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BBC RD 1987/13 

tsks tta 

1 B ■ 1 1 a * T 



£7/270 















September 1987 



BAND II VHF ANTENNA FOR HOME 

CONSTRUCTION 



S. Wakeling, B.Sc. 



Research Department, Engineering Division 
THE BRITISH BROADCASTING CORPORATION 



BBCRD19B7/13 



U DC 621.396.67 



BAND II VHF ANTENNA FOR HOME CONSTRUCTION 
S. Wakeling, B.Sc. 



Summary 

A wideband Band II Yagi antenna, suitable for home construction, has been 
designed using computer techniques. The antenna is intended for use as a domestic 
receiving antenna for VHF/FM sound broadcasting at frequencies in the range 88 to 
108 MHz. Scale-model antennas were constructed and measured to verify (he performance 
predicted by the computer. Finally, a full-scale Yagi antenna was constructed and Us 
radiation patterns measured The measurements on the new antenna and on some 
commercially made antennas, which all had performances inferior to the new antenna, 
throw doubt upon the practicability or usefulness of the directivity characteristic in CCIR 
Recommendation 599. 



Issued under the Authority of 



Research Department, Engineering Division, 
BRiTiSH BROADCASTING CORPORATION 

(RA-234) 



H- 



C-/1 




Head of Research Department 

September 1987 



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 



UNIVERSAL DECIMAL CLASSIFICATION 

II is intended thai the UDC reference (see Summary page, 
top right) will, from 1988 onwards, be omitted from BBC 
Research Reports. The classification is not generally used 
elsewhere within the BBC. Should, however, the lack of 
this reference be of particular concern to you would you 
kindly write to: 

The Research Executive, 

BBC Research Department, Kingswood Warren, 

Tadworth, Surrey, KT20 6NP, England. 



BAND II VHP ANTENNA FOR HOME CONSTRUCTION 

S. Wakellng, B.Sc. 

Section Page 

1. Introduction 1 

2. Antenna Requirements 1 

3. Computer Program 1 

4. Results of Computations 2 

4.1 Reduced element design 2 

5. Scale Model Yagi Antennas 2 

6. Full-Size Antenna Results 3 

6.1 Performance ot the original Yagi antenna 3 

6.2 Performance o1 the new wideband design 3 

6.3 Construction ot the proposed design 6 

7. COIR Recommended Patterns for Domestic VHP Receiving Antennas 6 

8. Discussion 7 

9. Conclusions 8 

10. References 8 

Appendix 1 BBC Engineering Information Department. Information Sheet 

No. 1104(4). VHP Radio Receiving Aerials (facingp.8) 

Appendix 2 CCIR Recommendation 599 11 

Appendix 3 Scale-model Yagi Antennas 12 

A3.1 Yagi antenna with a single dipole driven element 12 

A3.2 Yagi antenna design with a folded dipole 16 

A3.3 Sensitivity to constructional errors 18 



IRA-234) 



© BBC 2006. All rights reserved. Except as provided below, no part of this document may be 
reproduced in any material form (including photocopying or storing it in any medium by electronic 
means) without the prior written permission of BBC Research & Development except in accordance 
with the provisions of the (UK) Copyright, Designs and Patents Act 1988. 

The BBC grants permission to individuals and organisations to make copies of the entire document 
(including this copyright notice) for their own internal use. No copies of this document may be 
published, distributed or made available to third parties whether by paper, electronic or other means 
without the BBC's prior written permission. Where necessary, third parties should be directed to the 
relevant page on BBC's website at http://www.bbc.co.uk/rd/pubs/ for a copy of this document. 



BAND II VHF ANTENNA FOR HOME CONSTRUCTION 
S. Wakeling, B.Sc. 



1. INTRODUCTION 

The number of VHF radio programmes 
broadcast in the UK is likely to increase in the future. 
At present, the lower and mid-band frequencies of the 
Band II spectrum are almost fully occupied by the 
existing services. In accordance with the 1984 VHF 
Regional Administrative Radio Conference, new 
transmissions will increasingly be fitted into the upper 
frequency range of Band II, between 98 and 
108 MHz, as these frequencies become clear of 
existing mobile services^ 

Up to 1979, the BBC's Engineering Information 
Department recommended a VHF antenna design for 
home construction which covers the bandwidth 88 to 
98 MHz (Appendix 1 contains the design information). 
It is a Yagi antenna which has a limited bandwidth 
for satisfactory radiation patterns and input impedance. 
The deterioration in performance is usually more rapid 
at frequencies above the design frequency rather than 
below it, so the present Yagi design cannot be 
expected to cover the proposed bandwidth. A new 
Yagi antenna has been designed, with emphasis given 
to its operation over a wide (20%) bandwidth, 88 to 
108 MHz. 

