AN INVESTIGATION OF POSSIBLE INTERFERENCE TO TELEVISION IN BANDS I[ AND ~ FROM HIGH POWER RADAR INSTALLATIONS

Transcription

1 RESEARCH DEPARTMENT AN INVESTIGATION OF POSSIBLE INTERFERENCE TO TELEVISION IN BANDS I[ AND ~ FROM HIGH POWER RADAR INSTALLATIONS Report No. K-15 ( 1961/9) C.P. Bell, B.Sc.(Eng.), Grad.I.E.E. (R.D.A. Maurice)

2 » This Report Is the property ot the British Broadcasting Corporation and ma1 not be reproduced in any form without the written permission ot the Corporation.

3 Report No. K-l5 AN INVESTIGATION OF POSSIBLE INTERFERENCE TO TELEVISION IN BANDS IV AND V FROM HIGH POWER RADAR INSTALLATIONS Section Title Page SUMMARY 1 1 INTRODUCTION 1 2 TRANSMITTER CHARACTERISTICS 1 3 MEASURING EQUIPMENT 2 4 NATURE OF THE INTERFERENCE 3 5 SUBJECTIVE ASSESSMENT General, and Co-channel Interference Adjacent-Channel Interference Intermediate Frequency Bandpass Response of 625-1ine Receivers. 6 6 POWER SPECTRUM OF THE RADAR TRANSMISSION 7 7 FIELD STRENGTH/DISTANCE CURVES It- 8 8 POLARIZATION DISCRIMINATION 9 9 CONCLUSIONS 1 1 ACKNOWLEDGEMENT REFERENCES 12

4 June /9 ) AN INVESTIGATION OF POSSIBLE INTERFERENCE TO TELEVISION IN BANDS IV AND V FROM HIGH POWER RADAR INSTALLATIONS SUMMARY This report describes an investigation carried out to evaluate the extent to which 5 cm air~surveillance radar transmitters may be expected to interfere with television transmissions in the u.h.f. bands. Subjective assessments of the interference were made, together with measurements of the radar peak field strength at points along radials from both a high~power and a medium~power transmitter. It is concluded that l, although interference may occur on adjacent and image channel frequencies, it will, in general, be limited to regions where the radar transmitter is sited within the boundary of the affected service area. EOCceptional cases of high interference levels at considerable range may occur where either the effective transmitter or receiver aerial heights are great" The recommendation is therefore made that, wherever possible, the highest television channel in Band IV (574~582 Mc/s) and the lowest channel in Band V (6&-614 Mc/s) should not be employed where a 5 cm radar transmitter lies within the service area. 1. INTRODUCTION The proposed extension of television broadcasting into Bands IV and V introduces problems of interference from radar transmitters operating in the interband channels. At the I.T.U. Conference in Geneva in the use of the radio spectrum between 582 Mc/s and 6S Mc/s was allocated to 5 cm radar transmission. Since television broadcasting in this frequency range is not envisaged in the U.K., the only relevant types of interference to be expected are on adjacent and image-frequency channels. The proposed Band IV/V channel width is 8 Mc/s but, in the absence of any suitable commercially available British 625-line receivers, subjective tests were of necessity carried out with a 45-line receiver. This enabled observations in the vicinity of an existing radar site to be made using a Band I transmission as the source of the wanted signal. Since, however, the extent of adjacent~channel interference is directly related to the receiver bandpass response, a degree of uncertainty will exist in the results obtained and this will depend upon the standards adopted by the manufacturers in the development of Band IV/V receivers. 2. TRANSMITTER CHARACTERISTICS The two latest types of 5 cm airport radar developed by Marconi's Wireless Telegraph Co. Ltd. are known as the S264 and S264A. Transmitter characteristics are as follows:

