Tournai of Research of the National Bureau of Standards Vol. 46, No. 2, February 1951 Research Paper 2182

Effect of a Metal Mast and Guy Wires on the Performance of the 600-0hm Multiple-Wire Delta Antenna

Harold N. Cones

This report describes the res ul ts of meas ure ments made to dete rmin e the effect of a metal mast an d guy wires on t he radiatio n pattern a nd the effect of guy wires on t ile ter­minal impedance a nd the rad iation e ffi cie ncy of the 600-ohm mu lt iple-wire delta antenna. Model techniques were used to obtain radiat ion patte rns of ide nt ical antennas s upported in var io us ways. Curves are prese nted showin g t he te r mi na l impedance over t he frequency range 1 to 25 megacycles of full -scale a nten nas wit h a nd wi t hou t g uy wires. A graph o f t he quasi-radiat io n e ffici ency of t he ante nn a as a fun ction of freq uency is s hown .

I. Introduction

The 600-ohm multiple-wire delLa antenna 1 (fig. 1) used in ionosphere studies requires only a single large wooden mast. This mast, which is lc s than 80 ft. long, can ordinaril y be procured and erected in most locations. However , in some locations it may be desirable to u.se a sectionalized metal mast. As the mas I, lies in a neu Lral plane of Lhe antenna, it wo uld appear that if the antenn a was electrically and mechanically symmeLrical and was fed by a balanced transmitter there would be no radiation from a metal supporting mast. It is quiLe im­probable however that such a compleLely balanced condition will prevail. Any unbalance in Lhe sys­tem will cause a residual current flow in Lhe mast, which at resonance could become quite large caus­ing appreciable r adiation. I t would also seem desirable to usc guy wires to help support eiLher a wooden or m etal mast. Because the effecLs of a metal mast and guy wires on the radiation pattern and impedance of th e del ta an tenna are d iffi cult to predi ct analytically , experimental measurements were made to determine these effects. This report describes the results of these measurements.

II. Instrumentation

Radiation pattern meas urements were made by using model techniques. That is, all dimensions of the antenna were scaled down by a certain factor in Lhis case 30, and th e frequency was scaled up by the same factor. The model antennas were sup­ported by nonconducting or metal masts of a diam­eter corresponding to 1 ft. in diameter on a full-scale antenna. Since ordinarily a single mast supports one transmitting and one receiving delta antenna mounted at right angles to each other , guy wires when used were spaced as shown in figure 2. In all cases the guy wi.res were insulated from the pole and from ground. Wnen continuous guy wires were used . these wer e the only insulators. The sectionalized guy wires had insulators at a spacing equivalent to every 776 ft., or about 0.2 wavelength

I II. N . Oones, H . v. Oottony, a nd J . M. Wa tts , A 600·ohm multiple·wire delta antenna for io nos phere studies, J . Research N BS 44, 475 (1950) RP2094.

at 25 :Nfc. Pattern meas urements were made over a ground mat of metallic mesh. When the model was supported by a metal masL, Lhjs mast was electri cally con nected to the gro und mat.

The model an Lenna was used as a receiving an­te nna. A baLLery-operated high-frequency trans­mi tte r mod ulaLed at an audio frcq uency was mounted at Lhe apex of an A-frame and moved abou t the model in a semicircula r a rc of 13 ft. in rad ius. The angular displacement of the Lransmitter wiLh respect to ground was Lr ansferred to Lhe t urnLable of a recorde r by means of a sclsyn system. The signal received by the mod el was recl in ed and th e resul t lng audio-f requen cy volLage passed through a seri es of selective amplifiers and se rvomechanism Lo cause a radial displacemen t of a pen on the recorder tu rntable. Thus as Lhe transmitter was moved through an arc of ]80 deg. , Lhe verLical radiation pattern of the anLenn a was au tomatically ploLted.

F igure 3 is a d iagram of the model range showi.ng the paths of th e target transmi tter with reference to the antenna up.der test. The model antenna was located in the X -Z plane. Radiation patterns were measured in. th e X-Z and Y-Z planes. In measuring patterns in the X-Z plane the polarization of t he transmitting antenna was always such that the electric vector was tangent to the m eridian arc XZX'. In measuring patterns in. th e Y-Z plane, the electric vector was always parallel to the X axis.

The patterns obtained show the relative power th at would be radiated by the antenna at different vertical angles in a given plan e for a given frequency. The patterns were normalized, that is, the gain of the pattern plotter was adjusted for each frequency to give a full-scale deflrction at t he point of maximum gain. Pattern m easurements were made only for the frequencies equivalen t to 14 through 25 Mc because, w ith the scale factor of 30 which was used, the model range was too small for measurements at lower frequencies.

