IEEE TRANSACTIONS ON BROADCASTING, VOL. BC-28, NO. 3, SEPTEMBER 1982
EFFECTS OF POWER LINE RE-RADIATION ON MHE PATTERNS OF A DUAL-FREQUENCY MF ANTENNA
M.M. Silva and K.G. BalmainDepartment of Electrical Engineering
University of TorontoToronto, Canada M5S 1A4
Abstract
A detailed computer simulation of AM broadcast re-radiation from a power line is described and the compu-tations compared with measurements. The power line isin the vicinity of the London, England two-frequencydirectional antenna array of the U.K. IndependentBroadcasting Authority. A simplified equivalent compu-tational model is derived for the individual power linetowers, and the ground effect due to the tower footingsis estimated. The computational model includes theoverhead ground wire (skywire) but excludes the powercarrying wires. Computations are described for thepower line induced currents and for the antenna radia-tion patterns as modified by the power line currents.The comparison of the computed results with full-scaleradiation pattern measurements shows sufficient quanti-tative agreement to support the utility of computation-al predictions for power-line re-radiation. The fre-quency sensitivity of the re-radiation is studiedtheoretically and found to have a noticeable effect fordeviations of + 10 kHz.
Introduction
The scattering by re-radiating structures of AMradio signals in the medium-frequency broadcast bandhas progressively increased over the past few years.With the spread of urban population there has been acorresponding spread of urban structures, particularlyhigh rise buildings. Other large structures such aselectricity power lines and smoke stacks are to befound almost anywhere, not necessarily close to cities.Their dimensions can be of the order of a wavelengthin the 500-1600 kHz broadcast band and thus they couldbe effective re-radiators of radio waves, creating anundesirable distortion in the radiation pattern of anyMF transmitting antenna in the vicinity.
The broadcaster may have little choice but tobuild his station in proximity to such existing struc-tures. Furthermore, he may find over the years achange or an increase in the number of these structures,all of which will be outside his control. The formerproblem has been encountered by the Independent Broad-casting Authority in the United Kingdom, and the re-sulting experimental and theoretical studies that arosewill be of interest to those concerned with the in-creasing problem of scattering by large structures.
Historical Notes
Until 1972, radio broadcasting in the UnitedKingdom was provided at national and local level solelyby the British Broadcasting Corporation, on a non-commercial basis. The MF services were transmittedfrom antennas generally having little or no directivityand fuithermore those stations that shared the samechannels within range of skywave interference from eachother carried the same program.
The UK Sound Broadcasting Act of 1972 empoweredthe then Independent Television Authority (ITA) to es-tablish an additional service of radio broadcasting asan alternative to that of the BBC. This would bepurely for local as opposed to national coverage, inwhich major conurbations in the United Kingdom would be'served by independent consortia on a self-financing
E. T. FordMasts and Aerials Section
Independent Broadcasting AuthorityWinchester, England S021 2QA
commercial basis. The ITA, renamed the IndependentBroadcasting Authority (IBA), would be responsible forbuilding the transmitting stations and for ensuringthat the various Independent Local Radio operators main-tained specific standards for technical performance,advertising and program content.
Up to 60 independent MF stations were originallyplanned (37 of which are in service at present) andconsiderable re-use of the UK's available MF frequenciesbecame necessary. The IBA, finding little or no ex-perience in the UK and Europe of providing deep andstable co-channel protection nulls in MF antenna pat-terns, looked for examples and consultation to NorthAmerica and the many thousands of highly directionalarrays successfully being used to control co-channelproblems.
One of the most valued franchises for commercialradio, of course, was Greater London. Two program com-panies were granted licenses to operate independently,necessitating two separate channels and implying twoseparate transmitting stations. The IBA could foreseevery great difficulties in obtaining two antenna sitesat suitable ranges and orientations relative to CentralLondon, bearing in mind the high population density andthe possibility of objections from nearby residents.Moreover, a considerable saving in costs would accruefrom co-siting the services with a single transmitterbuilding and antenna array, albeit that the antennawould itself be more complicated owing to dual-frequencyoperation.