This Report describes the measurement of the 
existing recommended Yagi antenna and the design 
and testing of a new antenna. A computer program 
was used to predict the patterns for a number of Yagi 
antenna designs. Scale-model measurements were then 
made to verify the accuracy of the computed antenna 
performance. A full-size version was constructed and a 
matching network added to improve the input 
impedance. The performance of the final design has 
been compared with the original Yagi antenna and is 
recommended as a replacement for it. The BBC's 
Engineering Information Department will shortly issue 
this new design for home construction. 

2. ANTENNA REQUIREMENTS 

The new antenna design should comply with 
the CCIR directivity characteristic for domestic 
receiving antennas in Band II (Appendix 2) which is 
used for planning. It must cover the new band of 88 
to 108 MHz with a similar performance to the original 
Yagi antenna. However, it would be beneficial to be 
able to increase the front-to-back ratio to 20 dB which 
would reduce co-channel interference from transmitters 
on substantially different azimuths. 



Yagi antennas are inherently narrow-band 
devices and the mid-band gain has to be sacrificed to 
provide a reasonable performance over the required 
bandwidth. The input impedance of the antenna 
should be 75 il for domestic use. 

Its ease of construction is also important. For 
this reason a simple dipole is used for the driven 
element, although a folded dipole was used in one 
experiment. Also, the design must not be very sensitive 
to small errors in the element lengths and positions. 
The proposed constructional tolerances are + 10 mm. 

Many VHF services in the UK are now 
transmitted with mixed polarisation. This means that 
more care needs to be taken of its cross-polar 
discrimination and conductors which are not in the 
plane of the antenna will affect its performance. This 
has to be taken into account when the antenna is 
mounted and the feed connected. 

3. COMPUTER PROGRAM 

A computer program was written previously^ 
to calculate the performance of end-fire arrays. It 
computes the radiation patterns, element currents and 
input impedances given the element length and 
thickness and their relative spacing. The method used 
in the program is that of solving the matrix equation 



[Va] = [Z;,] [u] 



where V and I are the voltages and currents in the 
elements and Z is the matrix of the self and mutual 
impedances. The E-plane and H-plane radiation 
patterns are then calculated from the currents in the 
elements. 

A wideband Yagi antenna can be designed 
using an iterative procedure. The start point is a 
known narrowband antenna design. The length and 
spacing of the elements are altered by small amounts, 
usually in 10 mm steps, and the computer program 
calculates the horizontal radiation patterns (h.r.ps). 
This is done at several different frequencies to establish 
a band of frequencies over which the performance of 
the antenna is acceptable. The length and sparing of 
the elements are then altered again until the design 
gives acceptable patterns over the required frequency 
range of 88 to 108 MHz. 



(RA-234) 



4. RESULTS OF COMPUTATIONS 



4.1 Reduced element design 



The number of permutations of input para- 
meters is large and it is expensive and time consuming 
to make a really comprehensive investigation of all 
combinations of parameters. The priority is to design 
an antenna to cover the required bandwidth. It is 
suggested^ that if the directors are made shorter than 
the optimum gain length of just less than A./2 and the 
reflector is made longer, the bandwidth can be 
widened at the expense of the peak gain. Also, if the 
directors are longer than A/2 at the highest frequency 
at which the antenna is to operate, it will fire 
backwards. That is, the backward lobe will have a 
larger magnitude than the forward lobe. (This happens 
to the original antenna at 100 MHz). So the front 
director must be shorter than \/2 at 108 MHz. 

The final design is a compromise which covers 
the required bandwidth with the maximum possible 
front-to-back ratio and gain. 

The dimensions of the antenna giving the 
optimum compromise between bandwidth and gain is 
shown in Fig. 1 with the dimensions in wavelengths in 
Table 1. The wavelength, X, is taken at mid-band. 



