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A process control instrument comprises a control for generating or receiving a high frequency signal. A waveguide comprises a cylindrical housing closed at one end by a rear wall. A loop launcher is operatively connected to the control and comprises a wire having a first straight leg electrically co

A process control instrument comprises a control for generating or receiving a high frequency signal. A waveguide comprises a cylindrical housing closed at one end by a rear wall. A loop launcher is operatively connected to the control and comprises a wire having a first straight leg electrically connected at one end to the control and extending into the waveguide a first select length. A second straight leg is connected at one end to the rear wall and extends into the waveguide a second select length, greater than the first select length. A curved middle section connects the other ends of the first and second straight legs. An antenna is operatively coupled to the waveguide.

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We claim: 1. A process control instrument comprising: a control for generating or receiving a high frequency signal; a waveguide comprising a cylindrical housing closed at one end by a rear wall; a loop launcher operatively connected to the control and comprising a wire having a first straight leg

We claim: 1. A process control instrument comprising: a control for generating or receiving a high frequency signal; a waveguide comprising a cylindrical housing closed at one end by a rear wall; a loop launcher operatively connected to the control and comprising a wire having a first straight leg electrically connected at one end to the control and extending into the waveguide a first select length, a second straight leg connected at one end to the rear wall and extending into the waveguide a second select length, greater than the first select length, and a curved middle section connecting other ends of the first and second straight legs; and an antenna operatively coupled to the waveguide. 2. The process control instrument of claim 1 wherein the second leg is located at a center axis of the waveguide. 3. The process control instrument of claim 1 wherein the first leg is located off center in the waveguide. 4. The process control instrument of claim 1 wherein the first select length is about a quarter wavelength. 5. The process control instrument of claim 1 wherein the waveguide has a length of about three-quarter waveguide wavelength. 6. The process control instrument of claim 1 wherein the curved middle section has a radius of about 10 mm. 7. The process control instrument of claim 1 wherein the loop launcher is asymmetrically placed entirely on one side of an axis of the waveguide. 8. The process control instrument of claim 1 wherein the first leg is parallel with the second leg. 9. The process control instrument of claim 1 wherein the waveguide is filled with a dielectric material substantially surrounding the loop launcher. 10. The process control instrument of claim 1 further comprising a coupling cavity surrounding the waveguide for coupling the antenna to the waveguide. 11. The process control instrument of claim 10 wherein the coupling cavity is formed of metal to define an intermediate waveguide. 12. A process control instrument comprising: a control for generating or receiving a high frequency signal; a waveguide comprising a cylindrical housing open at a distal end and closed at an inner end by a rear wall; a loop launcher operatively connected to the control and comprising a wire electrically connected at one end to the control and extending into the waveguide and connected at another end to the rear wall; a coupling cavity comprising an open cylinder surrounding the waveguide and extending beyond the waveguide open end; and an antenna operatively coupled to the coupling cavity and the waveguide. 13. The process control instrument of claim 12 wherein the coupling cavity extends beyond the waveguide open end a length in a range of about 7, 9, 11 or higher odd multiples of quarter waveguide wavelength. 14. The process control instrument of claim 12 wherein the coupling cavity is of metal construction. 15. The process control instrument of claim 14 wherein the coupling cavity comprises a process connection. 16. The process control instrument of claim 12 wherein the loop launcher comprises a wire having a first straight leg electrically connected at one end to the control and extending into the waveguide a first select length, a second straight leg connected at one end to the rear wall and extending into the waveguide a second select length, greater than the first select length, and a curved middle section connecting other ends of the first and second straight legs. 17. The process control instrument of claim 12 further comprising a union nut operatively secured to the waveguide for threading relative to the antenna at any angular orientation. 18. A process control instrument comprising: a housing; a control in the housing for generating or receiving a high frequency signal; a waveguide comprising a cylindrical housing closed at one end by a rear wall; a loop launcher operatively connected to the control and comprising a wire electrically connected at one end to the control and extending into the waveguide and connected at another end to the rear wall to develop an asymmetrical radiated electromagnetic field; an antenna operatively coupled to the waveguide; and means for rotatably mounting the waveguide to the housing so that the housing and the loop launcher can be independently oriented relative to a process vessel. 19. The process control instrument of claim 18 wherein the means for rotatably mounting the waveguide to the housing comprises a waveguide adapter defining the rear wall of the waveguide and having a thread received in a threaded opening of the housing. 20. The process control instrument of claim 19 wherein a set screw in the housing maintains the waveguide adapter in a desired rotational position. 21. The process control instrument of claim 18 wherein the waveguide comprise a two piece assembly including a waveguide adapter operatively secured to the housing and a waveguide adapter tube extending from the waveguide adapter and defining the cylindrical housing so that the waveguide adapter defines the rear wall. 22. The process control instrument of claim 21 further comprising a conductor passing through the waveguide adapter for connecting the loop launcher to the control. 23. The process control instrument of claim 18 further comprising a union nut operatively secured to the waveguide for threading relative to the antenna at any angular orientation so that the housing and the loop launcher can be independently oriented relative to the process vessel. 24. The process control instrument of claim 23 wherein the waveguide adapter tube includes an annular shoulder and the union nut is operatively secured to the waveguide adapter tube between the shoulder and a snap ring. 25. The process control instrument of claim 18 wherein the loop launcher comprises an asymmetrical wire. 26. The process control instrument of claim 18 wherein the loop launcher comprises a wire having a first straight leg electrically connected at one end to the control and extending into the waveguide a first select length, a second straight leg connected at one end to the rear wall and extending into the waveguide a second select length, greater than the first select length, and a curved middle section connecting other ends of the first and second straight legs.