With ten co-channel stations to protect in the lK -five on each frequency - it was required that ground-wave nulls with depths of up to 24dB relative to themain lobe be provided at various angles to the northand west. A four-mast endfire array 'was required,occu-pying a ground-mat area of some 300 metres by 130 metres.
I The expected difficulties of obtaining a site ofthis size were inevitably encountered. The station hadto be within a reasonable range of the city in order tosatisfy the criterion of "local" radio, but this wouldplace it within the protected Green Belt of Outer London.Some 200'sites were investigated, most of which provedtechnically unsuitable or were contested by City Plann-ing Authorities. Finally in 1973, a mere three sitesemerged as possible candidates, and all three had highvoltage electricity power lines passing close by.
The IBA had previous experience with power linere-radiation effects at a few directional-antenna in-stallations. The resulting pattern distortion hadturned out to be acceptable and no detuning of the sup-port towers had been necessary. The London situationseemed by far the most severe, however, not the leastreason being that two frequencies were involved. Itwas understood that in North America tower detuning(usually at single frequencies) was often undertaken incollaboration with the power companies but this appearedto be a very time-consuming procedure. The disturbingprospect arose in the minds of the engineers that theresulting re-radiation problem might require the towersto be detuned at two channels simultaneously. Detuningeven a single chanmel was unprecedented in the UK, andthe administrative delays in negotiating the technicaldetails (as well as safety and maintenance matters)with the Electricity Board together with the time re-quired to install and adjust detuners, would jeopardise
the dates for which the commercial radio companies were
geared to commence their operations.Of overriding need was a computer model that would
predict the re-radiation from power lines and pylons.None was available to the IBA at that time, so.a first-order study was initiated as follows [1]. The proposedantenna sites were adjacent to power-line towers ofvarious heights between 27 and 32 metres and about 500metres away at their closest approach. At one site thelines would pass across in front of the main antennalobe. At the other sites, the wires would be -to theside and rear of the main lobe, but the situation was
complicated by the presence of an isolated 24-metresteel tower, for clay pigeon shooting, 700 metres infront of the antenna in the main lobe. At each site,the mutual coupling between the antenna masts and thenearest 28 pylons was computed using the Induced EMFMethod and assuming sinusoidal current distributions on
all the vertical radiators. The four theoretical cur-
rents for the antenna masts themselves were the designcurrents prepared by the consultant for the antennaarray in the absence of any re-radiators.
In order to make use of existing computer programsthe re-radiating structures had to be assumed to be ver-
tical radiators all of the same height as the masts,namely 71 metres. This first-order approximation-waspartly justified by the presence of the towers' horizon-tal top earth wires which, it was thought, would "top-load" them at MF frequencies, rendering them considera-bly higher electrically than their physical height.Horizontal currents were deemed not to contribute tofar-field vertical polarisation in the groundwave sig-nal, and were expediently ignored for the skywave con-
tribution. That such currents might alter the pyloncurrents according to pylon spacing had to be totallyignored as well. Although the computer simulation leftmuch to be desired it had the merit of providing insightinto which of the pylons would probably carry thegreatest induced currents and hence distort the nullsin the antenna pattern. Furthermore, by computing theradiation patterns from this giant array of 32 radiatingsources (which included the four antenna masts) addi-.tional insight was gained into the extent to which theantenna patterns would break up into a series of minorlobes.
The towers were "grounded" by a top wire (forlightning protection) but their bases consisted only of
the steel tower leg extensions set in concrete with no
special grounding provisions. Therefore the base impe-dances of the towers were assigned various values on
the order of 200 to 300 ohms magnitude with phase anglesranging between zero and -90.degrees. This had the
merit that unrealistically large estimates of the base
impedance magnitude could be made to offset the assump-tion of 71 metres physical height. Consequently the
final step in establishing this very approximate compu-tational model would have to be. the adjustment of the
base impedance values to produce a reasonable corres-
pondence between the calculated and measured far fields.
