High performance antenna systems for new VOA stations

IEEE TRANSACTIONS ON BROADCASTING, VOL. 34, NO. 2, JUNE 1988 167

HIGH PERFORMANCE ANTENNA SYSTEMS FOR NEW VOA STATIONS

Stephen W. Kershner Life Member IEEE

Kershner, Wright & Hagaman, P.C. 5730 General Washington Drive Alexandria, Virginia 22312

ABSTRACT

Advancements in curtain type HF broadcast antennas are reviewed, and new types of high performance antennas for VOA are described including vertical and horizontal beam selections, performance measurements, effects of finite ground and hills, radiation hazard zones, and transmission lines.

I. INTRODUCTION AND BACKGROUND

For some years the operators of international broadcasting stations have been developing an increasing need for directional antennas which will provide high performance as required to deliver effective signals to short-wave listeners in designated areas. In recent years the trans- mitter powers utilized for HF broadcasting have increased rapidly to a point where transmitters of 500 kW power are in common use. The high initial and operating costs of such transmitters and the requirement for providing better service to listeners has focused attention on the need for highly efficient antennas. The requirements have been further emphasized by the need to overcome jamming signals which have attempted to obliterate the programs of certain broadcasting organiza- tions.

This paper provides details of high performance HF curtain type antennas and transmission lines for installation at new Voice of America (VOA) stations in Morocco, Botswana, Sri Lanka and Thailand. The new VOA stations rill broadcast in the 6.1 to 25.6 MHz international frequency bands using several 500 kW transmitters. Kershner, Wright & Hagaman assisted VOA in the preparation of the antenna procurement specification as a member of the Broadcast System Engineering (BSE) team which included Holmes & Narver (prime BSE contractor), Atlantic Research Corporation, Inter- national Broadcasting Consultants and National TeleConsultants. The BSE contract was awarded for VOA by the U.S. Army Corps of Engineers, who provided design guidance and assistance.

A. < Curtain type antennas have been used for

many years for HF broadcasting. Early curtain antennas for VOA stations in the United States were designed by Weldon and Carr and installed between

1948 and 1956. These included the two band, 500 kW screen reflector antennas with power gains of 21 to 23 dBi that are still in use at the Green- ville, North Carolina station.'

Later designs for major Western HF broadcast stations were completed between 1952 and 1968 by Kershner & Wright' and the engineers of Radio Free Europe, Radio Liberty, VOA and Ammann and Whitney. These designs involved two band antennas capable of carrier power inputs up to 1000 kW and power gains of 20 to 24 dBi. Some of these types provided both horizontal beam slewing and selec- table vertical beam patterns obtained by switching the excitation of 4 to 8 vertically stacked dipole radiators per bay. The vertical beam selection feature was introduced to permit matching the antenna radiation to the most effective ionospheric transmission paths applicable to a given target area. The engineers of RFE/RL made further antenna design improvements for their stations in Munich and Lisbon. These new designs were installed in 1979-1983 and provide both vertical and horizontal beam selection and three band operation with two 250 kW transmitters combined in a novel triplexer network. The triplexer design is described elsewhere in this issue by Hugh Fallis of RFE/RL.

Meanwhile major broadcasters including the British Broadcasting Corporation, Deutsche Welle, and Radio France Internationale developed improved types of curtain antennas, sometimes working closely with antenna contractors in Europe, England and the United States. As a result several contractors now offer competitive designs that generally follow the CCIR curtain antenna definitions such as HRS 4/4/.5.3 These designs feature wide band performance over octave frequency ranges encompassing four to five frequency bands, power ratings of 500 kW plus 100% modulation, and up to five horizontal slew positions. RF switching to reduce horizontal or vertical aperture is also available on some designs.

B. DeveloDment Protect

A VOA contract was awarded in 1985 to TCI of Mountain View, California to develop a state-of- the-art antenna for HF broadcasting. The result is a prototype antenna4 that provides a number of in-

0018-9316/88/O6OO-0167$01.~ 0 1988 IEEE

168

dependent vertical and horizontal beam selections involving excitation and aperture controls, octave frequency ranges and a number of other fea- tures. Preliminary results of the prototype tests have substantially validated the design, and this project had considerable effect on the performance specifications recommended for the new antennas.

This new VOA-TCI antenna utilizes wide-band horizontal folded dipoles arranged in HRS 4 / 6 / . 5 modules. The radiator effective lengths, lowest dipole heights and horizontal and vertical radiator spacings are all X0/2 , and the reflector screen is spaced X0/4 behind the radiator, where X o is the design frequency wavelength. Separate feed lines are used for the three pairs of radiators in each bay, so excitation control is accomplished in dipole pairs. The relative currents in each vertical pair are further limited to +1, 0, or -1 to simplify the feed and switching system. Further details of the design are provided in another paper in this issue by Dr. Wilensky of TCI.

C. Prouaeation Studies

Studies performed by VOA and VOA contractors conducted in 1986 and 1987 provided substantial data on the vertical angles that provide the most effective ionospheric propagation to the target areas of the four new stations. The performance of different antenna patterns was also evaluated and compared for a wide range of target azimuths and distance ranges. VOA requirements cover a wide range of targets ranging from small countries to substantial portions of continents, with distance ranges from about 2000 kilometers to several thousand kilometers. The selection of antennas is thus complicated as most antennas must effectively serve small, medium and large size targets over a wide range of distances.

