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What’s your DAB+ reception like? Spasmodic? Subject to spitting
and dropouts? You need a decent antenna rather than rely on the
nearly useless extendable whip antenna fitted to most DAB+ radios.
This 5-element Yagi antenna is specifically designed for DAB+ radio
and should markedly improve your reception. In some cases it could
mean actually receiving DAB+ signals where there are now none!
Build this 5-Element
Yagi Specifically For
By LEO SIMPSON
M
any people these days have
DAB+ radios but they are often disappointed in the reception, even though they may live
quite close to the transmitters in Australia’s capital cities.
They buy a DAB+ radio because they
have been sold on the “clean, clear
digital sound” that it is supposed to
have. But often, the results are less
than expected, mainly for two reasons.
36 Silicon Chip
First, virtually all DAB+ radios come
with an extendable whip antenna
which does double duty for FM and
DAB+ reception. Truth is, these whip
antennas are at best a compromise for
either mode.
Nor do most DAB+ radios have an
antenna socket into which you can
plug an external antenna. So the average listener is stymied – even if they
want to improve reception.
And the second reason why DAB+
sound quality is generally disappointing is the that digital data rate (ie, at
the station) is simply too low for good
quality sound.
This means that a station broadcasting on FM and DAB+ will typically sound much better on good old
“steam-driven” FM stereo multiplex
transmissions.
(And before any of our readers email
siliconchip.com.au
us about the “steam-driven” comment,
think about it. FM transmitters are
“steam-driven”; or at least, most of
them are in Australia!)
Even so, a DAB+ radio fed with a
good signal will always have a quiet
background, no “spitting” and will
sound quite clean to most ears.
So this article will describe how to
build a good DAB+ antenna and also,
tell you how to fit an antenna socket
to your radio so you can feed the improved signal into it.
By the way, if you go on-line to see
if you can buy a DAB+ antenna, you
can’t. As far as we can tell, they are
simply not available. The closest you
will come is a DAB antenna from the
UK. Don’t buy one of these as they are
not cut to suit the DAB+ transmissions
in Australia. UK DAB uses the band
from 215 to 230MHz.
Vertical polarisation
All of the DAB+ transmissions in
Australia are vertically polarised.
That means that any receiving antenna
must also be vertically polarised. That
brings about an important constraint
about how the antenna is mounted on
a metal mast, as we shall see shortly.
As with the FM antenna presented
in last month’s issue, this is a 5-element Yagi design but is half the size
of the FM antenna because it operates
at roughly twice the frequency.
All of the DAB+ transmissions
in Australia’s capital cities (Adelaide, Brisbane, Melbourne, Perth
and Sydney) are in three bands:
202.928MHz (9A); 204.64MHz (9B)
and 206.928MHz (9C). Canberra and
Darwin have experimental transmissions centred on 211.648MHz (10B).
The Yagi antenna presented in this
article is designed to cover all the
DAB+ bands in Australia. It has a
measured gain of about +12dB with respect to a whip antenna. However, that
figure is with respect to a dipole which
has a ground plane. Unfortunately, the
whip antennas in DAB+ radios do not
have adequate ground planes so their
performance is even worse.
With good “line of sight” reception,
a 5-element Yagi antenna like this
should give reception at quite long distances from the transmitter, perhaps
100km or more. However, we have
not tested this aspect. We can vouch
for the gain figure though (as detailed
later in this article).
Tools you will need
Most enthusiasts will have all the
tools needed for this project. You will
need a hacksaw, electric drill and a
vice. It would also help if you have a
drill press but you can do without this.
Apart from an antenna clamp (Ubolt and V-block), no special hardware
or fittings are required.
Tube cutter
Last month we noted that a tube cutter is a very handy tool in an antenna
project such as this. You end up with
smooth square cuts with no swarf. We
The “Digital Cliff”
Unlike analog broadcast radio (AM or
FM) but very similar to digital TV, with
DAB+ radio you have either got a signal, or you haven’t. There’s virtually no
middle ground!
It’s the dreaded Digital Cliff – the metaphor is that if you don’t have enough
signal (or, perhaps surprisingly, too
much signal causing overload) digital
radio or TV falls off the cliff and you
get nothing.