Al-l pylons in this model had the same base impedanceand therefore would re-radiate directly in proportionto the incident illuminating signal from the main
antenna.
As one would expect, the predictions showed that
regardless of the phase of the pylon base impedance(and hence the re-radiated phase) the resulting antenna
patterns. would be broken up in the null regions by upto a dozen.narrow lobes at both channels. The computermodel highlighted the probable limitations in the angu-lar sectors over which nulls of sufficient depth could
be established in the groundwave and skywave- patterns.As a result, the site where pylons crossed in front of
the antenna was shown to be untenable. On the other
hand one of the other sites, while having not insigni-ficant re-radiation, appeared to offer the hope that
pylon detuning might in the end be dispensed with. Thesite was acquired 19 km north of Central London, andfrom its location the Antenna Pattern Specificationswere derived, as shown in Table 1. It should be notedthat all measurements and computations in this paperhave been carried out at the pre-1978 European channelfrequencies of 1151 kHz and 1546 kHz prevailing at thetime. After November 1978 all European MF channelswere changed to multiples of 9 kHz, the London stationbeing retuned to 1152 kHz and 1548 kHz.
Antenna Commissioning in the Presence of the Power Lines
The antenna was commissioned in late 1974, the aimbeing to find the minimum backlobe levels achievablewithout detuning any towers. At 1546 kHz, relative tothe main lobe, levels of -23dB -toward Bristol on bearing2630 and -17dB toward other co-channel stations over thearc 315° to 350° (East of True North) were achieved andfound to be acceptable. They were difficult to improveon and appeared to be limited by re-radiation effectsmuch as had been predicted-by the computer study.
At 1151 k-Hz, levels of -23dB were initially ob-tained over the arc 300° to 350°. However, it was cri-tical that a level not higher than -24dB be establishedtowards Birmingham over the arc 3010 to 3160, and byfurther adjustments to the currents on the antenna mastsa null of -35dB was generated toward central Birminghamwith -24dB at the extreme edges of the arc. No towerretuning was necessary. The antenna has been in servicein this condition since March 1975, and regular monitor-ing of critical nulls has revealed that whereas the-35dB null has drifted, particularly after the frequencychange in 1978, the co-channel protection limits arestill being met.
To measure the groundwave patterns in late 1974,field strength was plotted against range on 16 bearingsat ranges typically from 1 to 20 kilometres, involving240 monitoring locations. To obtain valid informationabout the depths of the nulls in the far-field over thearc 2600 to 3500, the measurements had to be made atranges of at least 10 kilometres in order to be tentimes further from the antenna than those pylons thatwere believed to contribute the larger part to the re-radiation. (The investigation of the Birmingham nullwas carried out as far as Birmingham itself, 145 kmaway). Note: Where measured results are plotted inthis paper the spread of uncertainty shown on the bear-ings 130 and 40° ETN reflects the very cursory examina-tions carried out in this angular sector, which has nosignificance to the service area or co-channel protec-tion.
In the intervening years there has been no evi-dence of any addition, removal or relocation of theelectricity pylons, so the IBA's re-radiation experiencemight have rested there, had it not been for the reali-zation in 1978 that the use of general-purpose thin-wirecomputer programs [2] such as that of Richmond [3] mightshed some light on this particular re-radiation problem.Recently other computer programs -have also been used inthe study of power-line re-radiation by Trueman andKubina [4]. In addition, attention should be drawn tothe pioneering work of Alford and French [5].
Computer Model Development
A computer model was developed in order to obtainsome insight into the re-radiation problems experiencedby IBA at the London site. The computations for themodel were carried out using a slightly modified versionof a computer program [3] which performs a frequencydomain analysis of thin-wire antennas and scatterers.The wire structure can be any interconnection of straight:wire segments. Piecewise sinusoidal-expansion and test-ing functions are used in the process of solving numeri-cally the integral equation for the current diistributions.
