US2424596A - Microwave wattmeter unit - Google Patents

Description

July 29, 1947.

H. E. WEBBER MICROWAVE WATTMETER UNIT Filed, Dec. 13, 1943 ll llllllllll llllllllllllllllllllllllllllllllllllllllllll r:

I i I INVENTOR E. [Me-535R Patented July 29, 1941 oer-ice MICROWAVE WATTMETER UNIT Hugh E. Webber, Williston Park, N. Y., asslgnor to Sperry Gyroscope Company, Inc., a corporation of New York Application December 1:, 194:, Serial No. 514,109

' 8 Claims.

Another object of the invention is to provide a a mounting arrangement for connecting the hot wire element at the closed end of a coaxial transmission line.

Still another object is to provide a by-pass condenser arrangement for electrically closing the end of a coaxial transmission line.

Other and further objects and advantages will become apparent as the description proceeds.

In carrying out the invention in its preferred form, a hot wire element, comprising a filament in an evacuated bulb is mounted within the outer conductor of the coaxial transmission line'near one end of the line, a cylinder or cup is mounted in the bulb at one end thereof to serve as the inner plate of a condenser, and the filament is connected between the cup and a pinterminal coaxially extending from the hot wire bulb. The 7 dimensions of the bulb are such as to conform very closely to the outer surface of the cup forming the inner condenser plate, and the dimensions of the outer cylinder of the coaxial line are such that there is relatively little clearance between the portion of the bulb surrounding the inner condenser plate, so that the inner surface of the outer conductor of the coaxial line serves as an outer by-pass condenser plate. The filament and the pin terminal, extending from the bulb, form a continuation of the internal conductor of the coaxial line so that the filament is at the current anti-node or current loop of a standing wave set up in the coaxial line by the admission of high frequency alternating current power thereto. The arrangement thus permits the high frequency power to be supplied to one end of the line and a direct current connection to be made at the other end without setting up any microwave reflections.

A better understanding of the invention will be afforded by the following description considered in conjunction with the accompanying drawing in which:

Fig. 1 is a side view, largely in section, of a hot 2 wire holder of a radio frequency wattmeter, showing portions or the apparatus cut by a longitudinal plane;

Fig. 2 is a sectional view of a hot wire unit as cut by a longitudinal plane through its axis;

Fig. 3 is alongitudinal sectional view correspending to Fig. 2, fragmentarily showing a hot wire holder and showing the manner in which the hot wire unit of Fig. 2 is mounted;

Fig. 4 is a circuit diagram of a bridge circuit in which the apparatus 01' Figs. 1 and 3 may be employed.

Like reference characters are utilized through the drawing to designate like parts.

Radio-frequency wattmeters have been proposed in which both alternating current and direct current are supplied to a hot wire or a filamentary element having a substantial change of resistance with temperature such as a Wollaston wire or a barreter wire, for example. The total power dissipation of the wire is kept constant by maintaining its resistance constant, variations in the alternating current power being ascertained by measurement of the variations in direct current powertrequired to maintain constancy of power dissipation and resistance.

Fig. 4 illustrates a suitable circuit for measuring and maintaining the constancy of hot wire resistance. The bridge illustrated in Fig. 4 comprises four arms li, l2, l3 and I connected in series parallel to a source of direct current l5, having a switch it and a current adjusting rheostat I1 for controlling the supply of direct current power to the bridge. The bridge includes the usual diagonal arm, comprising a null indicating galvanometer I8, and a sensitivity adjusting rheostat is connected in series between con- Jugate points 2| and 22 of the bridge. Two of the bridge arms II and I2 comprise resistors oi! substantially constant or fixed resistance. The bridge arm 13 includes a resistor 23 and a current-responsive instrument such as a milliammeter 24, for example, in series, the electrical dimensions being so chosen that the total resistance of the'elements 2i and 24 forming the arm l3 balances the resistances of the arms H, I! and H to produce a null deflection in the galvanometer II when the arm I4 is held at a predetermined resistance value.

The arm It comprises a hot wire element adapted to have both alternating and direct current applied to it and including a filamentary resistance element 25 which constitutes the actual resistance forming the bridge arm I4.

In order to obtain adequate sensitivity to var- 'ment will be highly responsive to variations in energy dissipated in the filament, and also for the purpose of protecting the filament against destruction and against erosion causing variations in resistance thereof, the filament 25 is maintained in a vacuum or inert atmosphere and enclosed in a gas-tight envelope, such as a tubular glass bulb 26.

The hot wire unit I4 is mounted in a coaxial transmission line with the hot wire 25 forming a portion of the inner conductor of the transmission line in order that microwave energy may be supplied thereto. For example, a illustrated in Fig. 1, a coaxial transmission line wire holder may be provided comprising a main line 21 and a tuning stub 28.

