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50 views17 pagesBBC Monograph 23
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/ 17
ENGINEERING DIVISION
MONOGRAPH
NUMBER 23: FEBRUARY 1959
The Crystal Palace Band I Television
Transmitting Aerial
by W. WHARTON, A.M.LE.E.
and
G. C. PLATTS, B.Sc.
(Planning and Installation Department, BBC Engineering Division)
BRITISH BROADCASTING CORPORATION
PRICE FIVE SHILLINGS BBC ENGINEERING MONOGRAPH
No. 23
THE CRYSTAL PALACE BAND I TELEVISION
TRANSMITTING AERIAL
e@ by
W. Wharton, A.M.LE.E,, and G. C. Platts, B.Sc.
(PLanoune ano Insratsation Devarraenr, BBC ENcINceRING DIvsi0N)
FEBRUARY 1959
BRITISH BROADCASTING CORPORATION FOREWORD
#8 is one of a series of Engineering Monographs
I] published by the British Broadcasting Corporation.
About six are produced every year, each dealing
with a technical subject within the field of television and
sound broadcasting. Each Monograph describes work
that has been done by the Engineering Division of the
BBC and includes, where appropriate, a survey of
earlier work on the same subject. From time to time the
series may include selected reprints of articles by BBC
authors that have appeared in technical journals. Papers
dealing with general engineering developments in broad-
‘casting may also be included occasionally.
This series should be of interest and value to engineers
engaged in the fields of broadcasting and of telecom-
munications generally.
Individual copies cost 5s. post free, while the an-
nual subscription is £1 post free. Orders can be placed
with newsagents and booksellers, of HRC PUBLICATIONS,
35 MARYLEBONE HIGH STREET, LONDON, Wt CONTENTS
Section Title Page
PREVIOUS ISSUES IN THIS SERIES : : 4
SUMMARY. 5
1, itRopUCTION 5
2. MECHANICAL AND ELECTRICAL DESIGN OF THE AERIAL SYSTEM. 3
2.1 General 5
Dad Pract 5
2.1.2. Theoretical Considerations 6
2.2 Single-Dipole Element for Upper Half ofthe Aerial 7
23. Double-Dipole Element for Lower Half of the Acrial 7
24 Screening of Tower 8
2.5 Main and Distribution Feeder Systems 8
2.5.1 Main Feeders 8
2.5.2 Distribution Feeders 8
3. MODEL AND FULL-SCALE PROTOTYPE AERIAL. MEASUREMENTS °
3.1. Scale Model Tests 9
3.2 Single-Dipole Element 9
3.3 Double-Dipole Element 10
4A. PHASING OF AERIAL TIERS : : 10
5. LINING UP THE AERIAL sySTEM : : n
5.1 General Considerations : nl
5.2 Upper Half of Aerial : : 2
53. Lower Half of Aerial : : 2
5.4 Phasing of Upper and Lower Halves j 2
5.5 Mutual Impedance between Halves of Aerial : R
6. AERIAL PERFORMANCE ‘ 13
7. conctusions B
8 ACKNOWLEDGMENTS, 13
9. REFERENCES : 3
A RECENT BBC TECHNICAL SUGGESTION 14 PREVIOUS ISSUES IN THIS SERIES
Title Date
‘The Suppressed Frame System of Telerecording BONE 1955
Absolute Measurements in Magnetic Recording SePTeameR 1955
The Visibility of Noise in Television ocroner 1955
‘The Design ofa Ribbon Type Pressuresgradient Microphone for Broadcast Transmission ecemmer 1955
Reproducing Equipment for Fine-groove Records FEBRUARY 1956
A VHLEJUILE. Field-strength Recording Receiver using Postdetector Selectivity aril 1956
The Design of a High Quality Commentator's Microphone Insensitive to Ambient Noise sun 1956
An Automatic Integrator for Determining the Mean Spherical Response of Loudspeakers and Microphones SUGUST 1956
The Application of Phase-coherent Detection and Correlation Methods to Room Acousties Novenmer 1956
An Automatic Spstem for Synchronizing Sound on Quarter-inck Magnetic Tape with Action on
35-mm Cinematograph Film JANUARY 1957
| Encineerine Training in the BBC wanctt 1957
An Improved "Roving Eye" -aPRit. 1957
The BBC Riverside Television Studios: The Architectural Aspects suLy 1957
The BBC Riserside Television Studios: Some Aspects of Technical Planning and Equipment ‘ocroner 1957
‘New Equipment and Methods for the Evaluation of the Performance of Lenses for Television pecesmen 1957
Analysis and Measurement of Programme Levels Marci 1958
The Design of a Linear Phase-Shift Low-pass Filter APRIL 1958,
The BBC Colour Television Tests: An Appraisal of Results May 1958
A UHE. Television Link for Outside Broadeasts JUNE 1958
|. The BBC's Mark IT Mobile Studio and Control Room for the Sound Broadcasting Service AUGUST 1958
Two New BBC Transparencies for Testing Television Camera Channels NOVEMBER 1958
2. The Engincering Facilities of the BBC Monitoring Service JANUARY 1959 THE CRYSTAL PALACE BAND I TELEVISION TRANSMITTING AERIAL
SUMMARY
This monograph describes the design, testing, and installation of the eight-tier Band I transmitting aerial at the BBC's
Crystal Palace Television Station. Structural considerations made it necessary to mount the upper four tiers and lower four
tiers of the aerial on sections ofthe tower having very different cross-sections and this raised many electrical and mechanical
problems whieh had not occurred in previous aerial desigus.
