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UDC 621.396.67 



mmm # 



THE QUEEN'S AWARD 



RESEARCH DEPARTMENT . REPORT 



U.H.F. relay station aerials: 

the compensation if a 

914mm diameter aerial cylinder 

by means of an inductile grating 



No. 



111/33 



Research Department, Engineering Division 

THE BRITISH BROADCASTING CORPORATION 



RESEARCH DEPARTMENT 

U.H.F. RELAY STATION AERIALS: THE COMPENSATION OF A 914 mm DIAMETER AERIAL 
CYLINDER BY MEANS OF AN INDUCTIVE GRATING 

Research Department Report No. 1971/33 
UDC 621.396.67 



This Report may not be reproduced in any 
form without the written permission of the 
British Broadcasting Corporation. 

It uses SI units in accordance with B.S. 
document PD 5686. 



Work covered by this report was undertaken by the BBC Research Department 

for the BBC and the ITA 



N.H.C. Gilchrist, B.Sc. 
J.H. Moore 



Head of Research Department 



(RA-87) 



Research Department Report No. 1971/33 



U.H.F. RELAY STATION AERIALS: THE COMPENSATION OF A 914 mm DIAMETER AERIAL 
CYLINDER BY MEANS OF AN INDUCTIVE GRATING 



Section Title Page 

Summary 1 

1. Introduction 1 

2. Theoretical considerations 1 

2.1. The inductance of a grating composed of straight wires or helices 1 

2.2. An estimate of the current induced in the wires of a grating surrounding an aerial 2 

3. Measurements on aerials in a 914 mm diameter glass-fibre cylinder with a surface grating .... 3 

3.1. Radiation pattern measurements 3 

3.2. Impedance measurements 4 

4. Conclusions 4 

5. References 4 



(RA-87) 



September 1971 



Research Department Report No. 1971/33 
UDC 621.396.67 



U.H.F. RELAY STATION AERIALS: 

CYLINDER BY 



THE COMPENSATION OF A 914 mm DIAMETER AERIAL 
^lEANS OF AN INDUCTIVE GRATING 



Summary 

When an aerial is mounted inside a dielectric cylinder, it is possible for reflec- 
tions within the cylinder to affect the impedance and radiation pattern of the aerial. 

Theoretical and practical aspects of the use of inductive gratings at the surface 
of such a cylinder are presented in this report, together with the results of measurements 
conducted on aerials in cylinders equipped with gratings, showing that a useful degree of 
compensation of the effects of the cylinder is practicable. 



1. Introduction 

Many of the u.h.f. television relay stations built to date 
employ the standard BBC cardioid transmitting aerial. 
This aerial consists of 16 vertically-polarised half-wave 
dipoles stacked vertically at intervals of one wavelength. 
The usual mounting arrangement consists of a 45-7 m high 
self-supporting tower with the aerial at the top enclosed in 
a 387 mm diameter vertically mounted glass-fibre cylinder, 
which acts both as a mechanical support and weathershield. 
Inspection and servicing of the aerial necessitates its removal 
from the cylinder and a pulley arrangement is provided for 
the purpose. Larger cylinders are in use at main stations, 
and inspection of aerials is possible from within the weather- 
shield. The use of an intermediate size of cylinder at relay 
stations would provide this inspection facility but it has 
been shown that a cylinder with a diameter greater than one 
wavelength may have a severe effect upon the radiation 
pattern and impedance of the aerial contained within it. 



where Y s is surface admittance in Siemens, 
W is thickness of sheet in metres, 
X is free-space wavelength in metres, 
e r is relative permittivity of the sheet; 
2TtW\fe 

X 



and 
provided that 



is small (i.e. the sheet must be con- 
siderably less than one wavelength 
thick). 



The susceptance obtained is capacitive, and may be 
tuned by an inductive grating of conductors parallel to the 
electric field vector and close to (or embedded within) the 
sheet. The effect of a grating composed of closely-spaced 
straight wires may be expressed as an inductive surface 
admittance given by the formula: 



1207TC? 

^y =j_^_log e (d/27rr) 
X 



(2) 



One solution is to use a double- or triple-walled 
cylinder arranged so that reflections tend to cancel, but 
this is costly. An alternative method is to tune out 
effectively the capacitive reactance of the cylinder by 
means of an inductive grid of wires. The practical applica- 
tion of this method is considered in this report. 



where Y„ is surface admittance in Siemens, 

d is spacing between the axes of adjacent wiresA . 

X is the wavelength, /metres 

r is the radius of each wire, } 

This formula can be applied when r/d is small and g?<X/4. 