95
The signal source is a set of ideal voltage generatorsat the bases of the antenna towers and the output dataincludes the current distribution, input impedances,radiation efficiency, total gain, near-zone fields, andradiation contribution to the total gain from the scat-terers (this contribution is called "scattered gain" inthis paper). "Gain" is in dB referenced to a shortmonopole.
The computer model consists of a four-mast arraywith specified base currents, a simplified equivalentpower-line tower model, an overhead ground wire, and anapproximate representation of ground effects due totower footings. All wires are assigned a conductivityof 107 mhos/m.
The antenna array masts are 70.1 m in height andequally spaced by 61.0 m along a line oriented 159.6°east of north. A problem encountered when modellingsuch a structure stems from the fact that the thin-wireprogram accepts only voltages as input data and notcurrents. Therefore it was necessary to convert thespecified base currents into the required voltages bycalculating the admittance matrix Y and using V = Y1 I,where V and I are respectively the base voltage andspecified base current column matrices. Tables 2 and 3show the voltages for the specified design currents andactual array currents, at the two frequencies ofinterest.. In the tables, the specified currents arethe original theoretical currents specified for thearray, and the adjusted array currents are those mea-sured at the mast bases subsequent to both some adjust-ment during commissioning and slight drift after commis-sioning. The adjustments are those already referred toin the introduction.
The power line towers varied in height and shapealong the line. Their heights averaged approximately30 metres and their widths at ground level averagedapproximately 4.5 metres, and so these average dimen-sions were used to define a "typical" tower of the typedepicted in Figure 1; in the computational model, alltowers were taken to be identical and having these"typical" dimensions. The conductors consisted of anoverhead ground wire (skywire) with diameter 1.4 cm andsix power-carrying wires with diameter 1.93 cm (thelatter not included in the computational model). Sincethe computer program allows only one wire thickness andsince towers and skywire are of very different conduc-tor thicknesses, an equivalent tower model using a thinwire with 1 cm radius was developed. A detailed towercomputational model (Figure 2) was first designed sothat it would be similar to the actual tower, and itsre-radiation properties were computed for various strutradii ranging from 0.01 m to 0.2 m, with the expecta-tion that the actual tower would be represented bystrut radii between 0.1 m and 0.2 m. A simplified model(Figure 3) was then postulated and its dimensionsobtained by comparing its scattered gain with the morecomplicated model both with and without a skywire., Theprocedure used to calculate the simplified model dimen-sions was suggested by M.A. Tilston (personal communica-tion) and consists of three steps:
(i) short circuit the top of both models by attach-ing a skywire extending a quarter wavelength on eachside of the tower top. Adjust the leg separation of thenew model so that the scattered gains of both towers arematched.
(ii) open circuit the top of both towers by removingthe skywires and attaching a pair of crossarms at thetop of the new tower model. Adjust the arm length sothat scattered fields are matched.
(iii) for verification repeat step one with the shortcrossarms now included in the simpler model.
Figures 4 through 7 show the scattered gains forthe two tower configurations with and without the sky-
wire. At 1151 kHz it was found that for the case with-out the skywire the scattered field produced by the de-tailed model with a wire radius of 0.2 m was approxi-mately 0.5 dB larger than the one obtained for the sametower with wire radius of 0.1 m, and very close to thegain obtained for the simpler model. At the same fre-quency but for the tower with skywire, the discrepancybetween the two scattered gains produced by the samemodel but different radii was 1.0 dB. Since these aretwo extreme cases (X/4 skywire produces a high currentat the tower top and no skywire produces a low current),one can say that the skywire thickness does not affectthe scattered gains a great deal so that a comparisonbetween different results can safely be made. At 1546kHz a similar behaviour was noted, as follows. For thecase without the skywire the scattered field produced bythe detailed model with a wire radius of 0.2 m was appro-ximately 0.8 dB larger than the one obtained for the sametower with a wire radius of 0.1 m arid 0.25 dB differentfrom the results of the simpler model. For the case withthe skywire the discrepancy between the gains produced bythe detailed model but with different radii was 0.9 dB,and the difference between these and the simpler modelwas 0.8 dB and 0.2 dB respectively. From these analyses,it was concluded that the triangular-frame computationaltower model of Figure 3 is indeed a good choice.