Preliminary measurements at 450 Mc/s using scale models indicated that four dipoles per tier for the upper half and
‘eight dipoles per tier for the lower balf of the aerial would be necessary to give a sufficiently circular horizontal radiation
pattern. These model measurements also predicted the spacing of the elements from the tower face and, in the case of
the lower half of the aerial, from each other. Based on this information preliminary designs for a single-dipole radiating
clement forthe upper half of the aerial and a double-dipole radiatingelement for the lower half ofthe aerial were prepared,
‘These designs were developed at ful scale and further model tests were carried out in parallel with the full-scale develop”
‘ment work, thus enabling the effect of changes in design to be rapidly assessed. These prototype investigations enabled the
behaviour of the complete aerial to be predicted with considerable accuracy and as a result the adjustment at site was
carried out relatively easily,
1, Introduction
‘The economic considerations governing the choice of
transmitter power and aerial gain for the Crystal Pulace
Television Transmitting Station resulted in a decision to
use a Band T aerial consisting of eight tiers of vertical di-
poles with a substantially circular horizontal radiation
pattern. Restriction of space available on the sive, how-
ever, necessitated a self-supporting tower and not a stayed
‘mast being used. The tower is illustrated in Fig. 1 and con-
sists of a support structure 440 ft high of square cross-
section tapering from 120 ft wide at the base to 14 ft 6 in
at the top. Above this are three parallel-sided sections, 9 ft
6in,, 6f1 6 in.,and 4fi4iin, square, each 72 fe high. Short,
tapered assemblies interposed between the
the total height to 672 fv and there is provi
a 40-ft pole-mounted aerial above this level, bringing the
{otat height o the maximum permitted figure. This form of
tower was chosen since preliminary investigations had
shown that it would be possible to erect the Band I zerial
partly on the upper part of the tapered support tower
{mean cross-section 17 fix 17 ft)and partly on the parallel-
sided section above (cross-section 9 [16 in, 9 106 in,) and,
thus leave the maximum possible space above the Band T
aerial for higher frequency aerials. Fig. | also shows in out-
line the Band I aerial arrangement adopted.
‘The transmitting system adopted forthe station consists,
of two separate chains eaeh comprising a 15-kW peak-
white vision transmitter and 2 3-75-kW sound transmitter
ogether with a vision/sound combining unit. The com-
bined sound and vision output signals of the two chains
are ed to the aerial switching arrangement shown in Fig. 2.
‘The normal system of operation is with the two chains
combined in the diplexer, the output of which feeds the
two halves of the serial via the splitter transformers. This
system of normal operation is adopted, rather than that of
Feeding the output of the two chains independently to the
‘wo aerial halves in order to reduce distortion of the re
ceived signal which would otherwise tend to occur at
‘minima in the field near the station,
In the event of a transmitter or combining unit faiture
the radiated field is reduced by 6B, but by switching out
the diplexer the drop in field is reduced to 3 dB. In the
event of an aerial failure bath chains of transmitters are
paralleled via the diplexer and connected to the remaining
half ofthe aerial; the lossin radiated fieldis therefore once
again 3 dB. Each half of the aerial system, bath feeders
and aerial elements, have therefore to be capable of carry-
ing the combined power of both transmitter chains. In the
initial planning it was decided to cater for a possible future
increase of vision power to 50-kW peak-white for each
vision transmitter, together with appropriate sound power.