Analysis of the data provided by the propagation studies showed that the vertical angle requirements generally range from about 2 to 22 degrees and are fairly independent of operating frequency. A comparison of the performance of 4 and 6 stack curtains showed that 6 stack curtains generally provided about 2 dB greater signal strength than 4 stack designs to targets beyond 2500 kilometers. It was also established that selection of the best available vertical and horizontal patterns for a given target area as a function of frequency provides an effective method to offset the substantial variation of radiation patterns over the octave bandwidth of the antennas. This technique requires multiple vertical beam selection by switching the excitation of the dipole radiators in each bay of the antenna along with limited control of horizontal apertures.

An early propagation study for Morocco5 in which the author was involved compared the performance of 2 , 4 and 6 stack antennas for 43 VOA specified language areas. Six stacks proved best for 23 areas (54%), 4 stacks was rated best for 19 areas ( 4 4 % ) and 2 stacks was best for only one area (2%) .

Analysis of the many vertical beam modes available with the new VOA/TCI antenna showed that the +++ and ++0 vertical beam modes were the most useful, and that there were limited requirements for the +00 mode for near targets and the ++- mode to maximize one and two hop service to small target regions. The mode symbols represent the relative currents in the dipole pairs of each bay, from low to highest pairs with "+" represent- ing equal co-phased currents, "0" for zero currents and " - " for equal anti-phased currents.

11. ANTENNA DESIGNS FOR THE NEW VOA STATIONS

A. VOA Reauirements

VOA requirements were defined as an initial antenna selection for each station along with initial site drawings showing antenna locations. These initial specifications included antenna sizes, vertical beam modes, and required horizontal beam angles.

The VOA requirements included antennas from HR 2 / 4 to HR 4 / 6 in size with HRS 4 / 6 types specified most often. It was decided early in the selection process to meet the frequency band requirements by supplying low band-high band antenna pairs for the 6-11 MHz bands and the 1 1 - 2 1 MHz or 13-26 MHz bands. This decision was reached after considering the performance limitations of such wide band designs due to the substantial variation of radiated beam characteristics and gain with frequency, the use of vertical and horizontal beam controls to improve the performance of the wide band designs as discussed in Section I(C), the additional site requirements and the cost of specifying three antennas for the required frequency ranges.

It was also decided that the antennas would be limited in horizontal aperture to 2 or 4 bays, with some antennas providing selection of either 2 or 4 bay operation. Six bay antennas were considered in early studies but were not finally considered due to site size limitations, the high costs involved, and the limited use of such narrow beamwidth designs. The VOA requirements do, however, include provision for a number of side by side HRS 4 / 6 antenna pairs to be operated as 8 bay phased arrays using two properly phased 500 kW transmitters. This mode of operation will provide a 6 dB increase in signal strength to high priority narrow beamwidth targets.

B. Comvuted Radiation Patterns

25

169

HR 4/6/0.5

-

I Y J x Vertical Beam Modes

Radiation patterns and power gains are a function of the antennas size and type, the antenna height, horizontal and vertical beam modes, and the operating frequency to design frequency ratio.

The design antenna gains and radiation patterns were determined using a computer program with the following assumptions:

Perfect ground plane Perfect reflector screen Dipole current magnitudes equal to +1, -1 or 0 as specified by the vertical beam mode Ideal dipole current phases (depending on horizontal slew angle) Dipole directivity based on thin wire sinusoidal currents and half-wavelength dipoles at the design frequency Antenna dimensions per CCIR HR standards Gain determined by integrating the radiation pattern over a half hemisphere.

The resulting patterns and gains have been compared to patterns computed by CCIR6 and to data provided by method of moment programs such as the Numerical Electromagnetics Code and TCI's proprietary WIRA programs, and the agreement is quite satisfactory and usually within +/- 1 dB.

C. Vertical Radiation Patterns

The preliminary specifications prepared for the new VOA stations require that most of the antennas be equipped with the following vertical beam control modes:

3 0 HR 4 /6 /0 .5

+++

VERTICAL ANGLE (DEG. 1

F I G . 1A - VERTICAL PATTERNS FOR 6 . 1 MHz

2/4 or 4 / 4 +os ++, ( + - I 2/6 or 4/6 +oo, ++o ++t, (++-)

The +- and ++- modes are included as options for consideration by VOA.

These excitation modes are included in the vertical beam modes proposed by TCI for the VOA antenna development project.

Figures 1A through 1D show the computed boresight vertical patterns for the new VOA HRS 4/6/.5 antennas for 6-12 MHz. The patterns are for co-phased bay excitation (zero slew), and a design frequency of 8 . 5 MHz. Patterns are shown for the 6.1, 7.2, 9.7 and 11.8 MHz bands and for five modes indicating the excitation of the lower, middle and upper pairs of dipoles of each bay. The patterns for high band antennas may be inter- polated from these low band patterns.

The +++ pattern is for co-phased equal current excitation of the six radiators of each bay. This mode provides the highest gain at low radiation angles as required for maximum target penetration for one or two hop signals.

The ++0 pattern is equivalent to an HR 4/4/.5 antenna with equal co-phased radiator currents. This mode provides lower gains than the +++ mode at low vertical angles and higher gains at higher vertical angles as applicable to short and medium to long range transmission.

The +00 mode is equivalent to a HR 4/2/.5 antenna and provides patterns suitable for short to medium range transmission.

VERTICAL ANGLE (DEG.