Table I - Dimensions of Proposed Design 



The BBC's original antenna design can 
constructed with 4, 3 or even 2 elements. 



be 





Dimension 


Length 
in wavelengths 


Length 


a 
b 
c 
d 


0.55\ 
0.5A 
0.42A 
0.375A 


Spacing 


e 
f 

g 


0.25X 
0.075A 
0.1 62 A 


Diameter 


h 


0.004A 



Computer predictions of the performance of 
the proposed wideband Yagi antenna show that the 
front-to-back ratio is reduced and the gain drops for a 
design with a reduced number of elements. Fig. 2 
shows the h.r.ps for 4, 3 and 2 element Yagi antennas, 
at 98 MHz. The results were similar at other 
frequencies in the band. 







1 


















' 


. 




t 1 




t 


■) 


( 






c 


i 




/ 












' 




-/— e 


' 


-— f — ► 


- g ^ 



feed 



Fig. I - Proposed Yagi antenna design. 



5. SCALE-MODEL YAGI ANTENNAS 

Initially, for ease of measurement, scale models 
were constructed to check the accuracy of the 
predicted results; the calculated and measured radiation 
patterns are given in Appendix 3 and show good 
agreement. 



This antenna operates over the required 
bandwidth with a predicted front-to-back ratio of 16 
to 17 dB and a gain between 4.5 and 5.5 dB. The 
computer predicts that the input impedance will be 
near 50 fl just below the centre of the frequency band 
but will become capacitive at the low frequency end 
and inductive at the high frequency end. The predicted 
performance is compared with the measured results in 
Appendix 3. 



Models were constructed with either a single 
dipole and Pawsey-stub balun or with a folded dipole 
and a trombone balun. These were used to investigate 
input impedance. Altering the length and spacing of 
the elements of the model indicated that the radiation 
patterns were not sensitive to constructional errors. 
However, the input impedance does change, parti- 
cularly if the length of the dipole, or the length of the 
director next to tlie dipole is not cut accurately, and 
hence mismatch losses increase. 



(RA-234) 



2- 



180 



-150 



180^ 




(a) 



(b) 



180' 



-150' 



(c) 



E -Plane 

H-Plane 



Fi^. 2 - HRPsfor Yagi antennas: 

(a) 4 element 

(b) 3 element 

(c) 2 element 



6. FULL-SIZE ANTENNA RESULTS 

Two full-size VHF antennas were constructed 
from aluminium tube. The first was made to the 
original narrowband Yagi design (see Appendix 1) 
with a single dipole, rather than a folded dipole, as the 
driven element. The second was a wideband Yagi 
antenna constructed according to the dimensions 
developed using the computer program. As before, a 
single dipole was used as the driven element making 
the VHF antenna a full size equivalent to the model 
described in Appendix 3. 

6.1 Performance of the original Yagi aerial 

HRPs of the original antenna were measured 
in the required frequency range 88 to 108 MHz. 
However, this antenna was designed to operate up to 
98 MHz and its performance degrades rapidly above 
this frequency. Fig. 3 shows the measured h.r.ps 
within the operating range of the antenna. The gain is 
fairly high, between 6 and 7.2 dB. By 98 MHz the 
front-to-back ratio has already decreased to about 
9 dB and it decreases further as the frequency 
increases, until, at 102 MHz, the antenna fires 
backwards. 

The input impedance was measured on a 
network analyser and the characteristic is shown in 
Fig. 4. The voltage reflection coefficient lies between 
40 - 50% over the 88 to 98 MHz band giving a 
mismatch loss of 0.8 to 1.2 dB. However, the gain of 
the antenna is sufficient to accommodate losses of this 
order and a matching network is not used. 

6.2 Performance of the new wideband design 

The h.r.p. measurements of the new antenna 
are shown in Fig. 5. As expected, the gain is fairly 
low, 4.7 to 6.3 dB. The mismatch loss must therefore 
be kept as low as possible to prevent the gain from 
being reduced. The cross-polar discrimination of the 
antenna was measured to be better than 26 dB on 
boresight. 

The input impedance characteristic, without 
any added compensation, is similar to that of the 
model, given in Fig. A-3. To improve the match to 
75 n, compensation is applied across the drive point. 
Tapping into a A./4 stub modifies the impedance 
characteristic, however, the compensating network 
causes a small shift in the impedance characteristic 
towards the open circuit end of the plot. To centre the 
impedance characteristic on 75 II, the transforming 
effect of the A/4 stub was employed. The feed was 
connected to the antenna 15 mm nearer the short- 
circuit than the dipole. The resulting impedance 
characteristic is shown in Fig. 6. 



