US2354809A

US2354809A - Transmission line load for high frequencies

Info

Publication number: US2354809A
Application number: US458801A
Authority: US
Inventors: Hallan E Goldstine
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-09-18
Filing date: 1942-09-18
Publication date: 1944-08-01
Anticipated expiration: 1961-08-01

Description

1944- I H. E. GCjJLDSTINE 2,354,809

TRANSMISSION LINE LOAD FOR HIGH FREQUENCIES Filed Sept. 18, 1942 I TLE L Z5 16 f/ 10 Z3 Z2 lllw H I l W 15 Z6 33 v INVENTOR ATTORI'NEY Patented Aug. 1, 1944 2,354,309 mNsmssIoN mm LOAD Fon man moms FREQ

7' Hallan 1:. Goldstine, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 18, 1942, Serial No. 458,801

' 10 Claims. (Cl. 201-64) The present invention relates to dummy loads for high frequencies and, particularly, for loads designed to match coaxial transmission lines over a wide band of frequencies.

An object of the present invention is the provision of a high frequency load which is compact in size.

Another object of the present invention is the provision of a high frequency load in which the power dissipated may be readily determined.

ing a diameter of .004 inch was used for the in- A further object of the present invention isto provide a high frequency load which is aperiodic in its aracteristics.

St a further object of the present invention is th provision of a high frequency load which is capa e of working over a wide band of frequen-- cles without the necessity of returning the apparatus for each frequency.

A further object of the present invention is the provision of a high frequency load in which a considerable amount of power can be dissipated without materially changing the electrical characteristics.

It is desirable when adjusting transmitters to determine the characteristics of the transmitter when working into a resistive load. A load having the proper resistance will absorb all of the energy applied thereto through a transmission line connected to the transmitter. If the load is so designed as to have an impedance equal to the surge impedance of the transmission line there will be no reflected wave and the trans mission line is said to be matched to the load. For high frequencies the surge impedance of low loss transmission lines is substantially resistive.

For adjusting the loading of a transmitter, its

modulation characteristics, etc., it is essential that the transmitter operate into a transmission line which is properly terminated.

Therefore, in accordance with the principles of the present invention the load which is used may be constructed in the form of a concentric line having a resistance equal to the surge impedance of the transmission line to which it is connected and having the further properties that its input impedance is purel resistive and does not change with frequency. A small concentric line, the inner conductor having a high resistance per unit length, may be used for the load,

The present invention will be more fully understood by reference to the following detailed description which is accompanied by a drawing in which Figure 1 shows one view of a dummy load constructed with the principles of the lnvenner conductor i3. The outer conductor H was a copper tube having an inner diameter of .023-

inch and an outer diameter of .028 inch. The inner and outer conductors were maintained in their proper coaxial relationship by spun glass insulation l5 which was wrapped around the inner conductor I: in the form of a close helix of several layers. The surge impedance Z0 of such a transmission line is given by the following relationship;

G-l-jwC' (1) where R is the resistance and L and C the inductance and capacity per unit length and G is the conductance per foot.

The capacity per foot of a transmission line having the dimensions as given above is of the order of 26 ,uaf. per foot while the inductance is about .106 th. per foot. The resistance per foot is approximately 43 ohms. The above given value of inductance is determined from the relationship h/ft.=61 10g. 5 x10 At 15b megacycles,- neglecting the term G of,

Equation 1, since its effect is small, the surge impedance phase angle is smaller, as shown by the following example:

By the correct selection of wire size and insulation thickness the surge impedance may be made to match any transmitter transmission line.

The attenuation is so high on the load line that short piece of line represents an input impedance that will not vary greatly with a change in frequencies. If the section of coaxial line which constitutes the dummy load is open circuited on the far end of the line the input impedance Zin is given by the following:

sinh 2aL-j sin 25L, cosh 2aL--COS 25L :here or is the attenuation constant in nepers per unit length and B is the wavelength constant in radians per unit length. If Z=R+jiwL and Y=G+iwC, 'y (propagation constant) =x fi, the real part of which is equal to a and the imaginary part equal 5,

With the constants given above for the small nichrome concentric line the input impedance varies about :4%, if the line has a length about 6 feet for an operating frequency of 150 me.

A convenient mechanical arrangement of the load is to wind the coaxial line H! into a small diameter helix l5, as shown in section in Figure 2. With the successive turns touching, and using such small line dimensions a very compact coil results. For example, the 6 feet of line mentioned above may be wound into a. 1 inch diameter helix having about 23 turns. The total height of the coil is then approximately .64

tion of time. Then calibrating power is applied to the conductor ll and the amount of power required to give the same amount of temperature rise is noted. Thus the power dissipation at radio frequencies may be determined.

