US2825874A - Artificial load for broad frequency band - Google Patents

Description

March 4, 1958 A. e. KANDOIAN 9 5 ARTIFICIAL LOAD FQR BROAD FREQUENCY BAND 3 Sheets-Sheet 1 Filed March 5, 1954 INVENTOR ARM/G g: xowoamn/ ATTORNEY 7 March 4, 1958 A. e. KANDOIAN 2,825,374

ARTIFICIAL LOAD FOR BROAD FREQUENCY BAND Filed March 5, 1954 5 Sheets-Sheet 2 (aasvsmwm- 0F 3 00mm) 40,40

RES/STANCE OF (0A0 AND DES/CAI VALUES 0F ANTENNA INVENTOR ARM/6, QZ/(A/VDO/AN I I I I III |II T I 2 3 4' 6 89|O 20 l I l I I I 5 7 F/QfQ. //v MEG/ICYCZ$- ATTORNEY 1 1-. A Mach 4, IQOS A. G. KANDOIAN 2,825,874

ARTIFICIAL LOAD FOR BROAD FREQUENCY BAND 3 t e 6 mm A $8 .h UXN e h S 3 a M 7 M E R 4 5 9 l 3 h C r h L d e l '1 F n m ATTORNEY United States Patent 2,825,874 ARTIFICIAL LOAD ll;R()AD FREQUENCY Armig G. Kandoian, Glen Rock, N. J., assignor to International Telephone and Telegraph Corporation, Nutley, N. J., a corporation of Maryland Application March 3, 1954, Serial No. 413,942 5 Claims. (Cl. 333-22) This invention relates to a dissipating load or dummy antenna and more particularly to a dissipating load for gperation over a relatively wide high radio frequency and.

For the purpose of testing various radio transmitter equipment it is necessary to provide artificial or dummy loads. The advantages in using dummy loads as transmitter test equipment are Well known. By the use of such devices, which preferably are made as compact as possible and capable of dissipating large amounts of energy, the output power and over-all performance of high power transmitters can be tested conveniently without emitting any radiations which could interfere with communications in the operating frequency hand. These dummy loads have usually been designed for each frequency at which the transmitter equipment was to be tested, since the antenna impedance changes with frequency.

However, in the design, testing and maintenance of a complete antenna tuning system, a dummy load is desired whose terminal impedance is substantially the same as that presented by the actual antenna to the input of the tuner. By using such a dummy load, whose impedance curve closely resembles that of the actual antenna being simulated and capable of handling the rated output power of the transmitter and having provision for determining the power absorbed, the tuning performance and power efliciency of the antenna tuner can conveniently be checked.

One of the objects of this invention therefore is to provide a dummy load which can be used to represent the impedance of a predetermined antenna over a wide range of frequencies.

Another object of this invention is to provide a novel type of dummy antenna having an impedance variation substantially equal to that of a predetermined antenna over a wide range of frequencies.

A further object of this invention is to provide an artificial dissipating load for operation over a wide range of frequencies which is compact and capable of handling large amounts of power.

A feature of this invention is the use of a coaxial line having a hollow outer and a lossy helical line as a center conductor whose length electrically is equivalent to the length conductor in the antenna being simulated. A liquid dielectric may be used along with cooling fins to dissipate the energy absorbed by the dummy load.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a cross sectional view partly in elevation of one embodiment of the high frequency dummy load of this invention;

Fig. 2 is a graphic illustration of the radiation resist-- ance in ohms of a 25-foot whip antenna and of alternate embodiments of the dummy load of this invention plotted versus frequency;

2,825,374 Cg Patented Mar. 4, 1958 Fig. 3 is a graphic illustration of the impedance of the dummy and actual antenna at various frequencies of interest, showing the correlation between the design and actual impedance values;

Fig. 4 is a cross sectional view partly in elevation of another embodiment of a dummy load for operation over a frequency band below the operating band of the dummy load shown in Fig. 1; and

Fig. 5 is a schematic diagram of the equivalent circuit for the embodiment of the dummy load shown in Fig. 4.

Referring to Fig. 1 of the drawing, one embodiment of a dummy load antenna in accordance with the principles of this invention is shown comprising a coaxial line 1 having an outer hollow conductor, shield or container 2 and a center or inner conductor 3. Coaxial with and inside of the outer hollow conductor 1 is a form 5, composed of a dielectric material such as a fiberglass, a ceramic or a plastic, on which the inner or center conductor 3 is wound to form a helix. The inner conductor 3 is formed of a relatively high resistance wire. A coaxial cable 5 has its inner conductor 5:: coupled to the input terminal 6 and its outer conductor 5b grounded by the straps 7a and 7b to the container 2. The space 8 between the inner conductor 3 and the outer conductor 2 may be filled with a dielectric liquid medium in order to increase the power and voltage handling capacity of the load. As the power is dissipated by the dummy load the dielectric medium in space 8 is heated and cooling fins it are utilized to assist in the dissipation of the heat.