F I G . 1B - VERTICAL PATTERNS FOR 7 . 2 MHz

170

2 5 -

HR 4/6/0.5 +++

F I G . 1C - VERTICRL PATTERNS FOR 9 . 7 MHz

The U - mode involves phase reversal of the upper pair radiator currents and equal magnitude currents for all radiators. This provides a high gain, medium angle, narrow beamwidth pattern suitable for one hop transmission to short and medium range targets of limited distance range.

Finally, the +-0 mode involves phase reversal of the middle pair of radiators and no excitation of the upper pair. This mode provides good gains at high radiation angles as suitable for some short range transmissions.

The patterns of Figures 1A to 1D show a large variation over the four frequency bands, with the vertical angle and vertical beamwidths decreasing with frequency, and the maximum gains increasing with frequency. This variation of pattern parameters is inherent in wide band antennas with the specified excitation modes.

The antennas specified for the new VOA stations include these vertical pattern modes to permit selecting the optimum vertical pattern required for best transmission to selected targets.

It is believed that the flexibility provided by the specified vertical beam modes will substantially increase the broadcast service provided by the new VOA stations. An analysis of propagation studies conducted during the Step 1 VOA design contracts shows that for many VOA target areas the 6 stack +++ mode provides a 1 to 3 dB signal increase compared to a 4 stack antenna operated in the ++ mode. The 6 stack low beam mode has also been shown to be of special importance for ex- tending one hop service to maximum range and frequency as required to reduce jamming effectiveness on west-to-east transmission during the early eve- ning hours.

30 HR 4/6/0.5 1

VERTICAL ANGLE (DEG.)

F I G . 1D - VERTICAL PATTERNS FOR 11.8 MHz

D. Horizontal Radiation Patterns

The new VOA four bay antennas are to be equipped with seven horizontal beam positions to provide horizontal beam slewing over a nominal arc of +/- 24 degrees at 8 degree steps. Five position slewing over +/- 24 degrees at 12 degree steps is required for each two bay section; thus, the seven position slewing is required only for combining the left and right bay pairs.

In addition, for some antennas switching is required to permit operation as a four bay or two bay antenna to provide selection of beamwidth. Two bay antennas (and the two bay mode for four bay antennas) are required to operate at nominal slew positions of +12 degrees, 0 and -12 degrees and with extended +/- 24 degree positions at relaxed VSWR specifications.

Horizontal slewing in 8 degree steps was recommended to permit adequately matching the horizontal pattern to the many sets of VOA target azimuths. Utilizing 12 degree steps for two bay combining and 8 degree steps for four bay combining is cost effective as only one additional RF transfer switch is required to provide the additional slew positions.

Figures 2A to 2D show horizontal radiation patterns for the four bay antennas for the ++0 vertical beam mode at the maximum vertical beam angle. Only positive slew angles (0, 8 , 16 and 24 degrees) are shown as the corresponding negative slew positions are mirror images. The patterns show effective slewing in near 8 degree steps even though the two bay combining involves only 12 degree steps. An analysis of side lobe levels shows acceptable performance and only minor increases due to the combination of 8 degree and 12 degree steps.

171

30 HRS 4 / 4 / . 5 (++) '

25 t

5

0 -50 -40 -30 -20 - 1 0 0 1 0 20 30 40 50

HORIZONTAL RNGLE (DEG.)

F I G . 2 A - HORIZONTAL PATTERNS FOR 6.1 MHz

30 HRS 4 / 4 / . 5 (++)

-50 -40 -30 - 2 0 -10 0 10 20 30 40 50 HORIZONTAL RNGLE (DEG.)

F I G . 2C - HORIZONTAL PATTERNS FOR 9.7 MHz

E. Desim Frea uency

The design frequency is chosen to provide optimum radiation pattern input and impedance over the range of operating frequencies. Design frequencies are normally between the geometric and arithmetic means of the minimum and maximum frequencies.

At the design frequency, the nominal radiator lengths and the vertical and horizontal radiator spacings should be X 0 / 2 , and the reflector screen to radiator spacing should be approximately X,/4.

For site layouts, the antenna widths were based on the geometric mean frequency as this requires the greatest width.

F. Ante

The VOA requirements include installing selected 4/6 antennas as adjacent pairs designed for future operation as an 8 bay phased array operating with two transmitters and the required

30 HRS 4 / 4 / . 5 ( + + I

2 5 -

- 2 0 m - 15

- U

5

0 -50 -40 -30 -20 -10 0 10 20 30 40 50

HORIZONTAL ANGLE (DEG.)

F I G . 2B - HORIZONTAL PATTERNS FOR 7.2 MHz

30 HRS 4 / 4 / . 5 ( + + I I

F I G . 2D - HORIZONTAL PATTERNS FOR 1 1 . 8 MHz

phase control and monitoring system. It is thus necessary to adjust the spacing between these 4 bay antennas to provide reasonable horizontal patterns and side lobe levels for 8 bay operation at horizontal angles up to +/- 2 4 degrees from boresight.

Close spacing provides better radiation patterns but undesirably high coupling between the adjacent antennas. It was concluded that the following center-to-center spacings should be utilized.

Design Frequency Antenna T v ~ e Pavel- 4 Bay Regular 3 . 2 5 4 Bay Array Pairs 2 . 7 5 2 Bay 2 . 2 5

These spacings are believed to represent the best compromise between coupling between adjacent antennas, 8 bay array performance and efficient al- location of site space.