(RA-234) 



■3- 




H- Plane 




88 M 



HZ \l 



98MHz 

102MHz 



J L 



J_ 



-1_ 



_L 



- 20 



T3 



30 



40 



20 40 60 80 100 120 140 160 180 20 40 60 80 100 120 140 160 180 
360 320 280 240 200 -— 360 320 280 240 200 -^ 

relative bearing , degrees relotive bearing , degrees 

Fig. 3 - HRPsfor original antenna design. 



150 



180 




■150' 



Fig. 4 - Input impedance characteristic for original 
antenna design (centred on 75 il). 




The final version of the broadband Yagi 
antenna is given, with its matching network, in Fig. 7 
and its dimensions are given in Table 2. 

The dimensions of the compensating stub relate 
to semi-airspaced domestic UHF cable which has a 
velocity factor of about 0.77. If a cable is used which 
has a significantly different velocity factor (i.e. greater 
than 10%), the length of the cable making up the 
compensating stub should be changed according to the 
following equation:- 

old length 
New length = . „_ — X velocity factor 

The velocity factor of antenna cable is usually 
quoted by the manufacturer. 

H- Plane 




001 



88MHz 

gSMHz 

108 MHz 



_L 



J^ 



10 



(A 

Hzo* 



Hi 
T3 



30 



40 



20 40 60 80 100 120 140 160 180 20 40 60 80 100 120 140 160 180 
360 320 280 240 220 -^ 360 320 280 240 220 - — 

relative bearing, degrees relative bearing , degrees 



(RA-234) 



Fig. 5 - HRPsfor wideband Yagi antenna. 
-4- 



Table 2 - Dimensions of Full-She Broadband Yagi Antenna 




30° 



-90° 



Fig. 6 - Impedance characteristic of a full-size broadband 
Yagi antenna with impedance transformation. 





Dimension 


Length 

m 


Element Length 


a 
b 
c 
d 


1.72 
1.5 
1.3 
1.16 


Element Spacing 


e 
f 

g 


0.77 
0.23 
0.5 


Diameter 


h 


0.01 


Compensating network 


i 

J 
k 


0.37 
0.11 
0.015 



feed 




o/c. 



1 -I- 



-^ 



m 




m 



^ 



i 



polythene 
mounting 
block 



boom 



{RA-234) 



Fig. 7 - Broadband Yagi antenna with matching network. 

-5- 



6.3 Construction of the proposed design 

The broadband Yagi antenna shown in Fig. 7 
was built using 10 mm (3/8") aluminium tube for the 
elements and 19 mm (3/4") tube for the boom. The 
elements can be connnected to the boom by drilling 
holes through each element and the boom and bolting 
them together. This is easier if the sections of tube are 
flattened slightly first. A piece of polythene was used 
to mount the dipole onto the boom. A hole drilled 
through the block of polythene allows it to slide onto 
the boom. 

Holes drilled downwards though the polythene 
block allow the elements of the dipole to be bolted on 
together with solder tags for connecting the feed. A 
photograph of the antenna is shown in Fig. 8(a). Both 
the feeder and the matching stub are made from 
lengths of 75 CI antenna cable soldered onto solder 
tags, bolted to the aluminium tube. Fig. 8(b) shows a 
photograph of the antenna around the feed. 

Another version of the antenna was built using 
standard 15 and 22 mm copper plumbing tubes and 
fittings. A photograph of the antenna is shown in Fig. 
9. This has the advantages that ihe copper and the 
feed cable can be soldered which gives a good 
electrical connection, less likely to suffer from 
corrosion effects, and the parts are readily available to 
the home constructor. 



The antenna should be clamped with the boom 
mounted horizontally, pointing towards the transmitter 
and oriented in the horizontal plane unless the sound 
transmitter is only radiating vertically-polarised signals. 
The balun prevents currents flowing in the feed cable 
from entering the feed point of the antenna. However, 
any conductor, including the feed cable, near the 
driven element will have currents induced in it and 
will re-radiate. Care must therefore be taken when 
mounting the antenna and routing the feed cable. 



