BBC RD 1987/13
1 B ■ 1 1 a * T
BAND II VHF ANTENNA FOR HOME
S. Wakeling, B.Sc.
Research Department, Engineering Division
THE BRITISH BROADCASTING CORPORATION
U DC 621.396.67
BAND II VHF ANTENNA FOR HOME CONSTRUCTION
S. Wakeling, B.Sc.
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
Issued under the Authority of
Research Department, Engineering Division,
BRiTiSH BROADCASTING CORPORATION
Head of Research Department
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
UNIVERSAL DECIMAL CLASSIFICATION
II is intended thai the UDC reference (see Summary page,
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Research Reports. The classification is not generally used
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this reference be of particular concern to you would you
kindly write to:
The Research Executive,
BBC Research Department, Kingswood Warren,
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BAND II VHP ANTENNA FOR HOME CONSTRUCTION
S. Wakellng, B.Sc.
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
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BAND II VHF ANTENNA FOR HOME CONSTRUCTION
S. Wakeling, B.Sc.
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
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
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.
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.
0.1 62 A
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.
-— f — ►
- g ^
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
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
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.
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
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
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.
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.
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
New length = . „_ — X velocity factor
The velocity factor of antenna cable is usually
quoted by the manufacturer.
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
Fig. 5 - HRPsfor wideband Yagi antenna.
Table 2 - Dimensions of Full-She Broadband Yagi Antenna
Fig. 6 - Impedance characteristic of a full-size broadband
Yagi antenna with impedance transformation.
Fig. 7 - Broadband Yagi antenna with matching network.
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
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
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
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.
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
1 1 1
1 1 1
20 40 60 80 100 120 140 160180"
angle relative to direction of moin response
—.I 1 1 1 1 1
\ ^N ^\
\ V \,x
■ — .,
1 1 1 \ 1 / 1 1 _
20 40 60 80 100 120 140 160180°
ongle relative 1o direction of main response
CCIR recommended pattern
Fig. 10 - Directivity characteristics of proposed broadband
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
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).
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
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.
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.
EID Information Sheet 1104(4)
VHF RADIO RECEIVING AERIALS
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.
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).
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.
The midpoint of the dipole, opposite to P, P, can be anchored to the boom.
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.
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
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
Appendix 2 reproduced wilh acknowledgement lo ike CCIR
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
1 1 1
1 1 I
1 1 1
1 1 1
o 0-2 -
■ ^ 0-1
1 1 1
1 1 1
J I I L
T \ 1 r
— 1 — 1 —
1 [ 1
1 1 1
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
Fig. A-2 HRPsfor the model Yagi antenna.
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
Fig. A -4 Normalised input admittance.
x + y = V4
Fig. AS Use oja k/4 compensating stub.
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
« — i
Fig. A-6 Yagi antenna with a single dipole driven element and matching network.
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
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
Fig. A-8 Types of balanced feeds for a folded dipole.
(a) cenlre-fed (olded dipole
(b) trombone (X/2) balun
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.
Fig. A-9 Input impedance of a Yagi antenna with a folded dipole.
— J -
-« f— »►
Fig. A-10 Scale model of a Yagi antenna with a folded dipole.
Table A-2: Dimensions of Model 1:3.9 Yagi Antenna with a Folded Dipole
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.
Printed by BBC RESEARCH DEPARTMENT, Kingswood Warren, Tadworth. Surrey, KT20 6NP