Of course, either digital radio or TV
might be trying to tell you it’s struggling,
with continual dropouts and break-up
when it’s just on the point of teetering
over the edge – most listeners or viewers would not tolerate this and turn off.
It’s often found that weather can also
push reception over the cliff.
The big advantage of this project is
that the gain of the Yagi can lift your
signal to the point where you get reliable levels; enough to give you good
reception.
The other side of this is that DAB+ is
currently transmitted only in the mainland capital cities – even those in closeby urban centres (Central Coast/Illawarra/Blue Mountains around Sydney
and Geelong/Mornington Peninsular
around Melbourne, for example) find
DAB+ signal levels are just too low for
reliable reception.
With 12dB of gain, we would expect
this antenna will bring a lot more listeners into the DAB+ family!
Compare this DAB+ Antenna with the
FM Antenna published last month: it’s
about half the size. Funny, that!
siliconchip.com.au
November 2015 37
The ends of the folded dipole are fabricated using 42mm
lengths of aluminium tubing shaped to mate with the upper
and lower pieces. They are held together with 50mm long
machine screws, nuts and split washers.
The dipole insulator plate has wing nut terminals to
connect 300Ω ribbon or a 300Ω-to-75Ω balun. The plate is
made from Perspex, Lexan or other acrylic material. The
square boom makes mounting easy.
This “Waterfall” screen grab shows the signal picked up by the 5-element
antenna from a location in Sydney’s Arncliffe, aimed at the Redfern DAB+
transmitter (<5km away). It shows the signal levels of the three bands (9A, 9B &
9C). This measurement was taken by the Signal Hound USB Spectrum Analyser
(from Silvertone Electronics [www.silvertone.com.au]).
used a Bunnings product, the Haron
Model STC330N. When using this cutter, it is important not to rush the job.
Mark the position of the cut on the tube
with an HB pencil and then position
the blade of the cutter precisely on the
mark, with the tube sitting between
the rollers. Apply very light pressure
with the knob of the cutter and then
measure from the end of the tube to
the blade of the cutter, to make sure
you are cutting to the exact length you
want (to be sure, to be sure!)
Actually, if you have not used one
of these cutters before, do a couple
of practice cuts on scrap plastic conduit, just to get the feel of the whole
procedure.
You are also likely to find that because the tube is very smooth and quite
small in diameter, it is hard to get a
grip on it as the cut deepens. Gripping
the tube with a rubber kitchen glove
makes it a lot easier.
Buying the alumimium
By comparison, this Spectrum Analyser display was taken with a vertical whip
antenna adjusted to 368mm long (1/4 wavelength) and with a substantial ground
plane. As you can see, the received signal level is about 12dB less than that
received with the 5-element Yagi described here.
38 Silicon Chip
For convenience we purchased the
10mm round aluminium tubing and
19mm square aluminium tube from
the local Bunnings warehouse.
They stock the 19mm square tube in
3-metre lengths and the 10mm tubing
in 1-metre lengths. So we purchased
six 1-metre lengths of the 10mm tube
and one 3-metre length of the 19mm
tube. Total cost: $33.98.
You might be able to purchase your
aluminium from a nearby metal supplier and in that case, they might
also cut it to the various lengths you
will need (perhaps for a small extra
charge?).
One drawback of buying tube from
Bunnings is that every item you pursiliconchip.com.au
ements, many antennas are in a poor
state. Aluminium may not “rust” but
it does oxidise, particularly in seaside
areas or in metropolitan areas where
there is a lot of industrial fallout.
Corrosion will also be a lot worse
if you don’t use the right screws and
nuts. We strongly recommend the
use of stainless steel screws, nuts and
washers throughout, whether for ma-
chase has an adhesive label attached
which is obviously meant to last for
longer than the life of the product! At
least, it seems that way when you are
trying to remove seven labels – it took
ages! At least I ended with highly polished tubes!
Screws & nuts
After a few years’ exposure to the elREFLECTOR
750mm LONG
FOLDED DIPOLE
692mm LONG
SECOND DIRECTOR
664mm LONG
FIRST DIRECTOR
670mm LONG
chine screws or self-tappers. They do
cost a little more but they last indefinitely. You will find a good array of
stainless steel screws available from
ships’ chandlers.