The following estimates of coupling for adjacent 4 / 6 antennas were made based on computa-

172

tions made by TCI for the developmental VOA antennas. Figures for antenna spacings of 2.75 and 3.25 wavelengths are estimates based on experience and extrapolation of the TCI data for spacings of 2.03 and 2.50 wavelengths.

Suacing Worse Case Couuling 2.03 wavelengths -18 dB 2.50 wavelengths -31 2.75 (Phased Array) - 35 3.25 (Regular) - 40

G . InDut ImDedance

A balanced input impedance of 300 ohms is specified for the new antennas. A VSWR of 1.6 or lower is required for all operating frequencies and all normal operating modes, with higher VSWR permitted for the special ++- and +- vertical modes and the -24 degree and +24 degree two bay slew positions.

The maximum VSWR value of 1.6 was chosen as the maximum permissible to insure a load VSWR of 2.0 or less for the transmitters, considering the insertion VSWR's of the open wire transmission line, the wide band baluns, the coaxial transmission lines and the coaxial switch matrix. For worst case combinations of these components the VSWR will probably exceed 2.0 but it is expected that such combinations will be rare and thus not significantly reduce overall performance of the station.

H. Beam Control Networks

A complex network involving a large number of RF switches is required to provide the specified vertical and horizontal beam modes. A HRS 4/6/.5 antenna with 4 vertical modes, 5 two bay slew positions and 7 four bay slew positions must be capable of operating in 48 different beam control modes including 30 normal beam modes that must meet the 1.6 VSWR specification.

The VOA/TCI developmental antenna is equipped with 5 position horizontal slew switches for each 2x2 antenna module. Separate vertical beam switching networks are provided for the left and right 2x6 modules, and a single switching network provides four bay combining and horizontal slewing.

Intelligent controllers are to be installed in shielded shelters near each antenna to provide for operation and monitoring of the RF switches that control the vertical and horizontal beam modes. These controllers include local control panels to permit emergency operation and main-

tenance checks. Serial data interfaces are also provided to permit operation and monitoring of the

antenna from remote panels in the control room or by the station's computer control system.

I. Performance Testing

The wide bandwidth and large number of operating beam modes specified for the new antennas complicates both the design and performance testing of the completed product. The specifications require first article, and final tests to verify VSWR, high power and radiation pattern performance. In addition, a design analysis must be provided that includes radiation pattern computations made using advanced method of moments computer programs such as the Numerical Electromag- netics Code (NEC III)7. The resulting patterns will show the effects of actual radiator currents based on the design and feed system, and the results must show reasonable agreement with the idealized patterns included in the specifications.

Airborne radiation pattern measurements including the main beam vertical and horizontal patterns will be performed on the first article antennas and selected antennas installed at the four VOA sites to provide a final verification of antenna patterns.

J. Effect of Ground Conductivity

A computer study was conducted to predict how the electrical properties of the soil would affect the radiation patterns of curtain antennas. The Numerical Electromagnetics Code was used to model 4/4/0.5 curtain antennas with design frequencies of 8.5 and 18.8 MHz. Radiation patterns were computed over perfect ground and finite ground at frequencies in the 6-11 and 13-25 MHz bands of both antennas. The finite ground patterns were computed using the Sommerfeld ground loss formulation per the NEC I11 code to determine both the near and far field ground losses and effects on the radiation patterns.

It was found that the ground characteristics have very little effect on the azimuthal pattern, and Figure 3 shows the effect on the vertical pattern for a frequency of 6.1 MHz. Similar computations at higher frequencies showed smaller effects for finite ground.

This study indicates that the differences between the patterns of a curtain over perfect ground and one over a ground with these constants are small. Since the conductivity at Morocco was found to be substantially greater than 10 mS/m, there should be no significant differences between the patterns produced there and the patterns which would be produced if the ground was perfectly conducting. Conductivities for the other VOA sites range from 1.5 to 1 3 mS/m, but the effect on radiation patterns should still be minor.

173

I Pipure 3

E,- 4

6 - 7.7 mah VERTICAL RADIATION PATTERNS FOR 4141.5 CURTAIN ANTENNA

Opumling Fr*quen'oq 0.1 UHz Dad91 Frequbncy 0.6 UHz

I

K. Effect of Hills (Morocco)

A group of hills lie 2 to 7.5 km to the northeast and east of the Morocco site and the peaks subtend vertical angles of up to 2 degrees from the proposed antenna locations.

A numerical analysis of the effects of the hills on the curtain antenna radiation patterns has been performed. A detailed discussion of this analysis is presented in Appendix A. The results suggest that the average signal reduction caused by the hills will be less than 1 dB.

L. Radiation Hazards

The antennas, open wire transmission lines and baluns generate high RF fields that involve biological hazards to humans. VOA has chosen ANSI C95.1-1982, American National Standard Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 300 kHz to 100 GHz as its radio frequency protection guide. Over the HF international broadcast frequencies, ANSI C95.1-1982 states a maximum permissible human exposure plane wave power density of 900/f2 mW/cm2 where f is the frequency in megahertz.

For curtain antennas as employed for HF broadcasting, the principal main beam electric

field is horizontally polarized, and increases with the height above ground. A height of 20 feet above ground was chosen for the radiation hazard analysis, and near field effects were considered, as the far field pattern has not fully formed at the applicable distances.

The computer analysis was based on 500 kW carrier power for single antennas and 1000 kW carrier power for future operation of two 4/6 antennas as an 8/6 array. The effects of high program modulation and the combined fields of up to three adjacent antennas were then considered, resulting in a reduction of the effective safe height to approximately 10 feet.