2. Theoretical considerations 

2.1. The inductance of a grating composed of straight 
wires or helices 

The effect of a dielectric sheet on an electromagnetic 
wave may be expressed as a surface admittance given by the 
formula: 



Y =—( e --\)W 
s 60X r 



(1) 



Formulae (1) and (2) may be used to estimate the 
dimensions of an ideal grating to compensate for the 
surface admittance of a dielectric sheet. The cylinder used 
for the measurements described in this report had a wall 
thickness of 9 mm, and was made from glass-fibre material 
having a relative permittivity of approximately 4-5. The 
ideal grating for compensating such a cylinder requires wires 
of about 0-001 mm diameter, closely spaced; they would 
thus be so fine as to be incapable of carrying the induced 
currents and would also be mechanically very fragile. 



(RA-87) 



It is, however, possible to use straight wires of 
reasonable thickness by increasing the spacing to 0-4 wave- 
length. The spacing is then outside the limits given for 
Equation (2) so that the grating can no longer be regarded 
as continuous. Nevertheless the impedance compensation 
afforded by the more widely spaced grating is just as 
effective, and it gives acceptable radiation patterns. Straight 
wire elements of diameter 0-31 mm (30 s.w.g. wire) would 
be suitable and although wire of this size is satisfactory 
from constructional considerations, the possibility of dam- 
age by lightning stroke has to be borne in mind. To in- 
crease the thickness of the wire and still obtain proper 
compensation it has been proposed 4 that the grating wires 
should be coiled into helices of suitable pitch and diameter. 
In practice if helices are used then thewire thickness may be 
increased to about 20 s.w.g. However, estimates show that 
the likelihood of lightning strikes of an intensity to fuse 
even the finer wire is very small and it is probable that the 
straight-wire grating would be quite satisfactory in practice. 

2.2. An estimate of the current induced in the wires of 
a grating surrounding an aerial 

An estimate of the current induced in the wires by 
the radiating dipole elements can be made from the induc- 
t 
f 



ve surface admittance of the grating by calculating the 
eld from the dipoles at the wires. A cross-sectional 
agram of a cylinder equipped with a grating is shown in 
g. 1. The surface current is distributed between the 
res according to their distance from the dipole axis. 

A Type A relay station radiates 625 W mean vision 
power and 200 W mean sound power per channel. For a 
total of four programmes therefore the mean power will be 
3-3 kW. This means that the total current through the in- 
ductive grating will be approximately 3-5 amps (r.m.s.). 
With the dipoles at a distance (a) of 150 mm from the 
cylinder wall, and with a grating of 20 wires the current in 
the wire closest to the dipoles is found to be 0-576 amps 




es on 
mounting spine 



grating* 
elements 

Fig. 1 - Cross-sectional view of cylinder 

(r.m.s.), with correspondingly lower currents in the wires 

more remote from the dipoles. This is well within the 

rating of 30 s.w.g. wire which has a fusing current of about 

15 amps in air. 

TABLE 1 

Grating Details 



Grating 


Type of 


Spacing 


Dipole-cylinder 


No. 


element 


between 
elements 


wall spacing 


1. 


Straight-wire 


240 mm 


191 mm 


2. 


Helical 


140 mm 


152 mm 


3. 


Straight-wire 


172 mm 


197 mm 


4. 


Helical 


114 mm 


197 mm 


5. 


Straight-wire 


T50 mm 


152 mm 


6. 


Helical 


97 mm 


152 mm 




Fig. 2 - Horizontal radiation patterns of Band IV aerial in 

914 mm diameter cylinder with straight-wire grating 

(grating 1 in Table 1) 

____„500MHz _..._,_ 600 MHz 

— — — — Limits of pattern set by templet 




Fig. 3 - Horizontal radiation patterns of Band IV aerial in 

914 mm diameter cylinder with helical-wire grating (grating 

2 in Table 1) 

— — — —600MHz ._-._.._ 600 MHz 

— — — — - L imits of pattern set by templet 




Fig. 4 - Horizontal radiation patterns of lower Band V aerial 

in 914 mm diameter cylinder with straight-wire grating 

(grating 3 in Table 1) 

_____ -600 MHz ■ _._.._700MHz 

— — Limits of pattern set by templet 




Fig. 6 - Horizontal radiation patterns of upper Band V 

aerial in 914 mm diameter cylinder with .straight-wire 

grating (grating 5 in Table 1) 

.._____ 700 MHz _, —,— 850 MHz 
——Limits of pattern set by templet 

Straight-wire elements consist of 30 s.w.g. enamelled 
copper wire. Helical elements consist of 23 s.w.g. 
enamelled copper wire wound on 6 mm diameter insulating 
rods, with a pitch of 6 mm. 