5 2 1 Type S264 Peak transmitter power: 5 to 6 kw Pulse length: 2 to 4 flsec Pulse repetition frequency (p.r.f.): 5 to 8 pulses per second Type S264A Peak transmitter power: 5 kw Pulse length: 2 to 4 flsec Pulse repetition frequency (p.r.f.): 26 to 55 pulses per second Alternative aerials may be used with either transmitter. These are known as "normal" or "high cover" types, with "free space" forward gains of 31 db and 3J db respectively. The actual forward gain depends upon: (a) The angle of elevation, which is variable, but usually set between +4 and +6. (b) (c) The height of the aerial above the ground, which will vary from site to site but will not be less than approximately 12 ft (3 6 m). Ground reflection effects, Which will depend upon the nature of the site chosen. The horizontal beamwidth of the aerial is approximately 2 between the -3 db points, with side lobes not greater than -23 db with respect to the main lobe amplitude. The aerial has a speed of rotation of 1 r.p.m., which may be reduced to 5 r.p.m. in high winds. Observations were made in the field in the vicinity of existing radar installations at London Airport (type S264) and at Rivenhall, Essex (type S264A). 3. MEASURING EQUIPMENT Measurements of the peak field strength of the radar transmission were obtained by the use of a receiver with a tunable r.f. head, with the video output displayed on a cathode-ray tube. The aerial used was a "bowtie" dipole in front of a corner reflector, this type being chosen on account of its wide bandwidth. The aerial itself having been calibrated, the voltage input to the receiver was measured by substitution of a pulsed signal from a Marconi TF 16 u.h.f. signal generator. For subjective tests a Bush Model 54 television receiver was modified to enable a signal from a German N.S.F. Band IV/V tuner to be injected into the first i.f. stage, thus permitting a u.h.f. signal to be superimposed on a Band I 45-line picture. The subjective effect of the interference depends upon the relative amplitudes of interfering and wanted signals existing in the video output circuit of the television receiver. Since the voltage gains of the Band I and Band IV/V tuners are not equal it was necessary to measure the overall receiver gains at both Band I and u.h.f. frequencies. This permitted a correction factor to be applied to refer known Band I and u.h.f. input signals to their relative levels in the video output stage.

6 3 4. NA TURE OF THE INTERFERENCE With the head tuned to the radar frequency the interference takes the form of white bars on the screen, followed by a recovery period, the length of the total disturbance in the horizontal direction being approximately 5% to 1% of picture width for a 45-line picture@ These bars are arranged in a regular pattern over the screen, the form of the pattern depending upon the pulse repetition frequency (p.r.f.l, which in general is not an exact multiple of the field frequency. This causes the pattern to be displaced in successive fields and therefore to appear to drift across the screen. Interference on the sound channel consists of a rasping note at the p.r.f., but in practice this is not significantly disturbing until the level of the interfering signal increases to a value which is approximately 1 db greater than that giving "perceptible" interference on vision@ At high interference levels, bars are visible on the screen throughout the whole period of radar aerial scan but, due to the great aerial directivity in the horizontal plane, interference is present only periodically, when the aerial beam sweeps through the receiving site, at levels 3 db below that required to give continuous interference@ Subjective assessment was made with respect to signal levels required to give "perceptible" and "slightly disturbing;' interference from the main lobe. At "perceptible" levels, the duration of the interference is given by the time taken for the main lobe to sweep through the receiving site@ For a beamwidth of 2 and a speed of rotation of 1 r.p.m. this period is 1/3 sec, i e@ less than the period of one complete picture. Thus the time limitation is set primarily by the image retentivity of the eye. Also, for this duration of time, the number of pulses visible for the maximum p.r.f. of the airport radar type S264A is 1 x 55/3 or 19 pulses. 5. SUBJECTIVE ASSESSMENT 5.1. General, and C~channel Interference A combination of field and laboratory tests was undertaken using, for the latter, a pulsed~signal generator as the interfering source. The nature of the interfering transmission was such that the interfering field strengths were frequently considerably greater than the wanted field strength. Under these conditions interference effects due to intermodulation can occur. The effect of such intermodulation may be neglected for interfering signals at the same frequency as the wanted signal, or at its image frequency, since interference will already be disturbing at levels much below those required to produce intermodulation in the receiver. Measurements were made of input signals at Band I and BandV aerial terminals at each place visited in the field tests, Band I values were obtained by the use of a standard field~strength measuring receiver, and the Band V values by means of the tunable receiver and cathode~ray tube display previously described. The overall gains of the television receiver having been measured on both bands, the relative levels of the two signals could be adjusted so as to refer them to a common input. In the laboratory tests, known input voltages were fed to the television receiver from Band I and Band V signal generators.