Don’t, on any account, use brass
screws. When used to attach aluminium elements these will corrode away
almost before your eyes. Nor do we
recommend galvanised, bright zinc or
THIRD DIRECTOR
670mm LONG
CL
B
26
HOLES FOR
V-BLOCK
CLAMP
(TO MAST)
26
BOOM:
1250mm LONG
120
A
A
A
F
A
CL
26
26
B
50
300
125
CL
ALL ELEMENTS CUT FROM 10mm OD ALUMINIUM TUBING
340
340
6
D
300
300
200
692
288
6
26
332
FOLDED DIPOLE INSULATOR
(MATERIAL: 3mm THICK ACRYLIC)
(HOLES 4mm IN DIAMETER)
28
288
26
12
332
6
DIPOLE UPPER ELEMENT
(1 REQUIRED)
(HOLES 4mm IN DIAMETER)
6
DIPOLE LOWER ELEMENTS
(2 REQUIRED)
(HOLES 4mm IN DIAMETER)
13mm LONG 8G STAINLESS
STEEL SELF-TAPPING SCREW
ELEMENT
19
27
E
BOOM
DIPOLE END SPACERS
(2 REQUIRED)
MATERIAL: 10mm OD
ALUMINIUM TUBING
M4 x 50mm SCREW
F
DIPOLE CENTRE SPACER
(19mm LENGTH OF
19mm OD PVC CONDUIT)
A
DETAIL OF DIRECTOR & REFLECTOR
MOUNTING TO BOOM
M4 x 50mm SCREW
BOOM
UPPER ELEMENT
UPPER ELEMENT
M4 x 32mm SCREW
M4 x 19mm SCREW
E
LOWER ELEMENT
M4 NUT & LOCKWASHER
B
FOLDED DIPOLE END DETAIL
(ALL DIMENSIONS IN MILLIMETRES)
M4 x 32mm SCREW
F
M4 x 19mm SCREW
D
M4 NUT & LOCKWASHER
LOWER ELEMENT
M4 NUT &
LOCKWASHER
M4 NUT & LOCKWASHER
FLAT WASHER
FLAT WASHER
M4 WINGNUT
M4 WINGNUT
CENTRE OF DIPOLE ASSEMBLY DETAIL
Fig.1: this diagram shows all the details of the 5-element antenna. At top is a plan view while the other diagrams show
hardware and element mounting details.
siliconchip.com.au
November 2015 39
This topside view of the antenna shows how the folded
dipole is attached to the square boom. Note the short
section of PVC conduit which acts as a spacer between
the underside of the boom and the dipole insulator plate.
Whitworth or other machine screws hold it all together.
cadmium-plated steel screws.
In seaside areas these can be visibly corroded with just a few days’
exposure.
In rural areas, away from the sea or
city pollution, you can probably get
away with galvanised screws but the
antenna will last longer if you paint it.
Starting work
Constructing this antenna is quite
straightforward. If you have all the
materials available you can probably
The reflector and director elements are attached to the
boom using self-tapping screws. Ideally, all screws, nuts
and washers should be stainless steel to avoid corrosion.
To check that screws are stainless, take a magnet with you
when buying them. If they attract, they ain’t stainless!
do it in a couple of afternoons.
Fig.1 shows all the details of the
5-element antenna. It shows the dimensions of all the elements and the
various hardware bits you will have
to make to assemble the antenna. At
top is a plan view showing the length
of all five elements and their spacing
along the boom.
Before you start, make sure you have
obtained all the aluminium and hardware listed in the Parts List. You will be
frustrated if you get half-way through
and find you can’t progress further because you lack screws or some other
item. Get ’em all before you start.
You need to cut the 19mm square
tube (the boom) to length and then
mark it for drilling and this is where
it is quite easy to make mistakes.
If you are experienced in metalwork
and have access to a set of vee-blocks
and a drill press, you could substitute
25mm diameter tubing for the boom.