Impedan ce measurements of full-scale antenna were made with a balanced recording impedance meter that automatically plots the absolute magni­tude of input impedance of an an.tenna over the frequ ency range 1 to 25 Mc. A block diagram and brief description of this instrument have been

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published. 2 Basically, it consists of a sweep fre­quency generator with ' a constant-current output, the voltage across the output terminals being, therefore, directly proportional to the impedance impressed across the output terminals. The instru­ment is calibrated by substituting noninductive resistors for the unlmown impedance. The absolute magnitude of the unknown impedance at any frequency can be determined by interpolating between the calibrating lines.

This instrument can also be used to obtain an indication of the radiation efficiency of a nonresonant antenna. The curve plotted by the impedance meter when an antenna is connected to its output terminals shows in arbitrary units the voltage across the antenna imput terminals. The curve also shows the impedance of the antenna, as the voltage is directly proportional to impedance. Since the input voltage and the input impedance of the antenna are lmown, the input power (in arbitrary units) can be computed for frequencies at which the antenna is purely resistive. These are, for all practical pur­poses, the frequencies at which the impedance is either a maximum or a minimum. If measurements are made of the voltage across the terminating resistor, the power dissipated in the resistor (in the same arbitrary units) can be determined. Knowing the power input to the antenna and the power dissipated in the terminating resistor, a quasi-radia­tion efficiency of the antenna (taken as the quotient of the power input less the power dissipated in the terminatin.g refistor and the power input) can be evaluated. This quasi-radiation efficiency always exceeds the true radiation efficency, since it in­cludes the copper, ground, and dielectric losses as power radiated.

III. Results of Radiation Pattern Measurements

Radiation pattem m easurements were made of four identical antennas supported by a noncon ­ducting mast , by a metal mast, by a metal mast and continuous guy wires , and by a metal mast and guy wires that were broken up by insulators spaced at distances equivalent to 7 ~2 ft.

Figure 4 shows the radiation patterns obtained in the X-Z plane for these four antennas. Refer­ring to the figures, it is seen that at 14 Mc the side lobes that are characteristic of the multiple-wire delta antenna at this frequency are effectively sup­pressed, at least in this plane, by the continuous guy wires. At 15 Mc, the metal mast causes a marked change in the usual pattern. Here again the contino us guy wires suppress the side lobes in this plane. From 16 through 20 Mc, the method of sup­port does not appear to affect the pattern appreci­ably, although continuous guy wires slightly increase the amount of low angle radiation at 16 Mc and slightly decrease it at 17, 18, 19 , and 20 Mc. At 21 Mc, side lobes appeal' for the antenna with the con-

2 H . N. Cones, Impedance characteristics of some ex perimental broad·band antennas for vertical ·incidence ionospbere sonnding, J. Research NBS 43, 71 (1949) RP2006; see Rlso reference listed in footnote 1.

tinuous guy wires. At this frequency the long guy wires are 2 wavelengths long and the short guy wires are 1 wavelength long. Also at this frequency, the metal mast is approximately 1% wavelengths long. From 22 through 25 Mc the patterns are very nearly alike, although the use of the sectionalized guy wires results in a slight change in the radiation pattern at 24 and 25 Mc.

The difference in the radiation patterns observed at 15 Mc between antennas employing nonconduct­ing and metal masts (both without guy wires) is at­tributed to the resonance of the mast close to this frequency. This effect would not be observed ex­cept for the existence of an exciting voltage that would have to arise from an unbalance incidental to the model antenna system under test. Therefore, as the unbalance is likely to vary in different installa­tions it may be expected that the resonance effects discussed above will l ikewise vary in magnitude.

Figure 5 shows comparative radiation pattern s in the Y-Z plane for all four methods of support. At anyone frequency the patterns are substantially alike.

It is seen that tho use of continuous guy wires is very effective in suppressing side lobes in the X -Z plane at 14 and 15 Me and to some extent effective in reducing low-angle radiation at other frequencies. They do not appear to affect the pattern in the Y-Z plane.

IV. Results of Impedance Measurements

Impedance measurements were made of full-scale antennas. As no metal mast was available, tests were limited to determining the effect of continuous guy wires on the terminal impedance of the antenna.

The following impedance measurements were made: 1. Measurements of an antenna without guy wires; 2. measurements of an antenna with long continuous guy wires, that is, guy wires fastened to the top of the mas t; 3. measurement of an antenna with long and short continuous guy wires, that is, guy wires fastened to the top of the mast and guy wires fastened to the mast 35 ft above the ground.

Figure 6 shows the results of these measurements . The long guy wires affected the impedance only near the frequency where the guy wires were a half wave­length long. No change is noted at any multiple of a half wavelength. The short guy wires did not appear t.o have any effect on the impedance.

