE INVENTION 
In the invention, a simple, inexpensive positioning tool is disclosed that 
permits a receiving device to be accurately positioned at the focus of the 
receiver quickly by one person. The tool is attached to the receiving 
device and clamped into position on it. Because the tool is lightweight it 
does not produce any appreciable deflection of the support, which may be a 
waveguide or an inflexible coaxial cable 
In a preferred embodiment, three converging legs on the tool extend toward 
the reflector. These legs may be solid or hollow and are fitted with 
either a telescoping sleeve or telescoping rod, respectively. The sleeves 
or rods are extended to the reflector and may be clamped at a desired 
degree of extension. Preferably, the sleeves or rods bear fiduciary marks 
so that all may be extended the same length. By knowing the correct axial 
length of the reflector to the focal point and adjusting the legs of the 
tool accordingly, the tool will indicate whether the receiving device 
needs to be adjusted and, if so, in what direction from the reflector. 
When the receiver is precisely aligned with the axis of the reflector, the 
sleeves or rods will be symmetrically disposed about the center point of 
the reflector. In the preferred embodiment, the three indicated points 
show in what direction the receiving device must be moved in order to 
obtain the focal positioning desired. When that positioning is obtained, 
the desired symmetrical spacing is achieved. The open construction of the 
legs of the tool does not interfere with, nor is its use prevented by the 
waveguide or coaxial cable typically extending from the receiving device 
and passing through the center of the reflector. 
A stabilizing support Joined to the legs intermediate of their length helps 
to maintain the accuracy of the tool. The tool is simple in construction 
and use, rugged and inexpensive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A typical satellite downlink receiver antenna 1 is shown in FIG. 1. The 
receiver includes a convergent reflector 3, typically a parabola. A radio 
frequency transmission line 5, which may be a waveguide or stiff coaxial 
cable passes through the center of reflector 3. The opposite end of 
transmission line is bent to support electronic equipment 7, such as a 
preamplifier to boost the strength of the received signals. A receiving 
device 9, such as a feedhorn or other appropriate antenna, is located at 
the end of the transmission line and pointed toward reflector 3 to gather 
the signal reflected from and focussed by reflector 3. Because of the bend 
in the transmission line, this assembly is referred to as a "button hook." 
A portion of a typical receiving device 9 is shown in FIG. 2. Typically, 
receiving device 9 has a receiving aperture 11 covered by a dielectric 
window within a circular hood 13 surrounded by a peripheral flange 15. 
In FIG. 3, a frontal perspective view of a preferred embodiment of the 
inventive positioning tool 21 for locating a receiving device at the focus 
of a convergent reflector is shown. Tool 21 is shown mounted on receiving 
device 9 which is partially shown in broken lines 
Tool 21 includes a collar 23 open at one side to slide over and 
peripherally engage hood 13 of receiving device 9 Three legs, 25, 27 and 
29 are joined to collar 23, for example by welding. Legs 25, 27 and 29 are 
preferably equally spaced around collar 23; that is, radii drawn from the 
geometric center of collar 23 to each leg are spaced 120.degree. from each 
other. Each of the legs forms the same acute angle 31 with collar 23, so 
that the legs converge in the direction of the convergent reflector when 
the tool is mounted on the receiving device. A stabilizing brace 33 
disposed transversely to legs 25, 27 and 29 and attached to each of them 
steadies the legs and maintains the accuracy of the tool. The brace 
embodiment shown is preferably a welded rod or tube, but a solid plate may 
also be used. With a button hook type receiver and transmission line the 
brace need not be modified to accomodate the transmission line if the 
brace is placed near enough to the receiving device. Other constructions 
may require a cut-out in the brace to avoid interference between the tool 
and transmission line. 
In the embodiment shown in FIG. 3, each of legs 25, 27 and 29 are tubular 
and each contains a rod that telescope within its respective leg. Legs 25, 
27 and 29 receive rods 35, 37 and 39, respectively. The rods may be 
extended so their ends are near the surface of a reflector to indicate 
whether the receiving device is at the focus of the reflector as 
hereinafter described, or retracted into their respective legs for 
storage. 