In fact, you could use 25mm stainless
steel round tubing which is readily
Current DAB+ Transmission Sites around Australia (as at 5 October 2015)
City
Transmitter Location
Adelaide
TXA Crafers Site Tower 115 Mount Lofty Summit Road CRAFERS
Brisbane
Brisbane
TXA T-Site Tower 445 Sir Samuel Griffith Drive MOUNT COOT-THA
Broadcast/Comms Tower Digital Distribution Australia Site Mount Mee Rd MOUNT MEE
Melbourne
Melbourne
Melbourne
Melbourne
Melbourne
TXA Ornata Road Site Tower 12 Ornata Road MOUNT DANDENONG
Broadcast/Comms Monopole Pioneer Concrete Site 213 Boundary St PORT MELBOURNE
Broadcast/Comms Tower Roof 101 Collins Street MELBOURNE
Crown Castle Site Bald Hill off Swans Rd DARLEY
Tower Broadcast Australia Site Eyre Road MOUNT DANDENONG
Perth
Perth
Perth
Crown Castle Site Cnr Mulgrave Loop and Bergen Way MINDARIE
Central Park 152 to 158 St Georges Terrace PERTH
TXA Carmel Site 255 Welshpool Road East CARMEL
Sydney
Sydney
Sydney
Sydney
Sydney
Sydney
Sydney
Sydney
TXA Artarmon Site Tower 192-196 Hampden Road ARTARMON
TXA Willoughby Site Tower 15 Richmond Avenue WILLOUGHBY
Broadcast/Comms Tower Sydney Tower Westfield Centrepoint 100 Market Street SYDNEY
Broadcast/Comms Monopole Crown Castle Site Plateau Park off Blandford St COLLAROY PLATEAU
Broadcast/Comms (Optus) Tower Sydney Water Board Site off Plateau Rd BILGOLA PLATEAU
Broadcast/Comms Tower Rooftop Tower 1 GCA Building 1 Lawson Square REDFERN
Broadcast Monopole Aust Radio Network Site 754-768 Hawkesbury Rd HAWKESBURY HEIGHTS
Crown Castle Site Badgelly Hill off Badgally Rd GREGORY HILLS
Canberra and Darwin currently have limited “experimental” transmissions
40 Silicon Chip
(source: ACMA)
siliconchip.com.au
available from plumbing supply stores
but it is expensive and not easy to work.
Centre-punch the boom for all holes
prior to drilling. The boom is 1250mm
long and the total of the element spacings along the boom is 1100mm – see
the plan diagram on Fig.1. Mark the
hole centre position for the reflector element first, 125mm from one
end of the boom, and then work your
way along.
If you have a drill press which lets
you drill all the element holes square
through the boom you are fortunate. If
not, mark the hole centre positions on
both sides of the boom and drill from
both sides. If you don’t get the element
holes lined up properly, you will have
the elements skew-whiff.
A few words of advice on drilling is
appropriate here. Drilling in thin wall
aluminium tubing can be a problem
and many people tend to end up with
holes that are more triangular than
round. The way around this problem is
to drill all the large holes (ie, all 10mm
holes) under size and then ream them
out to the correct size using a tapered
reamer.
Don’t drill the larger diameters with
too high a speed otherwise there may
be a tendency to produce “triangular” holes.
If you have a bench drill which allows you to set slower drilling speeds,
so much the better. Either way, it is best
to drill the element holes to 10mm and
then slightly increase each hole with
a tapered reamer so that each element
is held firmly in the boom.
Reaming larger holes
Be careful when reaming holes out
though because it is quite easy to get
carried away and then end up with
holes that are oversize. Use a scrap
piece of 10mm tubing to test when
the holes specified at 10mm are the
correct size.
Each director element and the reflector is held in the boom with a selftapping screw, as shown in diagram A
of Fig.1. Drill a 3mm hole at the centre
point of each element but only through
one side. Don’t mount the elements on
the boom yet though because the dipole
should be assembled and mounted on
the boom first.
You need to keep a mental image
of how the finished antenna will appear. All the directors, the folded dipole and the reflector will all be vertical (ie, perpendicular) but the holes
siliconchip.com.au
for the clamp, at the reflector end of
the boom, will be horizontal.
Confused? Take another look at the
photos of the finished antenna.
Now cut the 10mm diameter tubing
for the director, three reflectors and
parts for the dipole. Remember the
old adage about “measure twice and
cut once”. It’s hard to lengthen elements that are too short. Note that two
of the directors are the same length,
ie, 670mm while the other is 664mm.