7. CCIR RECOIWIMENDED PATTERNS FOR 
DOMESTIC VHF RECEIVING ANTENNAS 

The CCIR recommends a directivity curve for 
domestic VHF receiving antennas (Recommendation 
599), shown in Appendix 2. Somewhat surprisingly 
this template applies to both E-plane and H-plane 
patterns and is intended to be used in transmitter 
service planning for stereo sound broadcasting. The 
BBC recommends that stationary VHF receiving 
antennas be mounted horizontally so their E-plane 
patterns should comply with the CCIR template. 

The proposed broadband Yagi antenna des- 
cribed in this Report was designed with the aim of 
meeting the stereo directivity characteristic and 
producing a satisfactory gain. However, it was found 
impracticable to produce a design meeting this 





(«; 



Fig. 8 - Photographs of (a) the full-size VHF Yagi antenna built using aluminium tube and 
(b) a close-up of the feed arrangement 



(RA-234) 




Fig. 9 ' Photograph of the full-size VHF Yagi antenna built using copper tube. 



criterion over the whole band and on all azimuths. 
Although the proposed antenna meets the back-to- 
front ratio required, its main beam is too wide. Fig. 10 
indicates the measured performance of the antenna at 
a clear site. For planning purposes some aUowance 
also has to be made for the apparent loss of directivity 
under practical conditions as a result of local 
reflections. 

A number of commercially available, VHF 
receiving antennas with up to 6 elements were 
measured at the same clear site. They all failed to 
meet the CCIR template over the full band 88 to 108 
MHz but most do comply over a more restricted 
frequency range. This CCIR planning template is 
therefore appropriate for the E-plane performance of 
existing narrowband domestic VHF antennas but the 
proposed 4-element Yagi antenna will not meet the 
template for the increased bandwidth, 88 to 108 MHz. 

Nevertheless this antenna design is recom- 
mended for stereo VHF reception in Band II despite 
the fact that it does not comply with the CCIR 
template. Consideration should rather be given to 
revising the CCIR template to encompass practical 
antenna designs. The slightly wide main beam suggests 
that the planning template could be changed so that 
the linear fall-off meets the -12 dB limit at 70" rather 
than 60°. A minor change in the planning template 
will have consequences, but the effects on UK 
planning will be small. However, the change in 
template is greater in the H-plane and therefore the 
consequences may be greater where vertically polarised 
transmitters are planned. 

8. DISCUSSION 

A foar-element Yagi antenna has been dev- 
eloped to operate as a domestic receiving antenna for 
VHF transmissions between 88 to 108 MHz. The 
antenna was constructed using a single dipole. A 



E-Plane 



dB 



-20 



1 


1 1 1 


1 1 1 


1 


1 \\\ 


y-:?4^ 


1 / 



20 40 60 80 100 120 140 160180" 
angle relative to direction of moin response 

H- Plane 





dB 
-10 



-20 



-•^ 


—.I 1 1 1 1 1 


1 


- 


\ ^N ^\ 


- 


\ V \,x 


■ — ., 




\ ^v^,^-- 







\ / 




1 


1 1 1 \ 1 / 1 1 _ 


1 



20 40 60 80 100 120 140 160180° 
ongle relative 1o direction of main response 



CCIR recommended pattern 



108MHz 

98MHz 

88MHz 



Fig. 10 - Directivity characteristics of proposed broadband 
Yagi antenna. 



(RA-234) 



-7- 



folded dipole was not used because it is more difficult 
to construct and has the same performance as the 
single-dipole design. The full-size antenna gives an 
acceptable performance over the required bandwidth, 
see Table 3. 



Table 3 - Performance of Proposed Wideband 
Yagi Antenna 



Frequency 


88 MHz 


98 MHz 


108 MHz 


Gain 


5.3 dB 


4.7 dB 


6.3 dB 


Front-to-back 

ratio 


16 dB 


14 dB 


13 dB 


Mismatch loss 


0.4 dB 


0.2 dB 


0.4 dB 



The fronl-to-back ratio of the antenna is lower 
than the 20 dB figure aimed for but slightly better 
than that of the original recommended design. The 
gain of the broadband antenna is lower than that of 
the narrowband antenna. However, the new design 
includes a matching network which gives lower 
mismatch losses than the old design, so that the overall 
performance of the proposed broadband design is 
similar to that of the antenna recommended originally 
for narrrowband use. 