Table I shows the maximum radiation hazard distances from the center of typical curtain antennas selected for the new VOA stations. For each type antenna, the front distances are given from a point at the center of the antenna. The distances are based on near field computations for all required beam modes and frequency bands. The worst case electric fields were used to determine the radiation hazard limits, as it was established that the magnetic field components were relatively equivalent or smaller than the electric fields.

TABLE I

RADIATION HAZARD DATA FOR TYPICAL VOA CURTAIN ANTENNAS

DISTANCE /kW) TYPE (MHZ)

500 4/6/. 5 6-11 < 180 500 4/6/. 5 11-21 < 250 500 4/6/. 5 13-26 300

500 4/4/0.5 6-11 < 180 500 4/4/0.5 11-21 220 500 4/4/0.5 13-26 245

500 2/6/. 5 6-11 < 180 500 2/6/. 5 11-21 < 210 500 2/6/. 5 13 - 26 < 250

1000 8/6/. 5 6-11 < 250

1000 8/6/. 5 13-26 440 1000 8/6/. 5 11-21 375

NOTE: Data are based on ANSI C95.1 radiation hazard signal limits taken at a height of 20 feet above ground level for a single antenna with 500 kW power input (carrier only). Considering multiple antenna operation (three overlapping beams) and average program modulation (70%) the effective height is reduced to 10 feet.

174

The radiation hazard distance is maximum at the horizontal angle of maximum radiation and for the VOA antennas the computed distances were considered to extend over an arc of +/- 30 degrees relative to the boresight azimuth.

Behind the antennas, the field strength will depend to a large extent on the type of reflector screen offered by the antenna contractor. Assum- ing the worst case operation of a 4/6 antenna operating at 500 kW with a front to back ratio of 10 dB, the radiation hazard distance was determined to be less than 160 meters, and this distance has been used for all antennas. Examina- tion of 8 bay operation with 1000 kW indicates that 180 meters is satisfactory for this case also.

111. OPEN WIRE TRANSMISSION LINES

Open wire transmission lines are required for all stations but Morocco. The lines are designed for balanced 300 ohm operation at a carrier power of 500 kW (6-26 MHz), VSWR up to 2.0, 100% modulation peaks and an average power equivalent to 70% tone modulation. Copperweld line conductors are specified to avoid work hardening experienced with copper conductors, the increased conductor loss, and the maintenance problems experienced by VOA with aluminum conductors.

It is recommended that the number of conductors for the transmission lines be limited to 4 or 6. Four wire lines have been widely used for 500 kw applications, and 6 wire lines are considered to be acceptable. Transmission lines Using two multi-wire cages formed by 6 to 8 small wires per cage are not recommended due to reports that corona arcs sometimes burn through the small wires and cause major line damage.

A. Conductor Loss

The following table shows conductor loss data for 300 ohm lines, assuming that the conductors are copper and equally share the total line current. For a six wire line, the conductors will likely have unequal currents and slightly higher loss. Four wire lines with .75 to 1.00 inch conductors or six wire lines with .50 to .67 inch conductors provide acceptable conductor loss.

4 WIRE 6 WIRE LINE LINE DIA . 0.500 0.333 0.23 0.30 0.40 0.48 0 . 7 5 0 0.500 0.16 0.20 0.27 0.32

DIA . LOSS (DB PER KILOMETER IIN.) IIN.) 1-

~.~

1.000 0.667 0.12 0.15 0.20 0.24

B. Ground Loss

Ground losses result from ground currents induced by the magnetic fields at the ground sur-

face. A static field analysis shows that for balanced lines with horizontally spaced conductors, the ground current is zero under the line center; and for typical line heights, maximum ground currents flow about 2 to 3 meters on each side of the center line. As the line height is increased, the ground currents and the resulting losses decrease. The ground currents are also a function of the line spacing and decrease as the spacing is reduced.

Brown Boveri has presented a method of computing the ground losses for high frequency transmission lines.' This method is based on quasi- static near-field theory and is thus of ques- tionable validity.

A study of the literature did not provide an acceptable method for computing ground losses under balanced HF transmission lines. It was established, however, that Professor R.G. Olsen of Washington State University had studied the losses of wire lines above a finite ground at frequencies below the HF band. Dr. Olsen was thus retained to provide a solution related to VOA requirements. Details of Olsen's method are included elsewhere in this issue.

Figure 4 shows the ground loss based on Olsen's method and NEC solutions for a typical transmission line as a function of height above ground and frequency. Analysis indicates that the

I Picure 4

COMPARISON BETWEEN GROUND LOSS

COUPUTED USlNO OLSEN'S METHOD AND NEC

175

line height should be from 5 to 6 meters in order to minimize ground losses.

An analysis using the Brown Boveri method shows general agreement with Olsen for frequencies up to 12 MHz, but provides losses about 50% higher at 22 Mhz. In both cases, the losses decrease rapidly as the line height is increased from 3.5 to 6 meters.

On the basis of this analysis, the following line heights are recommended.

Minimum Height 5.0 meters Average Height 6.0 meters

C. Other Losseq

Other losses include insulator losses, losses related to common mode currents or volt- ages, and radiation losses.

Insulator losses will be less than O.ldB per kilometer if high grade, low loss alumina or steatite material is used; and this requirement is included in the specifications.