Due to the attenuation of the radio frequency power along the length of line Ill, the power dissipation will not be equally distributed over the length of the line. The first foot of the transmission line will have more power dissipated than the second ioot, etc. Therefore, the line must be soarranged that the insulation It is not excessively heated nor is the inner conductor it burned out due to local overloads. This may be assured by using a lower specific resistance wire for the part nearest the input end; or the length of the line may be made longer and the resistance of the wire lower if it is permissible to make the power dissipation per unit of line length lower.

While I have particularly shown and described several modifications of the present invention, it should be clearly understood that the invention is not limited to these forms alone, but that modifications may be made.

I claim:

1. A high frequency load including a two-conductor line having at least one conductor with a high resistance per unit length for dissipating the energy applied to said line there being no inch. The coiled coaxial line has one end of inner conductor |3 connected to a plug ll of a coaxial line connector 20, while the casing 2| into which the helix is inserted is connected to the outer shell 22 of the coaxial line connector 20. The conductive portions of connector are maintained in coaxial relationship by means of insulators 23 which, preferably, are little aflectcd by heat. Plug It has a portion 2l' between insulators 23 of greater diameter than the central holes in the insulators to maintain the insulator spacing and to hold the plug in position against end thrust. One insulator rests against a shoulder in casing 2| while the other is held in place by a split ring 33 in a groove in connector 22. The concentric line "I, coiled within the cavity in casing 2| is embedded in a slug 2! of some metal of high heat conductivity such as tin or zinc. A thermometer 26 is inserted in the metal slug for measuring the temperature rise under load. Casing 2| may further be provided with radiating fins 28 for dissipating the heat developed. In some cases it may be desirable to employ forced air cooling or fluid circulation cooling. The open end of conductor |3 is connected to a terminal 30 which is supported in position on casing 2| by an insulator 3|. The calibrating power, which may be direct current or low frequency alternating current, may be applied to the conductor l3 by means of terminal 36 and plug I8.

In use, the temperature rise is measured by means of thermometer 28 and recorded as a fLll'lQ-v electrically conducting path between said conductors.

2. A high frequency load including a two-conductor line having at least one conductor with a high resistance per unit length for dissipating the energy applied to said line and means for coupling a source of high frequency energy to one end of said line, said line being open at the other end, the resistance of said conductor increasing along the length thereof from said one end.

3. A high frequency load including a coaxial line having an outer sheath and an inner conductor, said inner conductor having a high resistance per unit length for dissipating the energy applied to said line there being no electrically conducting path between said conductors.

4. A high frequency load including a coaxial line having an outer sheath and an inner conductor, said inner conductor having a high resistance per unit length for dissipating the energy applied to said line and means for coupling a source of high frequency energy to one end of said line, said line being open at the other end. the resistance of said conductor increasing along the length thereof from said one end.

6. A high frequency load including a coaxial line having an outer sheath and an inner conductor, said inner conductor having a. high resistance per unit length for dissipating the energy applied to said line. said line being coiled into a helix and immersed in a substance of high heat conductivity.

8. A high frequency load including a coaxial line having an outer sheath and an inner conductor, said inner conductor having a high resistance per unit length for dissipating the energy applied to said line, said line being coiled into a helix and immersed in a substance of high heat conductivity and means for measuring the heat developed in said load.

'I. A high frequency load including a coaxial line having an outer sheath and an inner conductor, said inner conductor having a high resistance per unit length for dissipating the energy applied to said line and means for coupling a source of high frequency energy to one end of said line, the resistance of said conductor increasing along the length thereof from said one end, said line being coiled into a helix and immersed in a substance of high heat conductivity.

8. A high frequency load including a coaxial line having an outer sheath and an inner conductor, said inner conductor having a high resistance per unit length for dissipating the energy applied to said line and means for coupling a source of high frequency energy to one end of said line, the resistance of said conductor increasing along the length thereof from said one end, said line being coiled into a helix and immersed in a substance of high heat conductivity and means for measuring the heat developed in sistance per unit length for dissipating the energy applied to said line, said line being coiled into ahelix and arranged within a casing, said coiled helix being immersed in a substance of high heat conductivity and means associated with said casing for removing therefrom the heat developed in said line.

10. A high frequency load including a coaxial linehaving an outer sheath and an inner conductor, said inner conductor having a high resistance per unit length for dissipating the energy applied to said line, said line being coiled into a helix and arranged within a casing, said coiled helix being immersed in a substance of high heat conductivity and means associated with said casing for removing therefrom the heat developedin said line, said last mentioned means including cooling fins on said casing.

HALPAN E. GOLD STINE.