7 p 4 The procedure was to attenuate the wanted signal in order that it might be representative of the weakest field likely to require protection. Assuming servicearea contour limits of 3 mv/m for Bands IV/V,2 a typical aerial gain of 1 db and feeder loss of 3 db, we find that at 6 Mc/s 1 mv represents the appropriate value of peak-white open-circuit receiver input voltage, and this was therefore chosen as the standard of wanted signal. Table 1 summarizes assessments made by this method: TABLE Ratio of Interfering to Wanted Signal in decibels o Assessment Disturbing interference on both static and moving pictures. Disturbing interference on test card; slightly disturbing on moving picture with a high degree of contrast. Perceptible, but not disturbing on test card; imperceptible on moving picture except in densely black areas. Imperceptible. The above assessments were made with continuous interference from the pulsed-signal generator. Comparison between the continuous interference and that from the radar transmission indicated that the transient nature of the latter, due to the aerial rotation mentioned previously, permitted the level of unwanted signal to be about 5 db greater than that indicated by the above figures. Thus, under typical viewing conditions, interference would become slightly disturbing when the level of the interfering pulses was approximately 5 db to 1 db lower than that of the wanted picture signal. No significant change was made to the above assessments when the interfering pulse length was increased from 3 psec to 4! psec; this latter length on a 45-line scan being considered as equivalent to a 3 psec pulse on a 625-line scan Adjacent-channel Interference Primarily, this is due to the frequency of the superheterodyned interfering signal falling within the "skirts" of the receiver intermediate frequency (id.) response. There is, however, a possibility of intermodulation due to overloading of the r. f. stage by interference having a frequency which after heterodyning falls outside the limits of the i.f. bandwidth. To investigate this a test was carried out as shown in the schematic diagram of Fig. 1. For various levels of wanted signal input, the unwanted signal was tuned across the receiver bandpass to determine the frequency limits at which interference became "perceptible". '!his was repeated for various other known unwanted signal amplitudes. Available signal generator output powers restricted the maximum ratio of unwanted/wanted signal to +6 db.

8 5 6 Mc/_ SIGNAL GENERATOR CRYSTAL MODULATOR N.S.F. TUNER IF. 45-LINE TELEVISION RECEIVER o VIDEO 6dB 6 Mc/s SIGNAL GENERATOR PULSE UNIT A.F. OSCILLATOR Fig. I - Schematic diagram showing method of measuring adjacent-channel interference Values obtained are shown in Table 2: TABLE 2 Wanted Ratio of interfering signal to wanted signal in Bandwidth in megacycles per second over which input decibels for "just interference exists for the quoted ratio of voltage perceptible" inter- unwanted/wanted signal inputs (peak ference when tuned to white) the mid-band frequency OdB +1 db +2 db +3 db +4 db +5 db +6 db 7 flv -1 db mv -15 db ("disturbing" at -1 db) 2 mv -15 db mv -2 db The bandwidths tabulated above are quoted to the nearest megacycle per second. Fig. 2 (abscissa and ordinate scales "A") is obtained by plotting the frequency separation, in megacycles per second, to a base of the ratio of unwanted to wanted signal, interpolating, where required, within the limits of measuring accuracy to obtain a series of smooth curves. The ordinate scale, expressed in terms of frequency separation required between wanted and unwanted transmissions for the interference to be "just perceptible" represents half of the bandwidth over which interference occurs. The assumption here made is that an interfering signal separated by a certain frequency from the upper limit of the wanted television channel causes equal subjective interference to that produced by a signal spaced an equal frequency from the lower limit of the wanted channel. This assumption may not be entirely valid for small frequency separations, but should not be significantly in error for larger frequency separations. All the curves of Fig. 2 have a maximum rate of change of slope occurring at approximately the same absolute level of interfering signal rather than at anyone