Making the dipole
The folded dipole is made from five
pieces of 10mm alumini
um tubing,
three long and two short. The detail
of its assembly can be seen from the
diagram at the bottom of Fig.1. The
two short tubes, shown as diagram E
on Fig.1, are cut and shaped so that
they key in with the top and bottom
elements of the dipole. Further detail
is shown in the accompanying photos.
The top and bottom pieces of the
dipole are held at each end with a
60mm long M4 screw (or 3/16-inch
Whitworth), together with a nut and
lock washer. At the centre, the lower
halves of the dipole are terminated on
an insulating plate (shown in diagram
D of Fig.1). This plate is made of 3mm
acrylic (Perspex or Lexan). The dipole
halves are each secured to the insulating plate with a 19mm long M4 (or
3/16-inch Whitworth) screw, nut and
lock-washer.
Terminals for the dipole are provided with two 32mm long M4 or 3/16inch Whitworth screws, each fitted
with a nut and lock-washer plus a wing
nut and flat washer.
The insulating plate is secured to
and spaced off the main boom via a
19mm length of 19mm PVC conduit,
shown as a “folded dipole spacer” in
diagram F of Fig.1.
The top piece of the dipole is secured
to the boom with a 60mm long 3/16inch Whitworth or M4 screw, nut and
lock-washer
The details of the dipole insulating
plate and fixing to the boom can be
seen in the accompanying photos. Note
that while we used white Perspex, you
could use a piece of polycarbonate if
that is what you have on hand. However, note our remarks on painting,
later in this article.
By this time the antenna looks just
about complete. You need to add the
antenna clamp, to enable it to be attached to the mast. This must be
Parts List –
DAB+ Antenna
Aluminium
1.25 metres of 19mm square tubing
with 1.2mm wall thickness
4.5 metres of 10mm diameter tubing
with 1mm wall thickness
Hardware
1 120 x 40 x 3mm white Perspex
1 stainless steel or galvanised U-bolt
and V-clamp to suit mast
4 8G x 13mm self-tapping screws
3 M4 x 60mm or 3/16in Whitworth
screw (round head)
2 M4 x 32mm or 3/16in Whitworth
screws (round head)
2 M4 x 19mm or 3/16in Whitworth
screws (round head)
7 M4 or 3/16in nuts
2 M4 or 3/16in wing nuts
7 M4 or 3/16in lock washers
2 M4 or 3/16in flat washers
Note: all screws, washers and nuts
should be AS316-grade stainless
steel
1 19mm long spacer cut from 19mm
conduit or similar
Miscellaneous (sizes/lengths to suit)
Mast and wall mounts or barge-board
mount (hockey stick style)
300Ω to-75Ω in-line balun (Jaycar
Cat LT-3028 plus matching
F-connector)
Quality 75Ω coax cable (Jaycar WB2006/9, Hills SSC32 or equivalent)
Plastic cable ties
Silicone sealant
mounted at the end of the boom (ie,
behind the reflector) and oriented to
allow the elements to sit vertically.
You will also need a 300Ω-to-75Ω
balun to match it to 75Ω coax cable.
You could use 300Ω ribbon if you
wish and omit the balun but to obtain
the most interference-free signal, we
recom
mend coax cable for your installation.
Unfortunately, many antenna clamps
are sold with a cadmium-plated and
passivated finish (which look like a
“gold” finish). This is barely adequate
for inland areas but rusts quickly in
sea air. We may seem to be paranoid
about corrosion but since the SILICON
CHIP editorial offices are only a kilometre or so from the seaside we are
very aware of just how quickly metal
hardware can rust and corrode.
November 2015 41
Connecting your DAB+ Antenna
OK, so you’ve finished your DAB+ Yagi and now you’re
ready to connect it to your receiver. Most DAB+ tuners do have an antenna input socket, so that makes
it easy.
But the vast majority of DAB+ radios (especially the smaller ones and virtually all portables) are
not equipped with any form of antenna input, apart
from the whip antenna.
So how can you connect an external antenna?
What are the connection options?
The V-block/U-bolt clamp should ideally be stainless steel
or at worst hot-dip galvanised (don’t use Cadmium-plated
[or “passivated”] steel – you can see them rust before your
eyes!). It must mount between the reflector (seen here on
the right) and the end of the boom, so that the mast doesn’t
interfere too much with signal. Note the orientation – it
mounts so that the elements are vertical when the clamp is
attached to the mast.