Each main feeder and half-zerial system, together with the
relevant part of the feeder switching system, has therefore
been designed to carry. total power of 100-kW peak-white
vision and 25-kW sound carrier.
2. Mechanical and Electrical Design of the
Aerial System
General
Practical Considerations
lly, model tests at one-tenth scale (450 Me/s) were
carried out to determine an arrangement of dipole radiat-
ing elements which would give a sufficiently circular hori-
zontal radiation pattern for each half of the aerial. Thein-
vestigation showed that acceptable results would be ob-
tained with an arrangement of four dipoles per tier for the
upper half of the aerial and eight dipoles per tier for the
ower half of the aerial (see Fig. 1). It was further deter-
mined that for both halves of the aerial the elements in
each tier should be drivenwith currents of thesameampli-
tude and phase. The tests showed that the dipole elements.
for the upper half of the aerial were required to be 0-2 2
from the tower face, whilst the two dipoles on each face
used for the lower half of the acrial needed to be O°5 2
apart and 0-2 from the tower face.
Since only one dipole per face per tie: was required for
the upper half ofthe aerial it was decided that each of these
dipoles should be designed as a single self-contained ele PROVISION FOR
FUTURE AERIALS
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Fig. 2 — Schematic of aerial, feeders, and switching
ment, For the lower half of the aerial, however, where two
dipoles per face per tier were required, it was decided 10
develop an element comprising two dipoles fed by a com-
‘mon coaxial feeder.
‘The spacing necessary between tiers of vertical dipoles
for optimum gain is approximately one wavelength. Due
to mechanical considerations, however, itwas nccessary to
reduce the spacing between tiers of the upper and lower
‘halves of the aerial to 0825 Aand 0-90 A, respectively. The
again varies very slowly with tier spacing in the region of
the optimum and the loss of gain due to the reduction in
tier spacing is negligible.
2.2 Theoretical Considerations
The variation of the conductance presented at the centre
ofa resonant half-wave dipole (either a single or a folded
dipole) is determined by the effective cross-sectional area
‘of the dipole limbs, and in practice i is found that no me-
chanical difficulty exists in achieving a limb cross-section
resulting in a sufficiently constant conductance. The
‘ceptance presented by the dipole varies considerably how=
ever, and has a negative slope over the band. To obtain a
‘broad-band admittance it is therefore necessary to con
rect a parallel resonant circuit across the dipole feed point
with @ susceptanee slope such that it cancels the dipole
susceptance. This method of improving the dipole aémi
tance characteristic is referred to as susceptance compen-
sation. The design of each type of element was therefore
‘based on the use of dipole limbs of sufficient cross-sectional
area and the incorporation of the simplest and most com-
pact arrangements possible for susceptance compensation. 22. The Single-Dipole Element for the Upper Half of the
Aerial
‘The final design of the full-scale dipole element is shown,
in Fig. 3, together with a schematic diagram illustrating the
principle of operation. The dipole limbs are skeletonized.
lo give low wind-loading and their length can be varied by
‘means of the adjusting tubes protruding through the end
plates. The parallel resonant compensating circuit is rea-
lized by mounting the dipole limbs on short-circuited stubs
which form an inductance, the required capacitance being
provided by a short length of open-circuited coaxial feeder
connected across the dipole limbs. As illustrated in Fig. 3,
the short-circuited part of the structure is split into three
in the practical arrangement, the outer arms forming the
‘means of mechanical attachment to the tower legs.
The following controls are available for adjusting the
dipole impedance to match its 2-in. diameter, 70-ohm feed-
cer over the band:
(@) Thedipole resonant frequency can be varied by adjust=
{ng the fength of the dipole limbs.
(®) The conductance can be adjusted about a mean value
of 14 mmho (70 ohms) by moving the driving point
horizontally relative to the dipole limbs. This is achiev-
ed by the provision of a telescopic joint in the 2-in,
diameter feeder.
(© The susceptance compensation can be controlled by
jmultaneous adjustment of the position of the short
cireuits and the value of the coaxial capacitance.