9 6 I I-- CURVES DRAWN FOR QUOTED VALUES OF WANTED SIGNAL OPEN-Cl RCU IT VOLTAGE AT 17 RECEIVER INPUT I1 I 'I 2mV 5mViI / VI / I '1/ / /1 / ~oo"v I I1 V/;I V /./ ~ ~ ~ ----::: ~ V ~ ~ ~ ~ :-: ;;:.;... I I I RATIO UNWANTED/WANTED SIGNAL, db se ALE A I I I! I I RATIO UNWANTED/WANTED SIGNAL, db (WITH 5dB ALLOWANCE FOR TRANSIENT OCCURRENCE) sea L E 8 Fig. 2-Frequency separation between wanted and unwanted signals for "just perceptible" interference particular ratio of unwanted to wanted signal. This may be assumed to be due to the onset of intermodulation effects at signal levels above which limiting and non-linear amplification occurs in the first stage of the receiver. The abscissa scale "B" of Fig. 2 represents operational conditions with a rotating radar aerial producing periodic interference. The discontinuous nature of the interfering signal permits its relative level to be 5 db greater than that measured in laboratory tests, and the abscissa scale is accordingly transposed by 5 db to allow for the reduced degree of subjective interference Intermediate Frequency Bandpass Response of 625-line Receivers Li ttle information is at present available as to the bandpass characteristics of 625-line commercial television receivers likely to be used at u.h.f. in the U.K.

10 7 However, extrapolation of the protection ratio curves published by the C.C.I.R. Meeting of Experts at Cannes (1961)3 indicates that the results plotted in Fig. 2 may be related to 625-line receivers at points on the "skirts" of the bandpass response by the addition of a constant factor of about 4 Mc/s to the bandwidth. This allowance is valid only for interfering frequencies not within the limits of the wanted television channel, but, as previously mentioned, no mutual sharing of television and radar channels is envisaged in the U.K. The ordinate scale of Fig. 2, modified to refer to 625-line receivers, is shown as scale "B", whose origin is transposed by 2 Mc/s with respect to that of scale "A". If the wanted signal field strength, and the ratio of unwanted to wanted signal be both known, then Fig. 2 may be used to determine whether interference will occur at any specified frequency separation. For an interfering signal frequency at the edge of the radar band, interference will occur in the adjacent television channel when the ordinate exceeds 4 Mc/s (for a 625-line receiver). Interference will occur in the second channel from the edge of the radar band when the ordinate exceeds 12 Mc/s. 6. POWER SPECTRUM OF THE RADAR TRANSMISSION Using a receiver of nominal 1 Mc/s i.f. bandwidth, the amplitudes of the radar field strength relative to the centre frequency were measured in 1 Mc/s increments for both the type S264 transmitter at London Airport and the type S264A at Rivenhall. The responses obtained are tabulated below, together with a reference response obtained from a signal generator input, pulsed by means of a Solartron pulse generator type SPS1. Table 3 also shows spectrum measurements obtained by Marconi's Wireless Telegraph Co. Ltd. using a Polyrad spectrum analyser. It is thought that the measurements refer to the actual transmitter output power rather than that radiated from the aerial. TABLE 3 Spectral bandwidth between points of quoted relative amplitude Relative Pulsed signal London Rivenhall Type S264A spectrum as ampli tude generator input Airport type S264A measured by Marconi's (reference) db type S264 Wireless Telegraph Co. Ltd. Mc/s Mc/s Mc/s Mc/s ~ 2ft ~ 2~ 4ft 5! ~ -4 4! 4! 6 Not measured Not measured Not measured Not measured