The design of a wideband Yagi antenna 
involves a compromise. In general, increasing the 
bandwidth reduces the gain. Some of the reduction in 
gain has been offset by lower mismatch losses, but at 
the expense of a more complicated feed arrangement. 
The broadband 4-element Yagi antenna has a fairly 
wide main beam and so does not conform to the 
CCIR planning template, but this is not thought to be 
a significant defect. It may be possible to design a 5- 
element Yagi antenna which will comply in the 



E-plane only but the 4-element antenna cannot meet 
the template over the whole band. 

The antenna can be constructed completely 
using aluminium or copper tube, 75 fl domestic 
antenna cable and fixings. It is not unduly sensitive to 
constructional tolerances but the mismatch loss will 
increase if the two critical element lengths (b and c) 
are not cut accurately (preferably to within + 1 mm). 

9. CONCLUSIONS 

A 4-element Yagi antenna has been developed 
as a domestic receiving antenna for stereo sound 
broadcasting in Band 11. The antenna has a satisfactory 
gain and is recommended as a replacement for the 
original antenna design issued by the BBC's 
Engineering Information Department for home 
construction. 

The directivity characteristic of the antenna 
does not comply with the template recommended by 
the CCIR which is used for planning. It was found 
impracticable to produce a 4-element Yagi design 
meeting this directivity criterion over the whole band 
and on all azimuths. It is recommended that this 
problem be raised in the CCIR so that the template is 
reviewed for the E-plane and a separate template is 
established for the H-plane. 

10. REFERENCES 

1. ITU, 1984. Final Acts of the Regional Admini- 
strative Conference for the Planning of VHP 
Sound Broadcasting, Geneva, 1984. 

2. THODAY, R.D.C., 1976. A Wideband Band II 
Aerial. BBC Research Department Report No. 
BBC RD 1976/25. 

3. JASIK, H., 1961. Antenna Engineering Handbook. 
New York, McGraw-Hill Book Company, 1961. 
Section 24.6. 



(RA-234) 



Appendix 1 
EID Information Sheet 1104(4) 



VHF RADIO RECEIVING AERIALS 



Engineering 
Information 



Dimensions for Home Construction 

The following notes are intended to assist listeners who wish to construct an aerial for vhf radio reception. 
The data given below apply to well constructed aerials using good materials, and it must be emphasized 
that the performance of multi-element aerials depends on accurate alignment of the component elements. 



Boom 




^•To transmitter 



The diagram shows a plan view; the boom and all elements must be arranged in the horizontal plane. 
If the boom is of metal the feed points P, P, which should be about one inch apart, must be insulated 
from It. A suitable material for the element is aluminium alloy tube of W outside diameter. The down- 
lead should be vhf coaxial cable. 

The recommended dimensions are as follows (A, B, C and D are for length plus diameter). 



A, 5'4" 



B, 5'0" 



C,4'9" 



D, 4'6' 



X, 2'6'^" 



Y,r5' 



The aerial can be constructed to the above dimensions to have two (A and B), three (A, B and C) or 
four elements (A, B, C and D). For monophonic reception, two-element aerials are adequate at most 
places, whereas for stereophony, three of four elements may be required. 



Engineering Information Dept, Broadcasting House 
London W1 A 1 AA 01 -580-4468 Ex 2921 



Inforination Sheet No. 

1104(4) October 1979 



For optimum matching to the downlead in aerials liaving more than two elements, the dipole (B) should be 
constructed in folded form, as indicated in the diagram below. 



4 101 




Downlead 



The midpoint of the dipole, opposite to P, P, can be anchored to the boom. 



Appendix 2 

CCIR Recommendation 599 

DiRECTiVITY OF ANTENNAS FOR THE RECEPTiON OF SOUND BROADCASTiNG iN BAND 8 (VHF) 

(Question 46/10, Study Programme 46L/10) (1963-1986) 

The CCIR unanimously recommends thai the characteristics of directivity of the receiving antennas of 
Fig. 1 can be used for planning sound broadcasting in band 8 (VHF). However, for portable or mobile reception 
of sound broadcasts, no directivity of the reception antenna should be applied in planning. 




-20 

0" 20 40 60 80 100 -120 140 160 180' 

angle relative lo direction of main response 
Fig. J - Discrimination obtained by use of directional receiving antennas. 

— monophonic-sound broadcasting. 

^— ^— ^ stereophonic-sound broadcasting. 



Note 1. it is considered that (he discrimination shown will be available at the majority of anlenna locations in built-up areas. At dear sites in open 
country, slightly higher values will be obtained. 