The effect of a perfectly conducting ground may betaken into account by introducing the system's image in-to the thin-wire model. The problem becomes more compli-cated when a distributed ground of finite surface impe-dance is considered. Such a ground could not be simu-lated using the computer program available, so that onlyground effects localized around the tower footings couldbe considered by representing each footing with a lumpedimpedance. Figure 8 shows an approximate representationfor a tower ground connection, where p is the radius ofthe tower footing. Monteath [6] derived an expressionfor the change in input impedance Z'- Z when perfectly-conducting ground is replaced with ground having finiteconductivity, where Z' and Z are respectively the inputimpedance in the finite and infinite conductivity cases.The expression is a function of the tangential magneticfield at ground level and the surface impedance. Sincethe magnetic fields for the towers are unknown an exactsolution cannot be obtained for Z' - Z. So, instead, a"worst case" situation for re-radiation is assumed.This case is obtained if one assumes that the currentdistribution on the tower is the same as that of a thinvertical monopole antenna one quarter of a wavelengthhigh and therefore having a current maximum-at groundlevel. The surface impedance n is a function of theradius p and is given by
r = 0 for p < pO= n for p > p0
where T 1is a constant (see Figure 8).
For the quarter-wavelength antenna and the abovesurface impedance the change in input impedance is
Z'- Z = 472 Fe Ei{-2j60(r -)}
2j, PI_+ e Ei{-2j60(r0
where Z = X/4 is the antenna height.
The values of Z' - Z were calculated for= 11.5 x 10 3 mhos/m, C = 20, p = 0.144 m. The re-
r lsults obtained for each tower leg were: for 1151 kH{z,Z' - Z = 20.7 + j 13.4, and for 1546 kHz, Z'- Z = 23.8+ j 14.6. The impedance for the tower was then calcu-lated by considering the impedances of the four legs as
96
being in parallel. The values obtained were 5.17 + j3.34 and 5.95 + j 3.66 at 1151 kHz and 1546 kHz respec-tively.
The clay pigeon tower is the isolated steel towersituated directly in the main beam. Due to its positionso close to the antenna it is strongly illuminated andtherefore it has quite large induced currents. Thistower is 24.4 m in height and 6 to 7 m wide. Four com-putational models were considered, the first and secondhaving configurations shown in Figure 3, with heightsof 30.0 and 24.4 m respectively. The third and fourthmodels are depicted in Figure 9 with 6 and 7 m widthsrespectively. Table 4 shows the total base currentsinduced in each of the tower models. From this tableone can see that, for the latter three lower models thecurrent has approximately the same magnitude, and thisvalue is lower, by as much as 30% at 1151 kHz and 40%at 1546 kHz, than that for the first model. However,the actual clay-pigeon tower had three fairly extensivemetal shooting platforms near the top, the effect ofwhich would be to raise the effective height, so it wasjudged that model # 1 would be the best representationto use.
The relevant part of the power line was taken toextend over a region with a 1 km radius. Figure 10shows the cowers' positions with respect to the antennaarray for a radius of 2.5 km. The route PFB is locatedmostly on the main lobe and therefore all of it wasincluded in the model. The route PMD, although locatedacross the back lobe, was also considered in part by in-cluding PMD 26 to PMD 29, plus PF 70; the clay pigeontower is also shown. The computational model of thepower line extended for about 3600 meters in totallength.