SUSCEPTANCE COMPENSATION
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Fig. 3 — Single-dipote element for upper half of aerial
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Fig. 4— Doubtesdipole element for lower half of aerial
(@) The coaxial capacitance, in addition to controlling the
susceptance compensation ia association with the
short-circuited stubs, may be adjusted to offset any ex-
cess oF deficit of capacitance across the centre of the
dipole. This is, in effect, an adjustment of the mean
susceptance of the dipole.
Devicing heaters of tubular form are fitted to the base
plate of each dipole limb, each heater being of S00-W load-
ing, ic. a total heating power of 1 KW per dipole.
23. The Double-Dipole Element for the Lower Half of the
Aerial
‘The final design of the full-scale double-dipole element
is shown in Fig. 4 together with a schematic diagram lluse
trating the principle of operation. As in the case of the
single element the limbs are skeletonized to reduce the
‘wind load. The dipoles are folded, each having a driving-
point impedance of approximately 280 ohms, and derive
acertainamount ofcompensation from the shor'-cireuited
transmission lines formed by the folded limbs. The ‘dead”
sides of the folded dipoles are attached to a beam, univer-
sally drilled, so that it ean be attached to the tower at any
one of the four tier levels by a pair of standard brackets
The driving points of the dipoles are connected together
by a balanced transmission line consisting of two fight
angles supported from the beam on polythene insulators.
‘The balanced transmission line is extended beyond the di- pole driving points for approximately a quarter-wave-
length and then short-circuited. A If-in, 70-ohm copper
feeder is bonded along the lower ofthe two angles forming
the balanced transmission line and is connected at the
centre point of the line between the two dipoles. The di-
poles are separated by a half-wavelength and, since the
characteristic impedance of the balanced transmission line
is 200 ohms, an approximate match to the 70-ohm feeder
is obtained at the centre point.
The following controls are available to match the dipole
clement to its Ij-in. diameter, 70-ohm feeder:
(@) The dipole resonant frequency can be varied by adjust-
ing the length of the limbs with the screwed rods pro-
truding from the end-plates.
(® The mean conductance can be adjusted by adding
capacity plates on the balanced transmission line be=
tween the dipoles, thus changing its characteristic im-
pedance. To ensure that an addition and not a reduc-
tion of capacitance is sequired a capacity sleeve is
placed on tho inner of the coaxial feeder an eighth-
‘wavelength back from the drive point,
(©) The susceptance compensation can be controlled by
the simultaneous addition of capacity plates across the
dipoles and the adjustment of the bslanced trans-
mission line short-cireuits.
(@) As in the case of the single-element the mean suscept-
ance can be controlled by adjustment of the capacit-
ance across the dipoles.
Inorder to increase the mechanical stability and simplify
the distribution feeder system, the main supporting beams
are extended so as to meet at the comers of the tower, thus
forming a continuous cat-walk at each tier level.
As an experiment, it was decided not to fit the double-
dipole elements with de-icing heaters, initially. The design,
however, permits the installation of tubular heaters on the
parallel transmission finein the region of the dipole driving,
points should this eventually prove necessary.
24. Screening of Tower
In order to prevent the feeders and steehwork internal to
the tower from affecting the aerial impedance and also to
reduce the possibility of any internal resonances within the
tower, screening conductors are installed on all four faces
of the tower over the aerial aperture. The screening con-
sists ofsix equally spaced vertical conductors per face over
the region of the upper half of the aerial and eight quasi-
vertical conductors per face over the region of the lower
half of the aerial. The number of conductors used is the
‘maximum permissible on wind-loading grounds and each
conductor consists ofa f-in. diameter S.C.A. cable bonded.
to the tower wherever it crosses a structural member.
2.8 Main and Distribution Feeder Systems
23.1 Main Feeders
Tn order to carry the maximum power ever likely to be
employed and also to minimize tosses, the two main feed-
cers are S-in. diameter coaxial copper air-spaced feeders of
51-5 ohms characteristic impedance. The performance of
‘these feeders is as follows:
(a) Each feeder is capable of carrying simultancously a
vision power of 100-kW peak-white on 45 Mc/s and a
sound carrier power of 25 kW on 41-5 Mc/s 100 per
‘cent amplitude modulated. The loss in each feeder is
Jess than 0-075 dB per 100 ft
(b) With the feeders terminated in matched louds theinput
reflection coefficient is less than 1-5 per cent, which
results in delayed signals caused by impedance discon-
{inuities being well below the level of visibility
252. Distribution Feeders
‘The use of two main feeders necessitates the provision
‘of separate distribution feeder systems for the two halves
of the aerial. The performance of these systems is as fol-
ows:
(a) Each half of the distribution feeder system is capable
of carrying the same power as its associated main
feeder.