Studies of radiation losses including clas- sical analytical methods and solutions using the Numerical Electromagnetic Code show that radiation losses are also negligible (less than 0.1 dB per line) at frequencies up to 26 MHz for typical spacings of opposite phase conductors (0.4 meters). This assumes operation at typical VSWR (up to 2.0) and straight lines to minimize radiation losses that occur at turns.

Common mode (unbalanced) currents and volt- ages on the transmission lines will increase the ground losses and radiation losses. For a long transmission line, it is likely that substantially all of the common mode power will be lost. Common modes caused by the transmission line may be kept negligible by careful control of line symmetry and conductor lengths. Assuming total common mode currents 20 dB below the balanced current, the power loss would be about one percent. With two such common mode sources (balun and antenna) per line, the loss will be two percent or approximately 0.1 dB.

D. Loss Budeet

The following loss budget is recommended for the transmission lines, with losses given in dB per kilometer.

Beauencv (MHzl 6 U 22

Conductor Loss 0.14 0.18 0.27 Ground Loss 0.05 0.07 0.06 Misc. Losses .16 .20 .32 Total Losses .35 .45 .65

The conductor loss is based on a four wire line with one inch diameter Copperweld conductors with a 20% allowance for increased loss due to non-uniform currents, stranding effects and operating VSWR. %e miscellaneous loss is an estimate to cover common mode and insulator losses and uncertainties in the ground loss estimates.

E. Hieh Voltaee Desien

The conductor size must be chosen to avoid or minimize corona or arcing on the transmission lines when operating at the maximum expected volt- ages and under unfavorable weather conditions including rain, wind and air contamination.

The line voltage is determined from the impedance (300 ohms), the maximum power input (550 kW), modulation (loo%), and maximum VSWR (2.0). For these conditions, the effective (RMS) voltage is 36.3 kV between the line conductors.

For the above conditions, the average voltage gradient for each conductor was solved by static field theory with the following results.

CONDUCTORS PER LINE AVERAGE VOLTAGE GRADIENT 2 - 4 6

WIRE DIAMETER (INCHES1 Volts- 1.00 0.50 0.333 511 1.50 0,75 0.50 381 2.00 1.00 0.667 286 3.00 1.50 1.00 191

These figures are based on the assumption that all wires have equal charges. The unequal charges normally found on lines with six or more conductors will cause a slight increase in voltage gradients.

Also, the voltage gradient on each conductor will vary around the circumference; thus the maximum voltage gradient will be greater than the average values given above. For a four-wire line of one-inch diameter conductors and a horizontal spacing of 0.40 meters, the maximum gradient is about 14 percent greater than the average gradient, giving a maximum voltage gradient of 326 volts per millimeter for the stated conditions.

F. ODeratin- Exue rience

Discussions with established antenna manufacturers indicate the following recommended transmission line conductor sizes and average voltage gradients.

Average Conductor Voltage

Conductors Diameter Gradient Source flmLuEl- 0 Brown Boveri Marconi 6 0.339 562

4 0.630 453 Thompson-CSF TCI 4 0.721 396

16 0.122 584

176

The TCI line provides the lowest voltage gradient and thus the highest wire corona safety factor. The TCI line uses the same wire diameter and configuration as used for the RFE/RL 500 kW transmission lines at the Pals station in Spain, and for the 250 kW trunk transmission lines at the VOA Tinang, Philippine station. The RFE/RL lines have been operating successfully for many years; however, the VSWR is normally below 1.5 and the old generation Continental transmitters do not normally operate at full 500 kW power.

The transmission lines at VOA Philippines Station perform well except when conductive fila- ments from sugar cane burning fall on the lines and start corona arcs. Under these conditions, power must be reduced to 100 to 150 kW to stop the corona discharges.

VOA has also operated 500 kW transmission lines for many years at the Greenville station, and these lines have provided excellent reliability. The lines have a 300 ohm impedance and employ four wires with upper wires of .721 inch diameter and lower conductors of 0.50 inch copper tubing. These lines normally operate at a VSWR of 1.50 or less and at power inputs of less than 500 kW from the old Continental transmitters.

G . Recommendations

Considering the above factors and the VOA requirements for high reliability for the new stations, it is recommended that the maximum voltage gradient for maximum operating conditions be specified at 340 volts per millimeter. This will require approximately 1.0 inch diameter conductors for four wire lines, or .70 inch conductors for six wire lines. It is believed that this will insure good reliability for 500 kW operation during normal weather conditions including rain and wind, and for 200-250 kW operation during periods of local crop burning near the sites. It should be noted that crop burning is expected to occur at the Thailand site.

H. Transmission Line Coupling

An analysis of coupling between adjacent transmission lines was conducted and results show good agreement between computations based on static field theory and modeling using the Numeri- cal Electromagnetic Code (NEC). For perfectly balanced lines a separation of 8 times the line conductor spacing (3.2 meters for a 0.4 meter conductor spacing) provides -50 dB coupling between lines.

Coupling was also computed between lines with total common mode currents 20 dB below the balanced line current. For this condition the following coupling values apply for a 6 meter line height and a 0.40 meter conductor spacing.

SDacing Couvling

6 meters 36 dB 10 meters 41 dB

Somewhat higher coupling is expected when the operating VSWR is considered.

Based on this analysis it is recommended that the line spacing be 10 meters minimum. The master plans show the lines to fan out in straight line runs to the antennas thus meeting this requirement.