Calculations Using the Specified Antenna Array Currents
The first set of calculations was done for theantenna array with the originally specified base cur-rent distribution. All towers were modelled similarlyand no footing impedances were included. The power linetower base current magnitudes shown in Figures 11 and 12were observed to be highest at the towers PFB 4 to 6 at1151 kHz and PEB 3 to 6 at 1546 kHz. The lowest cur-rents at 1151 kHz occur from PMD 26 to PEB 3 and PFEB 7to PFB 9. The lowest currents at 1546 kHz occur from PMD26 to PFB 2 and PEB 7 to PFB 9. These current distri-butions appear reasonable, to judge from Figures 13 and14 in which the total and scattered gain patterns havebeen plotted with the power line and clay pigeon towerlocations superimposed. From the graphs one can seethat the low currents were obtained for those towerswhich are either in the back-lobe direction or locatedfarthest from the antenna array. The computed totalgains are also plotted in rectangular coordinates inFigures 15 and 16, which include the effects due to afinite ground conductivity as represented by the valuesof footing impedance already discussed. With footingimpedances included, a small decrease in the maximumcurrents was observed at 1151 kHz (Figure 11), but noappreciable changes in the currents were observed at1546 kHz (Figure 12). As for the ground effects ontotal gain, it was noted that at 1151 kHz the minor lobepeaks were changed by less than ½ dB (Figure 15) whileat 1546 kHz no effect was noticeable. These resultsshow that the tower footing impedances have littleeffect on the system's total gain, so that, for thecalculations to follow, the computer model did not in-clude footing impedances.
Frequency Sensitivity
The second set of calculations was done for thearray with the specified base currents but for frequen-cies of 1151 + 5 and + 10 kHz, and 1546 + 5 and + 10 kHz.The scattered gains were calculated and compared with
97
the results for 1151 kHz and 1546 kHz. It was foundthat the scattered field magnitude did not change sig-nificantly but all the scattered lobes were shifted byangles of about 100. The total gains were also computedand the results for the + 10 kHz cases are shown inFigures 17 and 18. These results show changes of theorder of 2 dB in minor lobe levels, changes which arelarge enough to indicate the importance of multi-fre-quency computations in the analysis of re-radiationproblems.
Calculations Using the Adjusted Antenna ArrayCurrents and Comparison with Measured Data
The adjustment of the antenna array base currentsduring commissioning has already been described, thepurpose of the adjustment being to satisfy the protec-tion requirements without resorting to detuning of thepower line. After the adjustment, there followed ashort period of time in which some drift in the systemphasing occurred. Then, the array base currents wererecorded and a set of far-field measurements taken byIBA. These currents, comprising the "adjusted" setalready described, were used in the calculation of theradiation patterns shown in Figures 19 and 20. Thesame figures also show the measured data. Agreement atboth frequencies between theory and experiment is seento be excellent over the main lobe and fairly good inthe minor lobes. It is clear from the calculations andmeasurements that a small angular shift in the minorlobes would result in the protection levels beingexceeded.
Conclusions
The agreement between theory and experiment is evi-dence that moment-method thin-wire computational methodsare useful in analyzing the re-radiation from power linesand lattice towers adjacent to directional MF antennaarrays.
The frequency sensitivity of the re-radiated fieldhas been analyzed, showing that in this case there is aquite noticeable variation over + 10 kHz. This esta-blishes the importance of checking frequency sensitivityin all re-radiation calculations.
Ground effects which are localized near the towerbases have been estimated and included in the calcula-tions as equivalent base impedances. In this case,ground effects proved to have a negligible effect on there-radiated field, but this conclusion does not neces-sarily apply to other situations where resonances inpower line cells might be significantly reduced in am-plitude by ground effects.
This work demonstrates a logical method for build-ing up an electromagnetic computational model for a
power line. The model employs a minimum of struts to
represent accurately a complex lattice tower, therebyminimizing the costs of performing the re-radiation com-
putation.
The practical example studied is a "borderlinecase" in which power line re-radiation is strong enough(in relation to protection requirements) to necessitatecurative measures in the form of antenna array adjust-ments, but it is not so strong as to require the attach-ment of detuning devices to the power line. Thereforethis description of such a borderline case could beuseful to others who must decide how to classify andsolve existing or potential re-radiation problems oftheir own.