In addition to using the inside of collar 23 for attachment of the tool to 
the receiving device, in the preferred embodiment three c-clamps 41, 43 
and 45 depend from the rear of collar 23. These clamps are visible in the 
rear view of FIG. 4 and two clamps can be partially seen in FIG. 3. Each 
clamp has a thumbscrew 47, 49 and 51 passing through a threaded hole in 
the clamp for clamping collar 23 to the receiving device, for example, to 
flange 15. 
An alternative construction of the legs and rods is shown in the partial, 
detail view of FIG. 5. There, leg 25 is shown and may be tubular or solid. 
However, the indicating finger is not a rod telescoping within leg 25 as 
previously described, but a sleeve 53 telescoping over leg 25. The sleeve 
has a lengthwise split 54 in order to pass over bracket 33 readily. FIG. 5 
also shows that leg 25 bears fiduciary marks 55 for setting its extension 
to a desired length. As discussed below, it is desirable to extend the 
indicating fingers the same length and the marks are important to 
achieving that end. In the alternative embodiment, the rods bear the 
fiduciary marks. 
It is also important to keep the extended fingers, whether rods or sleeves, 
at the desired length while the position of the receiving device is 
adjusted. FIG. 5 shows one embodiment of a retainer for the fingers. 
There, the retainer comprises a simple thumbscrew 57 engaging a threaded 
hole in sleeve 53. Tightening the screw to bear on leg 25 frictionally 
locks sleeve 53 in place. This embodiment also may be used when the finger 
is a rod, but there the screw engages a threaded hole in the leg 25 and 
bears on rod 35 to lock it by friction. Another retainer embodiment is 
shown in FIG. 6 for use with the rod type finger. There, a compression nut 
59 engages a threaded end on leg 25. An elastic washer 61 within nut 59 is 
compressed as nut 59 is tightened and locks rod 35 in place by friction. 
The use of the inventive tool is illustrated in FIG. 7. The tool is 
attached to the receiving device by slipping collar 23 over the receiving 
device and tightening the c-clamps. The fingers of the tool are then 
extended to near the surface of the reflector. Ordinarily, the distance to 
the reflector from the receiving device is set to the proper axial 
distance from the reflector to its focal point. However, this distance is 
often adjustable and can be set to the proper length by using the tool. 
The fingers of the tool are adjusted to provide the desired axial length 
and the receiving device is set at a distance greater then the focal 
length. Then the receiving device is moved toward the reflector until the 
fingers touch the reflector. The receiving device is then locked in axial 
position. 
More often, the receiving device is set at the proper axial length, but is 
off the reflector axis. Again, the tool is attached to the receiving 
device and all the indicating fingers are extended the same length to 
touch or nearly touch the reflector. If the receiving device is off axis, 
the fingers will not be symmetrically disposed about the center of the 
reflector through which the transmission line passes. This circumstance is 
shown in FIG. 7A where reflector 3 is shown schematically with the points 
indicated on the reflector by fingers 35, 37 and 39. The misaiming is 
readily detected from the asymmetrical distribution of the fingers about 
transmission line 5. The aim of the receiving device is then adjusted to 
bring fingers 35, 37 and 39 into a symmetrical disposition about the 
transmission line as shown in FIG. 7B. Because the fingers converge to 
encompass a relatively small area at the reflector surface, small 
asymmetries are easily observed and corrected. Only one person is needed 
to aim the receiving device, simply, quickly and accurately with the tool. 
The tool is so inexpensive to make that a home user of a satellite 
receiver can readily afford to have one. 
Various modifications of the tool are possible. While I prefer the three 
legged version to "triangulate" off axis errors, a two legged version that 
would alternately be used to correct each of two orthogonal axial 
misalignments could be built. The use of the tool is not restricted to 
receivers, but is readily applicable to transmitting antennas as well. In 
addition, it can be used to focus solar reflectors and other convergent 
reflectors for electromagnetic radiation outside the radio frequency 
spectrum. 
The invention has been described with reference to certain preferred 
embodiments. Various additions and modifications within the spirit of the 
invention will occur to those of skill in the art. Accordingly the scope 
of the invention is limited solely by the following claims.