11 8 The response of the type S264 transmitter is in close agreement with the reference response, being, in fact, appreciably narrower in the range ~25 dbto -35 db, whereas that of the type S264A is greater over the range -25 db to -45 db. It is considered that the slope of the spectral response will be significantly steeper than that of the "skirts" of a commercial television receiver Lf. response, thus permitting interference effects to be referred solely to the centre frequency. The radar transmission bandwidth will, however, be a dominant factor in the planning of radar transmission frequencies within their allotted band. 7. FIELD STRENGTH/DISTAt'fCE CURVES Measurements at 3 ft (9 1 m) above ground level were taken at various sites along one radial from both London Airport and Rivenhall. Although insufficient sites were measured to permit a statistical appraisal of the field strength distribution, in general the places chosen varied between those typical of local terrain and those favourable to reception, ~~th no instances where the path to the transmitter was obscured by large buildings near the receiving site. Comparison of the plots (Figs. 3 and 4) indicates the great importance of local terrain features with such low effective transmitting aerial heights. The/best fit" curve for the Rivenhall measurements lies below that for London Airport, although the transmitter e.r.p. is 1 db greater. Also shown are typical path profiles along the radials measured. Both paths, and in particular that in the vicinity of London Airport (Fig. 3), are relatively flat for the first 1 miles (16 km) from the transmitter. On the radial shown from London Airport, there are no irregularities greater than ± 1 ft (3 m) over the first 5 miles (8 km) of path. Between 6 and 8 miles (1 and 13 km), ~here the profile crosses the Thames, a fall of 4 to 5 ft (12 to 15 m) in height occurs, and this is shown by a reduction of 1 to 15 db in field strength at points measured in this region; similarly between 1 and 12 miles (16 and 19 km) (Tolworth to Ewell) a ridge rises to 1 ft (3 m) between Surbiton and Long Ditton. This is approximately 5 ft (15 m) higher than the lowest point measured and represents a reduction of about 1 db in field strength. Conversely, between 12 and 16 miles (19 and 26 km) the profile rises up the north face of the North Downs, exceeding 55 ft (168 km) above mean sea level between Banstead and Burgh Heath. This is reflected in field strength values which increase steadily over this region through a range of 2J db. The points shown on Fig. 3 for distances greater than 16! miles (26! km) were measured on a different radial, through Croydon and West Wickham. There is a sharp discontinuity in the mean ordinate values of the plotted points at 16! miles (m! km) due to the reduction of the receiving site height,. Comparing the above results with those obtained from the Rivenhall measurements (Fig. 4), which were made over ground with undulations of the order of 5 ft (15 m), it will be seen that although there is a wider scatter between successive points along the Rivenhall radial, it is easier to construct a "best fit" curve through the points. On the radial measured, optical paths from the transmitter extend as far as Brentwood, and this distance corresponds to the furthest point at which a measurable signal was obtained, the limit of receiver sensitivity being equivalent to a field strength of 1 mv/m.

12 9 14 2!:? I 1 uj > >= -<..J uj ~o '" 1 CO f\. II "... FREE SPACE FIELD, I r- r- _ - If RECEIVING AERIAL HEIGHT- 3ft (9-lm) \.. '\ ""'- K ~ ~ 7 6 o ~ 3OOr TYPICAL PATH PROFILE ALONG MEASURED RADIAL ~ l X Cl iij X DISTANCE IN MILES Fig. 3 - Field strength/distance curve for type S26~ transmitter at London Airport 8. IDLARIZATION DISCRIMINATION At about twenty sites, which were chosen as being typical of both urban and rural locations, relative field strengths were compared with the receiving aerial horizontally and vertically polarized. With one exception, the vertically polarized field lay within the limits of 1 db to 21 db below that of the horizontally polarized component, the median difference being -17 db. Since, however, the main-to-side lobe ratio of the transmitter aerial horizontal radiation pattern was very much smaller for vertical polarization, often being only of the order of a few decibels, the subjective discrimination is not so good as the figures quoted, since any interference is present for a greater fraction of the time.

13 = r- ~ --SPACE "FREE FIELD 1-- E ~ g '" ~ t e.j '" " 1 co r.~ "... Cl ~ 9 1;; " o.j '" u:: 8 7 \ \ RECEIVING I~o "\ ~ o~ ~ AERIAL HEIGHT, 3f~ (9-1 m) '" p ~ " 6 o DISTANCE IN MILES t- TYPICAL Plll.TH PROFILE ::l ALONG MEASURED RADIAL ~2F>~-;~~~=-~ ~ 1 ~ENTWOOD I go ~~~ 25 r. DISTANCE IN MILES ----==:::::. 3 Fig. ~-Field strength/distance curve for type S26~A transmitter at Rivenhall, Essex 9. OONCLUSIONS The form of the curves in Fig. 2 indicates that the extent of the band affected by adjacent-channel interference depends largely upon the absolute value of the interfering signal, rather than upon the relative level of wanted and unwanted signals. This is a consequence of intermodulation effects being produced by overloading of the receiver input stages. For example, an interfering signal of +4 db with respect to 5 mv wanted signal causes interference over the same frequency range as +55 db with respect to a 1 mv wanted signal. Thus, in those marginal cases of interference for which both wauted and unwanted signal levels are high, a significant improvement may be effected by means of attenuation in the aerial lead. This method