V. Results of Measurements of Quasi-Radia tion Efficiency

Figure 7 shows the results of measurements of the quasi-radiation efficiency of an antenna with con­tinuous guy wires and of an antenna without guy wires. As noted previously the quasi-radiation efficiency always exceeds the true radiation efficiency, as the copper, ground, and dielectric losses are included as power radiated. Although the true radiation efficiency is not known, the lower curve of figure 7 indicates the manner in which the quasi-

114

radiation efficiency chal1ges with frequency. Th e efl'ects of guy wires are apparently very small.

It was shown in a previous paper (see foo tnotc 2) that the value of the terminating resistor was noL particularly criLical for frequencies above 8 M c. Figure 7 shows that only a relatively small amount of energy is dissipated in the termin ating resistOl" at. frequ encies above 8 11c.

VI. Conclusions

1. The use of a metal mast to suppor t the multiple­wire delta an tenna causes li ttle change in the vertical radiation pattern in the frequency range 14 through 25 M c, except at 15 M c where large side lobes were observed in the particular model on which measurements were made. (See fig . 4.)

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2. In general the usc of continuous g uy wires appears to improve the ver t ical radiation pattern of the an tenna.

3. G uy wires that are broken up at shorLin tervals by insulators have no measurable effects on the vertical radiation pattern of Lhe antenn a excep t at the highest frequ encies in th e oper­ating range. E ven here the effects are minor.

4. In the case of wooden masts continuous g uy wires have no m easurable effect on the input impedance of the an tenn a except ncar the frequency where th e long guy wires are a half wavelength long.

5. In the case of wooden masts the use of guy wires does not appear to affect the effi cien cy of the an tenna as a radiator.

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OBLIOUE VI EW OF THE OBLIQUE ELEMENT

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FIG URE 1. Simplified diagram of the 600-ohm multiple-wire delta antenna.

No. 12 wire used for all elements.

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The patterns are taken in the X-Z plane, and the antenna is in the .X-Z plane. rr"he radial displacement is proportional to power. A, N onconducting rnast; B, metal mast; C, metal mast witb continuous guy wires; D , metal mast with guy wi.res that ha ve in sulator::: inserted every 7Y2 ft.

116

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F IGUR E 4-Continued. Normali zed radiation patterns in space f or a l1w ltiple-wi re delta antenna s1Lppol·ted bV an insulated mast and for a multiple-wil·e delta antenna supported by a metal mast with and without guy wires .

'1"'110 patLcrns arc taken in t he X-Z plano, and t he antenn a is in the X- Z plane. '-rhe raelial displacement is proportional to power. A , No nconduct ing m ast; B , lTI ctal mast : C, metal mast wi j,h co nt inuous guy wires; :0 , lllctal m ast wit h guy w ires t.hat have insulators inserted every 7Y2 i t.

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Tbe patterns are taken in the X·7 plane, and the antenna is in the X-7 plane . '1' ho rad ial displaceme nt is proportional to power. A, Nonconducti ng mast; B , n:: etal mast; 0, metal mast with con tinuous guy wires; D , metal mast. with guy wires that have insulators inser ted cycr y 7Y2 fL.

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FIGU RE 5. N ormalized radiation patterns in space for a multiple-wire delta antenna supported by an insulated mast and for a multiple-wire delta antenna w pported by a metal mast uith and without guy wires.

The patterns are taken in the Y-Z plane, and the an tenna is ill the X-Z plane. rrhe radjal displacement is proportional to power . A, Noncanducting mast; B, metal mast; C J n:.etal mast with continuous guy wires; D , m etal mast with guy wires that have insulators inserted every 7Y2 ft.

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FIGUR E: 5- Contimted. Normalized mdiation paterns in space Jor a multiple-wil'e delta antenna supported by an insulated mast and Jar a multiple-wire delta antenna supported by a metal mast with and without g1!y wires.

The patterns arc taken in the Y-Z plane, aLld the antenna is in the X-z plaLl c. The radial disp lacement is proportional to power . A, No nconducting mast; B, metal mast; Ot metal mast with COD tinuous gu y wires; D , metal mast with guy wires that have insulators insert.ed c \'cry 77'2 ft.

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FIGURE 6. J mpedance of 600-ohm multiple-wiTe delta antenna with and without guy wires .

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FIGU RE 7. N[easurements oj the quasi-radiation effic iency of a multiple-wil'e delta antenna with and without gWJ wires.

A, GOO-o hm multiple-wire delta antenna supported by a wooden mast, no guy wires used ; B, GOO-ohm multiple-wire delta antenna supported .by a wooden mast, co ntinuous guy wires used; C, 0, without guy wires; . , with continuolls guy wires.

W ASHINGTON, October 16, 1950.

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