In order to design a dummy load the impedance of the particular antenna to be simulated must first be determined. The impedance of a base fed whip antenna consists of the impedance of the base insulator in parallel with that of the radiating element. The base insulator can be viewed as a simple condenser having a given capacitance and power factor. As such its impedance is readily determined. The determination of the input impedance of the radiating element of the antenna, however, is much more difiicult basically due to the fact that the concept of impedance is primarily applicable to lumped circuits. Analysis of such circuits is best carried out by study of energy relations using voltage and current, but radiation on the other hand is essentially a distributed circuit phenomenon. The study of energy relations in distributed circuits is most readily analyzed by the use of the field theory which does not use impedance as a basic concept.

A thin vertical radiator over a ground plane has been analyzed by many people. The impedance has usually been computed theoretically by first assuming or evaluating by successive approximation the current distribution along the antenna. The fields are then computed by a process of integration. By means of a second integration the total stored and radiated energy is determined. The impedance is finally evaluated from the values of energy and the current at the base of the antenna. This sort of computation assumes the ideal case. The ground plane is assumed to be infinite and lossless. The physical dimensions of the antenna and base insulator are the only parameters considered.

Measurements made on an actual antenna will usually deviate appreciably from the theoretical values due to the finite conductivity and irregularities in the ground plane, the presence of nearby metallic objects and other peculiarities of the particular installation. Measurements made on the same antenna in two diilerent locations will seldom yield the same result and thus those skilled in the art advise that the best way to determine the impedance of a particular antenna is to measure it. In view of the above-mentioned considerations when it is desired to simulate an antenna it is necessary to make several sets of measurements of the impedance of the antenna to be simulated throughout the entire operating band. By averaging the various readings a set of hupedance values is obtained which can be considered to be. a fair estimate of the impedance of the antenna an average installation. For purposes of explanation let us assume that it is desiredv to simulate. a-25-foot base fed whip antenna intended for use between 3.00 kilocycles and 26 megacycles. It can be expected that the first resonance of such an antenna will be at 8 magacycles where a resistance of about 37 ohms will be measured. Referring to Fig. 3, curve A represents a set of impedance values for such a 25-foot base fed whip antenna. Referring to Fig. 2, curve A, it is seen that as the low end of the operating band is approached, the radiation resistance rapidly becomes negligible while at the same time the ground losses which are characteristic of the impedance site rather than the antenna become greater and greater. Because of this and the fact that the measuring equipment used is seldom capable of measuring small resistance values with reasonable accuracy, it is wise to accept only the measurements for frequency above the 8 magacycle resonance. The impedances representing the antenna below this frequency can be computed by obtaining the reactance of the radiating element through extrapolation by a measurement made at 2 megacycles considering the radiator as an open circuited transmission line having a constant characteristic impedance. The input resistance at the radiator can be determined and the antenna impedance computed as the parallel combination of the radiating element and the base insulator.

An antenna can be considered as an open circuited section of transmission line from which power is lost because of radiation. Thus, referring to Fig. 3, curve A, the graphic illustration therein shown represents the impedances of a 25-foot base fed whip antenna. The impedances shown by curves B and C represent the impedances of two dummy loads constructed in accordance with the principles of this invention. The impedance curve A of the 25-foot whip antenna can be synthesized by using an equivalent length of non-radiating transmission line; the radiation losses of the antenna being replaced by resistive or dielectric losses in the load. Thus, as shown in Fig. l the non-radiating transmission line 1 is utilized to synthesize the 25-foot antenna. The parameters available for utilizing a transmission line for the design of a dummy load are the length, losses and characteristic impedance of the line and the capacitance of the condenser simulating the base insulator of the synthesized antenna. The electrical length of transmission line 1 should be so chosen as to make the first resonance of the dummy load occur at the same frequency as the first resonance of the antenna being simulated. However, the physical length of the load need not be restricted by this limitation since by using a coaxial line 1 having a helical center conductor 3, the desired electrical length can be achieved in a compact physical size for handling the rated power output of the load.

For close fitting shields 2 the electrical length of the helical line 3 is approximately equal tothe electrical length of the wire making up the helix when unrolled to its full length in the dielectric medium contained in the space 8. The characteristic impedance of the helical line 3 is approximately equal to the product of the impedance of a coaxial line obtained by viewing the helix 3 as a solid center conductor and the ratio by which the physical length of the line is contracted by the spiraling of the center conductor 3.