(0) With cach of the branch feeders leading to the aerial
elements terminated in a matched load, the reflection
coefficient of the input impedance of each half of the
distribution feeder system is less than I per cent, This
performance ensures that the degradation of aerial
performance due to the distribution feeder system is
negligible.
‘The discribution feeder systems for the upper and lower
halves ofthe aerial use similar main transformers, but there
‘aro considerable differences in layout owing to the differ
‘ences in the tower cross-section. To illustrate the method.
vwherehy the impedances of the aerial elements are trans-
Formed to match the main feeder, Fig. 5 shows a sche-
‘matic layout of the distribution feeder system used for the
Tower half of the aerial.
For the upper half of the aerial the distribution feeder
system is mounted axially in the tower but for the lower
half of the aerial structural considerations made this form
of layout impossible. In the latter case, therefore, the
lransformers are mounted at one side of the tower and
Jie in a plane parallel to one of its faces. The tapering,
section of the tower and the layout of the distribution
Feeder transformers resulted in a variation in the lengths
of the If-in. feeders at each tier level and this, together
with the need for special phasing due to the tapering of
the tower, made it necessary to incorporate equalizing
oops of feeder in each of the 2-in. 35-obm feeders (shown
in Fig. 3)
Throughout the system, screwed joints tightened by
phosphor-bronze set-serews are used wherever possible to
joint adjacent lengths of the inner conductor and, where
Spring contact joints are necessary, all contacts are heavily
‘copper or silver plated. The outers of the feeders are fan
‘ged at the ends, bolted together, and sealed by synthetic
rubber rings. The system is supplied with dry air under
pressure from a dehydrator to ensure that no moisture
enters.
All horizontal feeder sections are protecied against fall-
ing ice and, in the case of the lower half of the aerial, use
is made of the open-mesh steel flooring of the plaiforms
for this purpose. | eco ss
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Fig. 5 — Schematic of distribution feeder system for tower
half of aerial
3. Model and Fall-scale Prototype
Acrial Measurements
3.1 Seale Model Tests
The full-scale design of the elements for both the upper
and lower halves of the aerial were checked by measure-
‘ments made on model aerials which enabled bandwidth
and impedance 19 be investigated. It was not possible,
however, to predict the precise settings of the impedance
matching controls due to the dificulty of scaling the full-
size elements sufficiently accurately. The great advantage
‘of the model measurements was that, due to the small size
of the model elements, it was possible to try out design
‘modifications ina minimum time and to build suflicient of,
the aerial system (ie, more than one tier) to investigate the
‘magnitude of the mutual impedances between elements
The latter measurements were of great assistance in setting
up the aerial and it is no exeggeration to say that the ac~
curate design of an aerial of this complexity would be vir-
tually impossible without the aid of model measurements
3.2. Single-Dipole Element
Initially, a mechanical design based on preliminary mode!
measurements was prepared, and a full-seale single ter of
Four dipoles, mounted ona 40-1 tower simulating the rele~
vvant section of the final tower, was then erected. Measure
‘ments were cartied out (0 determine precise mechanical
dimensions and the range of the impedance adjusting con-
trols was also investigated to ensure that sufficient range Fig. 6 — Admittance characteristic of one ter of dipole ele-
‘ments for upper half of aerial
of adjustment would be available to take up effects of
‘mutual impedance on site. Fig. 6 is a typical admittance
characteristic obtained for the complete arrangement of
one tier.
3.3 Double-Dipole Element
“The production ofa full-scale prototype ofa single tier
of double-dipole elements would have required a structure
approximately 20 ft>20 ft 40 ft high and four radiating
elements. This would have involved considerabledelayand
expense, but modeltestsconfirmed that testsonone double-
dipole element mounted horizontally over @ wire netting
earth sereen would be adequate to establish settings ofthe
controls.
Tn iew of the greater complexity ofthis element as com-
pared with the single dipole, full-scale prototype tests were
carried outin parallel with sale-modeltests. Inthe original
design the folded dipole limbs consisted of four tubes, wo
attached tothe support beam (dead side) and two attached
‘othe parallel transmission ine live side). Model tests
dicated, however, thatthe driving point impedances ofthe