The main line 21 comprises an outer conductor 29 in the form of a hollow cylinder, an inner conductor, comprising a rod 3| coaxial with the hollow cylinder 29 and extending from the lefthand .or microwave power input end 32 of the line 21 approximately to the junction with the stub line 28, and the hot wire unit l4. The latter comprises a lefthand axial pin terminal 33, the filament or hot wire 25 per se, and a righthand axially extending pin terminal 34. In the form of apparatus illustrated in Fig. 1 a by-pass condenser 35 is formed at the righthand or direct current input end 36 of the line 21 in order to close the end 36 of the line with respect to microwave alternating currents and to insulate the terminal 35 of the wire 25 from the outer conductor 29.

As shown in Fig. 1 the by-pass condenser 35 comprises a substantially cylindrical, somewhat tapered, metallic fitting 31 in contact with the pin terminal 34, surrounded by a sheet of insulating material 38 which is surrounded, in turn, by an inner surface of a metallic fitting 39 mating the fitting 31 and making direct contact with the outer hollow cylindrical conductor 29. Connections to a direct current circuit may then be made through metallic fittings 35 and 39.

For completing a direct. current circuit from the lefthand pin terminal 33 of the wire 25, to the outer conductor 29, the stub line 28 is provided, comprising an outer hollow cylindrical conductor 4| laterally extending from the main line 2'! and mechanically and electrically attached to the outer hollow clylindrical conductor 29 with an inner conductor 42 in the form of a rod connected between the rod 3| and a shorting plug 43. The shorting plug 43 may be adjustable in position for adjusting the electrical length of the stub line 28, for example, by threaded connections between the outer surface of the plug 33 and the inner surface of the hollow cylindrical conductor 4|.

For providing a sliding electrical contact between the plug 33 and the rod 42, spring contact fingers 44 may be provided.

Owing to the fact that the length of line 21 in terms of wavelengths will vary with variation in the frequency of microwave energy input at the lefthand end 32 of the line, it may be desirable to provide means for matching the impedance of the input circuit (not shown) to the line 21..

For example, an impedance ratio adjusting element such as a sleeve 45, slidable along the rod 3|, may be provided.

In order to avoid the necessity for tuning ad- 4 Justments'or impedance-matching arrangements, such as the device 45, and to make the radio frequency wattmeter broadband in characteristic, that is, accurately responsive to a relatively wide range of input frequencies, I prefer to mount the hot wire 25 as close as possible to the position of the current anti-node or loop in the standing wave pattern in the line 21. Preferably the wire 25 is mounted directly at such a current antinode. To this end the hot wire element I4 is so constructed that the wire 25 may have its righthand end 46 connected directly to one plate or terminal of the radio frequency by-passing condenser, and the inner plate of the by-pass condenser is mounted inside the bulb 26 of the hot wire unit l4. To connect the wire 25, it end 46 may be inserted through an aperture in the base 48 and secured by a plate 48 spot-welded to the base 48 within the cup 41.

For example, as illustrated in Fig. 2, a hollow cylinder, closed at one end to form a cup 41, may

be mounted within the bulb 26, the wire 25 being connected bet-ween the closed end or base 48 of the cup 41, and the pin terminal 33 which extends axially to the left and makes contact with the inner conductor rod 3|. As shown in Fig. 2, the actual connection may be made through a lead-in or supporting rod 49. In the arrangement shown in Fig. 2 an electrical connection from .the cup 41 to the righthand pin terminal 34 is made through a spider 5| which serve also for supporting the cup 41 within the bulb 26. Preferably the bulb 26 is of but slightly greater internal diameter than the outer diameter of the cup 41 in order to maintain a very small gap between the outer cylindrical surface of the cup 41 and the outer surface of the bulb 26.

In order to complete the by-pass condenser,

the inner plate of which is formed by the cup 41,

the hollow cylinder 29 may be reduced in internal diameter at the righthand end 36 to conform closely to the outer diameter of the bulb 26 as shown in Fig. 3.

For this purpose, a separate reducer 52 may be provided which serves also as a direct current connection. The reducer 52 is composed of suitable conducting material electrically and mechanically connected to the inner surface of the hollow cylinder 29, and may be threaded at the righthand end 53 to receive a direct current outer terminal 54. The hot wire pin terminal 34 may be extended through a bushing 55 to serve as a direct current inner or pin terminal. Direct current leads 56 and 51 may be made to the terminals 54 and 34 respectively. In the arrangement illustrated in Figs. 2 and 3, the coaxial line 2'! is terminated at the righthand end by the bypass condenser formed between the confronting surfaces of the elements 41 and 52, the actual length of these surfaces together with their radius being sufiicient to form a condenser of negligible impedance with respect to radio frequency. Since the righthand end 46 of the wire 25 is in direct contact with the inner plate 4! of the by-pass condenser, the wire 25 is in a current loop and its impedance causes no reflections from the righthand end 36 of the transmission line 21. Accordingly, an impedance-matching device sliding sleeve 45 (shown in Fig. 1) is not required and all of the radio frequency power applied to the lefthand end input end is absorbed in the hot wire 25.