The input resistance ofthe open circuited transmission line at its first resonance is equal to half the total resistance in the leads if the dielectric is lossless. The input resistance of the dummy load can, therefore, be matched to the required 37 ohms at the first resonance as shown by curve A Fig. 3, by winding the center conductor 3 with a high resistance wire such as Nichrome of the proper size to obtain a total resistance of about 74 ohms. Due to the low conductivity of this wire, there is practically no skin effect and the direct current resistance value may be used in choosing the wire size. The capacitance of the element respresenting the base insulator when properly chosen makes the second resonance of the load occur at the same frequency as the second resonance of the antenna. For high power loads a suitable length of coaxial cable 5 is utilized to represent the base capacitor since it has substantially the same power factor as the base insulator of the whip antenna being simulated and its capacitance can be chosen atwill.

The last step in the electrical design of the dummy load is the choice of its characteristic'impedance. This can be adjusted by (a) varying the ratio of the outer conductor diameter to the coil diameter until the load impedance at the second resonance frequency matches the impedance of the antenna at the second resonance freq n y o adjusting h pit h f coil o (c) by dis: placing the coil from the center of the outer conductor, in one embodiment of the dummy load designed in accordance with the principles of the present invention the resistance of the dummy antenna is correct at the second resonance when the characteristic impedance is about 10% higher than the geometric mean of the antenna pedance at the first and second resonance. T i 10% was chosen because itv approximates the decrease in the resonant impedance due to the loading of the base insulator.

The embodiment of the dummy load simulating a 25- foot base fed whip antenna shown in Fig. 1 is capable of dissipating 500 watt power output. The helical inner conductor is wound with No. 25 Nichcrome wire on fiberglass coil form 4. A liquid dielectric oil having substantially the same electrical properties as polystyrene fills the space 8 between the inner and outer conductor to help conduct heat away from the helix 3. Expansion of the oil is allowed for by providing bellows 9. With the cooling fins 10 shown the dummy load of Fig. l is capable of dissipating substantially 500 watts of power with a temperature rise of less than C. A shorting switch 11 is provided which converts the open circuited transmission line 1 into a coil short circuited to ground; Spring bellows 12 are provided to prevent the dielectric medium from leaking out of the shorting switch circuit. When the transmission line is shorted 60-cycle power may be fed into the transmission line through terminal 6 so that a thermal calibration of power dissipation may be accomplished. Absolute radio frequency power measurements can then be made by comparing a graph of temperature rise with reference curves obtained by using 60-cycle power. Relative power readings can be made by the use of an ammeter in the input lead.

Fig. 3 is a graphic comparison of the impedance curve of dummy loads shown in curves B and C and the impedance curve of a 25-foot base fed whip antenna of curve A. It should be pointed out that there is a fairly good correspondence between the whip antenna impedance curve A and the dummy load impedance curves B and C at frequencies above the first resonance. However, below the first resonance the load resistance does not ollo t r pidly ecreas n res sta ce of t n e a very well. This is graphically illustrated in Fig. 2 wherein the radiation resistance values of a 25-foot base fed whip antenna and two dummy loads are shown as curves A, B and C espe t e y t is seen a h s s nce of the antenna shownin curve A, Fig. 2, and the resistance shown in curve .C of the dummy load illustrated in Fig. 1 have a fairly good correspondence at frequencies above the first resonance of 8 megacycles, but below this frequency the load resistance does not follow-the rapidly decreasing resistance of the antenna. Thus, it is apparent that if a dummy load is needed below the first resonance it is necessary to construct a second apparatus having a resistance curve substantially like that shown in curve B, Fig. 2.