Acknowledgments
Acknowledgments are extended to the ConsultingEngineers - Cohen & Dippell, P.C., Washington DC, USA;and to the Antenna Contractor - formerly the Antenna
Division of EMI Sound & Vision Equipment Ltd.,-Hayes,England; now trading as Alan Dick & Company Ltd.,Cheltenham, England.
The advice of M.A. Tilston is gratefullyacknowledged.
This paper was contributed by permission of theDirector of Engineering, Independent BroadcastingAuthority, UK.
Support wAs provided by the Natural Sciences andEngineering Research Council of Canada, under GrantsG-0362 and A-4140.
References
[1] E.T. Ford, "A Dual-Frequency Highly-Directional MFAerial", IEE Conference Publication No.145, pp.215-218, Proceedings of the International Broad-casting Convention, London, England, 1976.
[3] J.H. Richmond, "Radiation and Scattering by Thin-Wire Structures in the Complex Frequency Domain",NASA report CR-2396, May 1974, and "ComputerProgram for Thin-Wire Structures in a HomogeneousConducting Medium", NASA report CR-2399, June 1974
[4] C.W. Trueman and S.J. Kubina, "Numerical computa-tion of the Re-Radiation from Power Lines at MFFrequencies", IEEE Transactions on Broadcasting,Vol. BC-27, No.2, pp.39-45, June 1981.
[5] A. Alford and E. French, "Some Observations Con-cerning the Re-Radiation of Radio Frequency Energyfrom Power Line Transmission Towers", Report dated6 Aug. 1966, prepared by Andrew Alford ConsultingEngineers, P.O. Box 2116, Woburn, Mass. 01888.
[6] G.D. Monteath, "Applications of the ElectromagneticReciprocity Principle". Pergamon Press, New York,1973.
[2] K.G. Balmain and J.S. Belrose, "AM Broadcast Re-Radiation from Buildings and Power Lines", IEEConference Publication No.169, pp.268-272, Pro-ceedings of the International Conference onAntennas and Propagation, London, England, 28-30Nov. 1978.
Table 1 - Antenna Parameters
SPECIFICATION* 1151 kHz 1546 kHz
ERP _ Direction ERP Direction
Groundwave ERP to central London +14 dBkW 1600° ETN +20 dBkW 1600 ETNservice area Minimum ERP at + 800 0 dBkW 80' to 240' +5 dBkW 80° to 2400
Co-channel Maximum ERP -10 dBkW 3010 to 3160 0 dBkW 261' to 265'protection Maximum ERP -5 dBkW 3240 to 348' +5 dBkW 315' to 348'
* An ERP of 0 dBkW represents a field strength of 300 mV/m at 1 km from the antenna,under lossless propagation conditions.
Table 2 - Antenna Array Base Currents andEquivalent Base Generator Voltages at 1151 kHz
(Currents are relative, with respect to one mastcurrent having an arbitrary reference value)
Fig.4 Scattered gain at 1151 kHz for the detailed andsimplified tower models without skywire.
a
C
m00
0C/)
-51
-53
-55 I-0
m 4.5 m
Fig.2 Detailed computational modelof power line tower.
180
Azimuthal angle 45(degrees)
Fig. 5 Scattered gain at 1151 kHz for the detailed andsimplified tower models with skywire.
99
Detailed model: wire radii 0.01 m, 0.1 m, 0.2 mSimplified model: wire radius 0.01 m
radiusL=O0.01 m /radius = 0.2 m
\radius=0.1 m
radius = 0.01 ml l l l l l l
30.0 m
360
- Detailed model: wire radii 0.01 m, 0.1 m, 0.2 mSimplified model: wire radius 0.01 m
radius = 0.2 m
,,radius= 0.1 m
radius = 0.01 m
,,radius=0.01 m
360I I i I
Detailed model: wire radii 0.01 m, 0.1 m, 0.2 m.----S Simplified model: wire radius 0.01 m
radius=0.01 m
radius= 0.2 m
1radius=0.01 m *radius=0.1 m
180
Azimuthal angle 4(degrees)
Mast
Earth
Fig.6 Scattered gain at 1546 kHz for the detailed andsimplif ied tower models without skywire.