I. Hazard Areas Near Oven Wire Lines and Baluns

Near field computations for 300 ohm open wire lines are shown by Figure 5. The results are based on operation with 500 kW power, 70 percent have modulation, a line VSWR of 2.0, and total common mode currents of 5 percent of the balanced line current. The graph shows the highest electric field components for heights up to 3 meters. The ANSI electric field limits for 12,22 and 26 MHz and also shown. Based on this analysis the following distances (from line centers) define the radiation hazard area for the transmission lines :

Low Band Antennas (6-12 MHz) 5 meters High Band Antennas (11-22/13-26 MHz) 7.5 meters

2 4

DISTANCE FROU LINE CENTER (Mmm)

MCdhUMI-rnTOtN RADIATION HAZARD FIELDS FROM 300 OHM OPEN WIRE

TRANSMISSION LINES

p a r - 5 m k w m - 2 0

cmnmuod.-5w smdlq- a m

I

I

; lH-UlrHald8(n**nl

1 I1

These figures are also considered to be represen- tative of the radiation hazard zones for the wide band baluns.

IV. CONCLUSIONS AND ACICNOULEDGUENTS

Specifications have been prepared for antennas to be installed at four new VOA stations. The largest antenna specified is a HRS 4/6/.5 CCIR type with low bandhigh band antenna pairs used to cover the required HF bands. The designs provide substantial flexibility in radiation beam characteristics with four vertical beam modes and seven horizontal b e p angles. It is believed that the technique of selecting the optimum radiation beam for ionospheric propagation to VOA target areas will provide a substantial improvement in the performance of the new stations.

The author gratefully acknowledges the assistance and guidance provided by Dr. Robert Everett and John Tat0 and other members of the VOA engineering staff, Hugh Fallis of RFE/RL, and Roy Bliss of the British Broadcasting Corporation. Others providing assistance with the project include Bernard Bale and Dr. Phillip Knight of IBC, Willie Kluehe and Arvo Mottus, formerly with RFE/RL, and the antenna engineering staffs of Brown Boveri, Marconi. TCI and Thomson-CSF.

1. "Antenna Engineering Handbook", Henry Jasik, Chapter 21, pp 28-29, McGraw Hill, 1961

2. "Curtain Antennas for High-Power HF Broadcast- ing Applications", Stephen W. Kershner, = Transactions on Br OadcastinF, June 1968

3. "Antenna Diagrams", CCIR, Geneva, 1984

4. "Antenna Electrical Design of Improved High Power Antenna Power System", VOA Contract IA- 22010-23, TCI, Mountain View, CA, March 1986.

5. "Propagation Analysis - Tangier Relay Station", VOA Contract No. IA-21911-23, Atlantic Research Corp., Alexandria, VA, 1986

6. "Antenna Diagrams"

7. "Numerical Electromagnetics Code (NEC), G.J. Burke, et.al., Naval Ocean Systems Center, San Diego, CA, 1981

8. "Transmission Line Study", Brown Boveri Co., VOA Contract IA-2160-23, Volume 111, Part 4.2, Section 2.5, November, 1985

APPENDIX A EFFECTS OF HILLS AT MOROCCO ON THE RADIATION'PATTERN OF AN HF ANTENNA

1.0 INTRODUCTION

There has been some concern regarding the effects which hills near the new VOA Morocco relay station will have on the vertical radiation patterns of the curtain antennas. A terrain analysis revealed that the most important hills lie to the northeast, east and south. The group of hills to the northeast is located 2100 to 7500 m from the antennas, and the angle between their peaks and the horizon as seen from the antenna site is approximately 2 degrees. The southern hill lies 1300 m away, directly in front of the 172 degree array, and is 2.2 degrees above the horizon. To estimate the impact of this irregular terrain on antenna patterns, a computer program has been written using Liepa's [1,2] expressions for the radiation pattern of a line source above a ground plane containing a semi-cylindrical boss. The program has been used to examine the perturbations caused by the hills for two different operating frequencies and source heights. The study was performed by Todd Pooton, formerly with Kershner, Wright 6 Haganan.

2.0 METHOD

A simplified model for an antenna radiating in the presence of a hLll is shown in Figure A-1. The model consists of a uniform line source over a perfectly conducting ground plane upon which is located a semi-cylindrical boss. As Liepa [1,2] has demonstrated, the pattern for this model, with the field normalized to that of a free-space line source is

~ ( 0 ) - 4lejkxcoSB jmCm(kro,ka)sin m0 sin mB0I m-1

where

J,(X) is the Bessel function of the first kind of order m, Hm(2)(x) is the Hankel function of the second kind of order m which is equal to J,(x) - jY,(x), and Ym(x) is the Bessel function of the second kind. Only the magnitude of the pattern function was of interest.

178

Y

Figure A-1

GEOMETRY OF SIMPLIFIED OBSTACLE PROBLEM

The numerical methods discussed in reference 3 were used to obtain the Bessel and Hankel functions. For some dimensions of interest, a large number of orders of these functions are required before Cm (kro, ka) will converge. If the functions of ka were computed for the same number of orders as the functions of kr,, the numerical range of the computer could be exceeded. Observa- tion of the Bessel functions suggested some valid approximations to limit the number of orders. For instance, Jm(ka) gets very small for large orders and can be set to a fixed value ( ~ x ~ O - ~ ~ O ) , and Ym (ka) gets very large for large orders and can be set to a large value ( 1 ~ 1 0 ~ ~ ~ ) . These maximums and minimums were chosen so that when they are squared, the resulting value would still be within the computer's range. The Bessel functions for ka were computed for fewer orders than those for kr,. Notice that as these extremes are reached, C (kro,ka) approaches Jm(x). Once

equal to Jm(x) for numerical convenience. The vertical pattern function rapidly converges to a solution once Jm(x) begins to diminish.