Note 2 The curves in Fig. l are valid tor signals o1 vertical or horizontal polarization, when both the wanted and the unwanted stgnals have the same 
polarization. 

Notes. The special Regional Conference, Geneva, 1960, and the European VHF/UHF Broadcasting Conference, Stockholm, 1961, and the African 
VHF/UHF Broadcasting Conference, Geneva, 1963, did not take (he directional characteristics of antennas into consideration for sound 
broadcasting 



Appendix 2 reproduced wilh acknowledgement lo ike CCIR 



(RA-234) 



11 



Appendix 3 
SCALE-MODEL YAGI ANTENNAS 

A scale model of the proposed Yagi antenna design was constructed using a scale factor of 1 : 3.9. 
Telescopic tube was used to make the antenna and allows the length and spacing of the elements to be altered 
quickly and easily. The model also allowed the driven element to be changed from a single to a folded dipole. A 
photograph of the model is shown in Fig. A-1. 

Baluns are used when connecting the feeds to the driven elements of the antennas. This is to prevent any 
currents induced on the outer braid of the co-axial cable affecting the directional properties of the antenna. If these 
currents are allowed to enter the feed point they would distort the radiation patterns and would reduce the front- 
to-back ratio of the antenna. 

A3.1 Yagi antenna with a single dipoie driven element 

A model was constructed with a single dipole as the driven element and the element length and spacing 
taken from Table 1. A Pawsey stub balun was used to connect the feed. This is the arrangement shown in the 
photograph of Fig. A-1. 

The h.r.ps of the antenna were measured across the frequency band and fairly good agreement was 
obtained between the measured and computed results, see Fig, A-2. The front-to-back ratio is between 14 and 
16 dB and the gain rises from 4.3 dB to 5.4 dB between the low frequency and high frequency ends of the band. 

The input impedance predicted by the computer program is shown plotted on a Smith chart in Fig. A-3. 
The measured input impedance is displayed on a network analyser, centred on 50 fl and is also shown in Fig. A-3. 
For domestic use the antenna should be matched to 75 fl which is marked with a cross (+) on Fig. A-3. The 
match can be improved by compensating the variation of the impedance with frequency using a matching network. 
The antenna could be used with the impedance characteristic shown in Fig. A-3 but a mismatch loss of up to 
1.2 dB would reduce the overall gain of the antenna. 




Fig. A- 1 Photograph of the model 
Yagi antenna. 



(RA-234) 



12- 



E- Plane 

10 



05 



344 MHz 



H-Plane 



344MHz 



02 



> 0-1 



2^0 05 



002 
001 



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20 40 60 80 100 120 140 160 180 20 40 60 80 100 120 140 160 180 
360 340 320 300 280 260 240 220 200 *- 360 340 320 300 280 260 240 220 200 
relative bearing , degrees relative bearing, degrees 



i^0 



measured 

computed 



(RA-234) 



Fig. A-2 HRPsfor the model Yagi antenna. 

-13 



150°, 



180° 



-150 




computed 

measured 



Fig. A-3 Input impedance characteristic for the model Yagi antenna. 

The input admittance, derived from the impedance characteristic normalised to 75 11, is plotted as a 
function of frequency in Fig. A-4. There is a susceptance slope, with the antenna being capacitive at low 
frequencies and inductive at high frequencies. A compensating network can be connected across the drive point 
which has a susceptance slope in the opposite direction, that is, capactive at high frequency and inductive low 
frequency. This can be achieved by tapping into a k/A stub, as shown in Fig. A-5. A length of co-axial cable was 
used to make the stub. The susceptance slope becomes steeper, i.e. more compensation is applied, as the tapping 
point is moved towards the short-circuit end of the stub. The antenna already has a Pawsey stub balun across the 




93 98 

MHz 
conductance 



108 




93 98 

MHz 
susceptance 



103 108 



s/c 



V. 



reactance slope 
increases 



Fig. A -4 Normalised input admittance. 



o/c 



I 



o/c 



s/c 



dipole 



x + y = V4 



(RA-234) 



Fig. AS Use oja k/4 compensating stub. 

- 14- 



drive point, consisting of a balanced two-wire transmission line ending in a short-circuit. It is possible to move the 
short-circuit along the balun to act as the short-circuit part of the matching network, as shown in Fig. A-6. The 
final dimensions are given in Table A-1. 