A direct current circuit is completed through the hot wire 25 from the lead 56 through the terminal 54, the reducer 52, the outer hollow cyl- 28, plug 43, the contact fingers 44, the stub line inner conductor, the conducting rod 3|, the pin terminal 33 (referring to Figs. 2 and 3), the conductor 49, the hot wire 25, the cup base 48, cup 41, the spider 5| and the pin terminal 34, back to the lead 51.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a radio frequency wattmeter, a coaxial line adapted to receive radio frequency power at one end and to be connected to a direct current circuit at the other end, said line having an outer hollow cylindrical conductor extending the length of the coaxial line and an inner rod extending from one end of the outer conductor partially toward the opposite end, and a hot wire unit extending from the inner end of the inner rod to the said opposite end of the outer conductor, said hot wire unit comprising a tubular bulb, a cylindrical condenser plate within said bulb, a powerabsorbing wire or filament connected directly to said condenser cylinder atone end, and having a connection from the other end to the inner end of said inner rod of the coaxial line, the bulb diameter conforming closely to the diameter of the condenser plate cylinder, and the internal diameter of the outer hollow cylindrical conductor at the said opposite end conforming closely to the outer diameter of the tubular bulb, whereby a bypass condenser is formed at the end of the powerabsorbing wire. I

2. A power absorbing hot wire unit for a radio frequency wattmeter, comprising a tubular bulb, a condenser cylinder mounted therein having an outer diameter conforming closely to the tubular bulb, a longitudinally extending pin terminal, and a power-absorbing filament within the bulb connected between said pin terminal and the said condenser plate cylinder.

3. Apparatus as in claim 2 wherein the condenser plate cylinder comprises a hollow cup and a second pin terminal is provided which extends axially from the opposite end of the tubular bulb and carries a spider supporting the condenser cup.

4. In a radio frequency wattmeter, a coaxial line unit comprising a hollow cylinder, 8. reducer in one end thereof, a hot wire unit mounted within the reducer, and an inner conductor extending within the outer conductor axially from said hot wire unit, said hot wire unit comprising a tubular bulb conforming closely in outer surface to the inner surface of the reducer, a. condenser cylinder within said bulb, conforming closely to the adjacent surface of said bulb, a power-absorbing wire or filament connected between said condenser cylinder and said coaxial line inner conductor, and means for making direct current connections between inder and said reducer.

5. A power-absorbing hot-wire unit for a radio frequency wattmeter comprising a bulb, a condenser plate mounted therein, a pair of terminals, and a power-absorbing filament within the bulb connected between one of the said terminals and said condenser plate, said condenser plate being connected between said filament and the remaining terminal.

6. In a power-absorbing unit connectable to a concentric transmission line,. a by-pass condenser said condenser cylcomprising a bulb adapted to fit within the outer conductor of the transmission line, a condenser plate contained'within said bulb and arranged to cooperate with the outer conductor of the transmission line to form a by-pass condenser, and means extending through said bulb for connecting said condenser plate to the inner conductor of the transmission line.

7. A power-absorbing hot-wire unit for a radio frequency wattmeter comprising a bulb adapted to fit within the outer conductor of a concentric transmission line, a pair of terminals, a powerabsorbing filament within said bulb connected to one 01' said terminals for connection to the inner conductor of the transmission line, and a condenser plate arranged within said bulb to cooperate with said outer conductor to form a bypass condenser, said condenser plate being connected between said filament and the other of said terminals.

8. A power absorbing hot wire unit comprising an evacuated bulb, a first condenser plate therein, a second condenser plate external to the bulb, and conductor means connecting said plates respectively to opposite sides of a potential source, said conductor means including a power absorbing filament contained within said bulb.

- HUGH E. WEBBER.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 2,314,764 Brown Mar. 23, 1943 2,081,044 Runaldue May 18, 1937 1,480,227 Stevenson Jan. 8, 1924 1,671,478 Marbury May 29, 1928 1,775,266 Bailey Sept. 9, 1930 1,597,835 Shrader Aug. 31, 1926 2,097,100 Linsell Oct. 26, 1937 1,517,466 Schaller et a1 Dec. 2, 1924 2,270,949 Hulster Jan. 27, 1942 2,097,519 Gabriel et a1 Nov. 2, 1937 2,342,254 Dallenbach Feb. 22, 1944 FOREIGN PATENTS Number Country Date 289,779 Germany Jan. 18, 1916' 350.170 Germany Mar. 14, 1922

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