Referring to Fig. 4 a low frequency dummy load designed using lump circuit elements which closely simulates the actual antenna impedance from the lowest frequency up to the first resonance is shown. Fig. 5 of the drawiug is a schematic circuit diagram of the low frequency dummy load shown in Fig. 4 having an impedance curve shown in Fig. 2 as curve C and a resistance curve shown in Fig. 3 as curve C. The radiati'v res B of Fig. 2 is approximated by shunting u l r 19 across a lossless line terminated in a short, that is electrically an eighth wavelength long at the first resonant frequency of the antenna. The shunt condenser 20 is a length of coaxial cable 21 similar to that the high ircquency load of Fig. 1. However, the series condenser is a vacuum capacitance 3 whose reactance at the low frequencies is equal to that of the antenna being simulated without the base insulator, that is, the reactance of the antenna alone at the lowest frequency is the same as that of the condenser 23. The shunt loaded resonant ircuit 24 with its resonance at twice the first resonant frequency of the antenna has a resistance curve shown in Fig. 2, curve B which is substantially equal to that of the antenna shown in Fig. 2, curve A. The elements are so chosen that the input resistance is 37 ohms at the antenna resonance and the reactance of the network at this frequency resonates that of the series condenser. The low frequency load is capable of dissipating up to 500 watts provided the input voltage does not exceed 22 kilovolts which is the rating of the vacuum capacitor 23. The coaxial cable 21 simulates the base insulator and its ends are covered with silicon grease and polyethylene tape to prevent corona. The vacuum condenser 23 is mounted in the front of cylinder The resistance network 31 is enclosed in this cylinder 3t) under the cooling fins 32 and the cylinder 39 is filled with oil to aid in cooling. The shunt resistance 31 is made up of a series-parallel combination of non-inductive resistors 33 that are mounted inside the fiberglass coil form 34 of the inductance 35. The shunt condenser 36 of Fig. 5 is the stray capacitance of the coil 35 to the shield or cylinder St). The most convenient method of measuring power with this load is by utilizing a radicfrequency ammeter. Thermal calibration using 60-cycle power is possible by disconnecting the shunt inductance and coupling the 60-cycle power directly into the resistors.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A dummy load for dissipating high frequency energy, simulating the non-linearly varying impedance of a predetermined antenna having a base insulator over a broad frequency range above and below the first point of resonance, comprising a coaxial line having an outer and inner conductive surfaced conductor, said inner conductor shaped to form a helical line having a lengt electrically equivalent to the length of conductor in said predetermined antenna for producing a non-linearly varying impedance for said load substantially identical to said non-linearly varying impedance of said predetermined antenna, a liquid dielectric medium in conductive contact with said inner conductor contained within said outer conductor, coupling means for applying energy to be dis sipated to said coaxial line and a section of transmission line having a central and outside conductor, both ends of said central conductor coupled to one end of said helical line and said outside conductor coupled to said outer conductor of said coaxial line to produce a capacitance characteristic for said load substantially identical to said base insulator.

2. A broad band dummy load having an impedance non-linear variation essentially the same as that of a predetermined antenna over a broad frequency range comprising an outer leafproof conductive container, a dielectric cylinder contained Within and coax al to said container, a relatively high resistance conductive surfaced Wire won "l in a spiral around said cylinder to form a helix whose length is electrically equivalent to the length of conductor in said predetermined antenna for producing a non-linearly varying impedance for said load substantially identical to said non-linearly varying impedance of s d predetermined antenna, a liquid dielectric in conductive contact with said inner conductor contained Within said container, means to short circuit one end of said helix to said outer container to permit calibration of said load, and coupling means for applying energy to be dissipated to said helix.

3. A broad band dummy load for dissipating high frequency energy and having an impedance variation essentially the same as that of a predetermined antenna over a relatively broad frequency range comprising an outer hollow conductive container, a dielectric cylinder inside of and coaxial to said container, 2. liquid dielectric contained within said container, a length of Wire, having a relatively high resistance, electrically equivalent to the length of conductor in said predetermined antenna wound on said cylinder to form a helix, resistive means coupled across said helix, a capacitor having one end coupled to said helix, a section of transmission line having a central and outside conductor, both ends of said central conductor coupled to the other end of said capacitor and said outside conductor coupled to said conductive container, and coupling means for applying energy to be dissipated to said junction of said transmission line and said capacitor.

4. A dummy load according to claim 3 wherein said resistive means comprises a plurality of resistor elements coupled in series-parallel relation and located Within said cylinder.

5. An artificial load, simulating the non-linearly varying impedance of a predetermined antenna, having a base insulator, over a broad frequency range, for dissipating high frequency energy comprising a conductive container, a conducting surfaced inner conductor coaxial with said container, said inner conductor comprising a lossy helical line having a length electrically equivalent to the length of conductor in said predetermined antenna arranged with respect to said conductive container to have a characteristic impedance which matches the impedance of said predetermined antenna at its second resonant frequency for producing a non-linearly varying impedance for said load substantially identical to said non-linearly varying impedance of said predetermined antenna, a dielectric medium Within said container in conductive contact with said inner conductor, coupling means for applying energy to said inner conductor to be dissipated in said dielectric medium, and capacitance means coupled to one end of said helical line and said conductive container to simulate a base insulator capacitance for said load substantially identical to the capacitance of said predetermined antenna base insulator.

References Cited in the file of this patent UNITED STATES PATENTS 2,294,881 Alford Sept. 8, 1942 2,562,921 Kandoian Aug. 7, 1951 2,588,832 Hansell Mar. 11, 1952 FOREIGN PATENTS 665,634 Great Britain Jan. 30, 1952

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