Tabk A-1: Dimensions of a 1:3.9 model Yagi Antenna with a Single Dipole Driven Element 





Dimension 


Length(m) 


Element Length 


a 
b 
c 
d 


0.438 
0.392 
0.331 
0.295 


Element Spacing 


e 
f 
g 


0.196 
0.058 
0.127 


Diameter 


h 


0.003 


Compensating network 


i 
J 


0.068 
0.045 



feeder 




dipole 



1^ 



M 




o/c 



matching 

stub 

« — i 



^Z. 



D 



i 



polythene 
mounting 
block 



boom 



(RA-234} 



Fig. A-6 Yagi antenna with a single dipole driven element and matching network. 

-15- 



The applied compensation curves the input impedance characteristic around the 75 Q point as shown in 
Fig. A-7. This gives a mismatch loss of only about 0.2 dB. 




Fig. A - 7 Input impedance characteristic for the model antenna 
with compensation 

A3.2 Yagi antenna design with a folded dipole 

The use of a folded dipole may improve the impedance match of the antenna. This is suggested for the 
original antenna design (Appendix 1). There are several ways of providing a balanced feed for a folded dipole. 
Fig. A-8 shows two methods which are easy to construct. 

The centre-fed folded dipole shown in Fig. A-8(a) has the feed co-axial cable connected across the centre 
of the folded arm. The feed cable is threaded through one half of the folded dipole and the inner is taken across 
the gap and connected onto the other half This makes it difficult to mount as the dipole is usually joined to the 
boom at the centre of the folded arm, where the feed cable enters the dipole. 

A trombone balun is shown in Fig. A-8(b). It has an advantage in that it is easily made from a length of 
feed cable and it allows the dipole to be mounted easily on the boom in the centre of the folded arm. The effect of 
a half-wavelength section of line across the feed point transforms the input impedance of the antenna. 

The folded dipole multiplies the input impedance by 4 and the trombone balun divides it by 4. However, 
the resulting input impedance is not the same as a single dipole because the folded arm of the dipole does add 
some compensation. 



3-C 



V 



(a) 



Fig. A-8 Types of balanced feeds for a folded dipole. 

(a) cenlre-fed (olded dipole 

(b) trombone (X/2) balun 



(b) 



j:pyri 1 



^ 



^ 



025X 



(RA-234) 



16- 



The input impedance characteristic of the antenna with the balun is inductive and a short open-circuit stub 
was connected across the feed points to compensate the antenna. The resulting input impedance characteristic is 
shown in Fig. A-9; the mismatch loss is low, about 0.3 dB. 

The final scale-model Yagi antenna design with a folded dipole as the driven element is shown in 
Fig. A- 10. The dimensions of the elements and the feed arrangement are given in Table A-2. 



150' 



180 




150 



Fig. A-9 Input impedance of a Yagi antenna with a folded dipole. 



feeder 




capacitive stub 

o/c 



balun 
— J - 



i 



-IX':^! 



-« f— »► 



(RA-234) 



Fig. A-10 Scale model of a Yagi antenna with a folded dipole. 

-17- 



Table A-2: Dimensions of Model 1:3.9 Yagi Antenna with a Folded Dipole 





Dimension 


Length(m) 


Element Length 


a 
b 
c 
d 


0.438 
0.392 
0.331 
0.295 


Element Spacing 


e 
f 

g 


0.196 
0.058 

0.127 


Diameter 


h 


0.003 


Compensating network 


i 
J 


0.045 
0.274 



A3.3 Sensitivity to constructionai errors 

The flexibility of the scale-model allows the performance of the antenna to be measured with small errors 
in the length and spacing of the elements. Tolerences of + 10 mm for the full-size antenna correspond to errors of 
the order of ± 2.5 mm in the model. 

HRPs of the antenna were measured with one or more element length or spacing altered by 3 mm. The 
patterns were compared at three frequencies; at the high frequency and the low frequency ends of the band and at 
the centre frequency. The small changes in the dimensions had very little effect on the radiation patterns, usually 
about 1 dB, but up to 2 dB difference in the magnitude of the back lobe. 

However, the input impedance is more sensitive to small changes in the length of the elements; in 
particular, the length of the dipole and the length of the director next to the dipole. Therefore, to keep the 
mismatch losses as low as possible, the length and spacing of these elements should be accurate to within about 
± 1 mm. 



(RA-234) 



■18- 



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