Detailed model: wire radii 0.01 m, 0.1 m, 0.2 mSimplified model: wire radius0.01 m_
eraclius=0.2m- ~~~~~~~radius=0.1 m
- 5 ~~radius = 0.01 m
xradius-0.01 m
180Azimuthal angle 0(degrees)
Fig.8 Power line tower ground connection:this is the situation analyzed todetermine lumped base impedance.
24.4,m
360
Fig.7 Scattered gain at 1546 kHz for the detailed and
simplif ied tower models with skywire.
4I11
K- A6or7 m6or7m
Fig.9 Third and fourth clay pigeon towermodels (8 segments).
NORTH
RoutePMD _22
,//.2 8~~~~
2564
3
Claypigeontowero
RoutePF 65
067
4 ~~~~~~~1000m
\ \ ~~Route\3 PFB
-->159.60 *K Four-mast
?xx antenna1 \array
TowardCentral London
Fig.10 Layout of power line towersand clay pigeon tower relativeto antenna array. Solid dotsindicate towers included incomputatio.nal model.
EAST
5 *,~ * \
67
9ELine terminatesin substation
100
m -51
C
@ -'53c0CD
0Cl,
-55 1C
_ -51
._v*0CJ
.g -53a)
co0C/o
-550
360
_ _
4
2
0 1 2 3
Distance along line (km)
Fig.ll Power line tower current magnitudes at 1151 kHzfor one of the legs vs. tower position alongthe line. Z is the footing impedance estimatedat each of the tower legs.
NORTH0
E0
-o
cm
E
-10
0
41
2
I I I I I I I
- Route PFB -
RoutePMD'4 5 Z0O
Route PF 6
27 89
26 28 29701 Z=23.8+ 1l4.6I I I I I I
1 2 3Distance along line (km)
4
Fig.12 Power line tower current magnitudes at 1546 kHzfor one of the legs vs. tower position along theline. Z is the footing impedance estimated at
each of the tower legs.
Gain (dB)-20 -10 0 +10
270
180
Fig.13 Total and scattered gains in dB, at 1151 kHz and using specif ied
antenna base currents, for the system without footing impedances.Power line towers and clay pigeon tower are superimposed.
101
E0
0IEc0L.
0
NORTH0
+10
270
180
Fig.14 Total and scattered gains in dB, at 1546 kHz and using specifiedantenna base currents, for the system without footing impedances.Power line towers and clay pigeon tower are superimposed.
mI-I
0
0 120 240 360Azimuthal angle 4(degrees)
Fig.15 Total gain computed at 1151 kHz, neglectingfooting impedances and using specified antennabase currents. Dots indicate influence ofincluding footing impedances on height ofminor-lobe maxima and minima.
-
m
ca
0 120 240 360
Azimuthal angle 4(degrees)Fig.16 Total gain computed at 1546 kHz, neglecting
footing impedances and using specif ied antennabase currents. The introduction of footingimpedances had a negligible effect at thisfrequency.
102
Ca
360
Azimuthal angle 15(degrees)
Fig.17 Total gain computed at 1151 ± 10 kHz,neglecting footing impedances and usingspecified antenna base currents.
0 120 240Azimuthal angle 4(degrees)
360
Fig.18 Total gain computed at 1546 + 10 kHz,neglecting footing impedances and usingspecified antenna base currents.
m.-I
.-_C)
0 120 240
Azimuthal angle (degrees)
oF
-10m
-20
-301-
360 0
-I
_ .4 1546 kHz
Co-channelprotection
levels
0
Calculated0 e Measured
I I
120 240 360
Azimuthal angle 4(degrees)
Fig.19 Total gain at 1151 kHz as calculated usingadjusted antenna base currents (solid line)and as measured (points). Co-channelprotection levels are indicated.
Fig. 20 Total gain at 1546 kHz as calculated usingadjusted antenna base currents (solid line)and as measured (points). Co-channelprotection levels are indicated.