14 11 of improvement is obviously not available to viewers on the fringe of the service area" Consider now the conditions at the service area limit, i.e, a field strength of 3 my/m giving 1 mv open circuit voltage at the receiver input. Fig. 2 (using ordinate and abscissa scales "B ) shows that interference would occur from a transmitter at the edge of the wanted channel, Le,. a frequency separation of 4 Mc/s for a 625-line system, when its relative field strength exceeds +14 db, or 15 my/m. Interference to the next adjacent channel (where the frequency separation is 12 Mc/s to the channel centre frequency), will occur at a relative level of +58 db, 1. e. a field strength of 2~ V/m, For a transmitter type S264A operating in typical flat terrain, e.g. Rivenhall, the distances corresponding to those field strengths may be obtained by reference to Fig. 4, and are, respectively, 12 to 16 miles (19 to 26 km) and 3 miles 4"8 km). If, however, the radar transmitter frequency be, say, 3 Mc/s from the edge of the broadcasting band these distances become 4 to 5 miles (6 to 8 km) and 1~ miles (2'4 km. Image--channel interference effects will depend upon the image suppression characteristic of the receiver, which may be of the order of 4 to 45 db. Since co-channel tests indicate interference as "perceptible" when the wanted signal is +1 db with respect to the interfering signal, a wanted field equal to 3 my/m will therefore suffer interference when the unwanted image-frequency field equals 1 to 2 mv /m.. repres en ting distances of 5 to 1 miles (8 to 16 km) for a type S264A transmi tter operating in flat terrain. In general, the terrain within a 1~ mile (2"4 km) radius of the transmitter will be flat and the above condition will apply. Also there are likely to be comparatively few viewers within this range if the radar installation is situated at the centre of the airfield. Thus the problem of interference in the second broadcast channel from the edge of the radar band is not considered serious, particularly if it can be arranged for the wanted transmission to be vertically polarized. However, a number of major airports, e.g. London and Gatwick, are situated in areas surrounded at a distance by fairly densely populated ranges of hills such as the Chilterns, and the North and South Downs. For example, a type S264A transmitter at London Airport operating 3 Mc/s from the edge of a local 8 Mc/s wide television channel would cause interference at Banstead to a wanted field of 5 my/m (assuming attenuation in the aerial lead to give not greater than 5 mv wanted sj,;;nal input to receiver). Under these conditions of great effective receiving aerial height, the interfering field may approach to within a few decibels of the "free space value and interference effects can be severe in channels immediately adjacent to the radar transmission band. It is therefore suggested that the two channels immediately adjacent to the radar band should not be used in regions where a 5 cm radar installation lies within the proposed service area. Similarly, television channels should not be allocated to regions where the image frequency is utilized by a radar transmission within the service area. The comments above have assumed all radar transmitters to be on or in the immediate vicinity of their associated airports, and thus in areas of flat low-lying terrain. This situation may, however, not always occur. For example, it is

15 12 believed that a type S264A transmitter may be located on a high site above Ventnor, Isle of Wight. This transmitter would have an effective aerial height on the seaward side of approximately 75 ft (223 m) thus producing "free space" fields for distances up to 35 to 4 miles (56 to 64 km) along the Sussex coast. Since the reciprocal problem of interference on radar displays due to adjacent-channel television transmitters may also be serious, it is obviously to the mutual advantage of the users of both bands that close co-operation between the relevant planning authorities should occur. 1. ACKNOWLEDGEMENT The assistance of Marconi's Wireless Telegraph Co. Ltd. is gratefully acknowledged, with regard both to the field strength measurements made at Rivenhall, Essex, and to the information supplied concerning transmitter characteristics of the type S264 and S264A installations. 11. REFERENCES 1. I.T.U. Radio Regulations, Geneva 1959, Ch. 11, Art Swann, G.F., "Field Strengths required for the Reception of Television in Bands I, Ill, IV and V", Proc. LE.E., Vol. 16, Part B, No. ro, November C.C.I.R. Meeting of Experts, Cannes, 1961, Document 64, Part 3. MM MY Printed by B.B.C. Research Department, KlngslJood Warren, Tadworth, Surrey