Hm I (~)/H,(~)(y)<lo-~~~, Cm(kro,ka) was just set

Necessarily, this analysis utilizes a simplifed model of a very complex topography. The actual terrain has multiple hills which can scat- ter the signal. However, this analysis is more detailed than the elementary uniform plane wave illumination of a knife edge. Knife-edge obstruc- tion analysis that includes phase effects and a line source has been shown to be in close agreement with this method. This method should provide a reasonable estimate of the hill effects and, un- less the results indicate that the hills would significantly impair coverage to critical targets, further analysis will not be warranted.

3.0 RESULTS

The first computations were performed to simulate 4/6/0.8 curtains aimed toward the northeast. Referring to Figure A-1, h was set at 2.05

wavelengths (the center height of the radiators) and the vertical angle to the boss was 2 degrees. The results of computations carried out for 2000, 3000, 6000, and 9000 m distances and for 9 and 18 MHz are presented in Tables A-1 and A-2. As the distance between the source and hill increases the low angle loss becomes larger, but the loss occurs over a smaller portion of the main beam.

TABLE A-1 GAIN DUE TO BOSS

Frequency - 9 MHz Angle to Boss - 2' Dipole Height - 68.3 m (2.05 wavelengths)

Vertical Anvle 1 2 3 4 5 6 7 8 9 10 11 12

Boss Distance (m) 2 o o o 3 o o o ~ ~ 0 .1 .4 .3 -.2 -.2 -1.4 -3.7 -.4 -.8 -3.0 -2.3 -.7 -1.4 -1.1 -.2 -1.0 -1.3 .6 . 7 -.9 -.3 .5 0 -.4 .8 .2 -.3 .5 .9 -.7 .5

1.1 -.l .6 -.5 .9 -.9 -.4 .1 -.4 -.l .1 .4 -2.2 .8 -.l -.3

TABLE A-2 GAIN DUE TO BOSS

Frequency - 18 MHz Angle to Boss - 2O Dipole Height - 34.2 m (2.05 wavelenghts)

Vertical Anele 1 2 3 4 5 6 7 8 9 10 11 12

BOSS Distance (m) 3000 2000 6000 9000

.3 .4 -.4 .3 -.5 -1.4 -5.3 -3.7 -1.5 -3.0 -1.4 -2.3 -1.9 -1.1 .1 -.2 -.7 . 6 .6 .7 .7 .5 -.6 0 .8 .2 .5 -.3

-.l -.7 -.4 .5 -.5 .6 -.l -.5 -.l -.4 .4 .1 .8 .1 .6 .4

-1.0 -.l -.6 .3

Computations were also made for the hill to the south (h-2.05 wavelengths, x-1300 m, 6-2.2 degrees). Once again, the low angle loss is greater for the high frequency case, and the o s - cillations between loss and gain are more rapid for the higher frequency.

To determine the effects of the hills on a 4/6/0.5 curtain, each computation described above was repeated but with h reduced to 1.75 wavelengths. There was very little difference between the two sets of results, s o , apparently, the attenuation is relatively insensitive to the radiator height.

Since such a large amount of data has been presented, it is helpful to examine the average

gains over the main beam for each case. Table A-3 shows the average and lowest (worst case) gain values for elevation angles between 2 and 10 degrees. Between 4 and 10 degrees the greatest attenuation was 1.9 dB, so the hills should not reduce the distant field strength very much except at the lowest angles.

TABLE A-5

SOURCE HEIGHT 2.05 WAVELENGTHS

Angle X f Avg. Gain Lowest Gain de& m IMW 2-10 de- 2-10 d a L a u

2 2 2 2 2 2 2 2

2.2 2.2

2000 2000 3000 3000 6000 6000 9000 9000 1300 1300

9 18 9 18 9 18 9 18 9 18

-0.1 -0.4 -0.4 -0.5 -0.5 -0.7 -0.6 -0.5 -0.4 -0.3

-3.5 -5.3 -3.0 -1.9 -1.6 -0.9 -0.7 -0.7 -1.1 -2.2

4.0 m S I O N

The effects of hills near the Morocco relay station on the radiation patterns of curtain antennas have been estimated by solving a simplified obstacle problem. The average attenuation due to the hills for angles of 2 to 10 degrees ranges from 0.1 to 0.7 dB depending on hill distance and angle.

The average gain reduction figures (.l to .7 dB) are believed to be the best measure of the net effect on the average signal loss to VOA target areas.

The worst case loss for angles between 4 and 10 degrees was 1.9 dB, and the worst case loss for angles of 2 and 3 degrees were 5.3 dB and 3.0 dB respectively.

REFERENCES

1. V.V. Liepa, “Numerical Approach for Predict- ing Radiation Patterns of HF-VHF Antennas Over Ir- regular Terrain,” Anten= Prooe., vol. AP-16, pp. 273-274, 1968.

2. V.V. Liepa, “Numerical Approach for Predict- ing Radiation Patterns of HF-VHF Antennas Over Ir- regular Terrain“, ESSA Technical Report IER 53- ITSA 51, 1967.

3. M. Abramowitz and I.A. Stegun editors, book of Mathematical Fu nctiong, Washington: Na- tional Bureau of Standards, pp. 385-386, 1970.

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High performance antenna systems for new VOA stations