3. Measurements on aerials in a 914 mm diameter 
glass-fibre cylinder with a surface grating 

3.1. Radiation pattern measurements 

Horizontal radiation patterns were measured on 
aerials consisting of Band IV, lower and upper Band V 




Fig. 5 - Horizontal radiation patterns of lower Band V aerial 

in 914 mm diameter cylinder with helical-wire grating 

(grating 4 in Table 1) 

__ 600 MHz _._„_700MHz 

— — — — — Limits of pattern set by templet 




Fig. 7 - Horizontal radiation patterns of upper Band V 

aerial in 914 mm diameter cylinder with helical-wire 

grating (grating 6 in Table 1) 

— — •——700 MHz —.-'— 850 MHz 

_______ Limits of pattern set by templet 

dipoles mounted on a supporting spine in a 914 mm dia- 
meter glass-fibre cylinder. The measurements were con- 
ducted with both straight wire and helical grating elements. 
The dipoles were positioned towards the edge of the 
cylinder, most results being obtained with a dipole-wall 
spacing in the range 150 - 200 mm, this position being 
chosen to leave sufficient climbing space for access and 
maintenance purposes. 

A selection of radiation patterns measured in Bands 
IV and V are shown in Figs. 2-7. Details of the gratings 
used in these measurements are given in Table 1; little 



difference has been found between results obtained with 
gratings on the inner and outer surfaces of the cylinder. 




Fig. 8 - Horizontal radiation pattern of upper Band V aerial 
in 9 14 mm diameter cylinder. Dipole-wall spacing 146 mm 

__ 730 MHz — — — — 790 MHz -■ 850 MHz 

The horizontal radiation pattern of a relay station 
aerial in a 914 mm cylinder without gratings is shown in 
Fig. 8; the dipole-wall spacing is 146 mm in this case. 
Comparison with Figs. 6 and 7 reveals the extent to which 
the radiation patterns have been corrected and brought 
within the limits defined by the templet. 



40 
330 



P 20 - 



£10 - 



1 


1 1 


A 

r 


limits of 

« aerial operating — *■ 

band 






/ 
/ 
f.-t. 



550 



600 



750 



650 700 

frequency, MHz 

Fig. 9 - Reflection coefficient of lower Band V aerial in 
914 mm diameter cylinder with straight-wire grating 
— ._ — aerial in free space 
— _. — ,„_ aerial in cylinder with no compensation 
— aerial in cylinder with straight-wire grating 

3.2. Impedance measurements 

The reflection coefficients of aerials for Band IV and 
both halves of Band V were measured with the aerials in 



40 



.9-30 



o 20- 



o. 10 - 



I 


i i 


4 * 




limits of 
* aerial operating — *■ 




- \\ 




band 


- 


\ 






/ 




/ 

/ ..* 




V \ /' f 




^v. .^^ Vy f 


i 


i i 




550 



750 



600 650 700 

frequency, MHz 

Fig. 10 - Reflection coefficient of lower Band V aerial in 
9 14 mm diameter cylinder with helical-wire grating 

— . — . — ' aerial in free space 
— ■ — ■— « aerial in cylinder with no compensation 
-! — —aerial in cylinder with helical-wire grating 

free space and also mounted within cylinders (with and 
without inductive gratings). Typical results are shown in 
Figs. 9 and 10, and it will be seen that reasonable reflection 
coefficients are obtained over the operating frequency range 
of a lower Band V aerial using both straight-wire and helical 
elements in a grating. Similar results have been obtained 
over Band IV and upper Band V. 

Although correction of the impedance of an aerial in 
a cylinder by the use of a grating is not complete at all 
frequencies in the operating band, a considerable improve- 
ment is obtained over the results obtained when the aerial 
is mounted in an uncompensated cylinder. 



4. Conclusions 

Experiments with two types of inductive grating 
attached to the surfaces of a glass-fibre cylinder have demon- 
strated their effectiveness in compensating for the surface 
admittance of the cylinder. Acceptable radiation patterns 
and reflection coefficients have been obtained over the 
operating frequency range of each aerial tested with this 
arrangement. 

It would be desirable, before equipping stations with 
cylinders compensated by gratings, to investigate the prob- 
lem of lightning damage, preferably by conducting high- 
voltage tests. 



5. References 

1. A vertically-polarized transmitting aerial for UHF relay 
stations. BBC Research Department Report No. 
E-122, Serial No. 1966/74. 

2. UHF relay station aerials inside 914 mm diameter glass- 
fibre cylinders. BBC Research Department Report No. 
RA-1 6, Serial No. 1968/18. 



3 Multiple wall weathershields for UHF aerials. BBC 4. BBC Patent application No. 7597/70. Reducing 

Research Department Report No. 1970/33. electromagnetic reflection from dielectric sheets. 



SMW/PF 



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