If you can, buy U-bolts and clamps that are stainless
steel, as used for car exhaust systems (or boat fittings), as
these will last a lot longer. At minimum, choose hot-dip
galvanised.
Be aware that zinc “plated” fittings are not as rust resistant as galvanised types. Zinc-plated fittings have a smooth
bright appearance while hot-dip galvanising is unmistakable – it has quite a rough appearance.
We also suggest that the ends of all the elements and the
boom be stopped up with silicone sealant. This will stop
them from whistling in the wind. (Commercial antenna
manufacturers tend to squash the ends flat for this purpose).
Better still, you can buy Delrin plugs to suit the square
aluminium tubing. These look neater.
If you live in an area where corrosion is a problem, it is
also a good idea to paint your antenna. If nothing else, the
dipole insulating plate should be painted as acrylic material does deteriorate in sunlight (ie, UV).
We suggest you leave the antenna for a month or so to
weather it and then paint it with an etch primer. Finish it
with an aluminium loaded paint.
Installation
When you have finished your antenna you need to carefully consider its installation. There is no point in going
to a lot of trouble making it if you don’t install it properly.
Try to install your new antenna well away from existing
TV antennas as these can have quite a serious effect on the
performance.
Similarly, nearby metal guttering, electric cabling, metal
roofing or sarking (ie, reflective insulation such as Sisalation) can have a bad effect on antenna performance.
And don’t forget the effect of a hot water tank which may
be lurking just beneath the roof tiles.
If you live on a busy street, try to install your antenna
as far away as possible from the traffic side of your house.
That will help minimise ignition noise from passing traffic.
Finally, install the antenna as high as possible above
the roof and guttering. If that is a problem, try to install
42 Silicon Chip
The antenna (and the coax lead in) require a
connection to both the antenna input and earth.
And with very few exceptions, there is nowhere on
the outside of the case to get an earth connection.
So you need to install a socket on the back of
the receiver which the cable from the antenna could
be plugged into.
Fitting a socket
Open up your receiver and find an appropriate
place to mount a socket. While the back panel of
our receiver is quite spartan, inside the possible locations are limited by existing fittings and
hardware.
Two main points to watch when choosing a
location are:
(a) mount it as close to the receiver’s input
terminals as possible to keep your input wires
as short as possible (we are using ordinary
hookup wire, not coax); and
(b) check inside the receiver to make sure
the mounting position will not foul any internal
hardware – and that you have enough room
to fit any washers/nuts to hold the sockets in
place.
The type of socket is up to you – those receivers
which do have input sockets often use standard 3.5mm
“audio” types because they’re nice and small. However, we
elected to use a standard “Belling Lee” socket (sometimes
called a “PAL” socket) as used on TV antennas, mainly
because it seemed appropriate to use the “normal” plug/
socket for 75Ω coax.
Fit the socket first, then wire the connections to the
receiver. We chose to connect to the bottom end of the
whip antenna mounting rather than the receiver’s PCB because it was the shorter connection; the “braid” connection from the socket connected to a solder lug under the
same self-tapping screw which connected the receiver’s
ground plane.
We couldn’t find a solder lug large enough to fit under
the antenna mounting, so effectively made one by winding a double loop of hookup wire about 7mm in diameter
and applying solder to that, making it rigid. We were fortunate when we removed the nut holding the whip antenna
in place, there was plenty of “meat” left on the thread to
allow our “solder lug” to fit on as well.
Our photographs explain all this in detail.
siliconchip.com.au
to your DAB+ Receiver
Opening just about any DAB+ receiver will reveal a PCB
containing the DAB+ receiver module (the silver box
middle of lower pic) with whip antenna connections
close by. We elected to attach our external socket to the
other end of the antenna cable because it was simpler.
The photo below shows an enlargement of the area where
we located the antenna socket. Be careful that you allow
enough room for the socket and for its mounting screws –
ours will end up very close to that vertical pillar.
And here is that antenna socket, wired into the mounting
hardware for the whip antenna (the blue wire) and to
a solder lug connecting to the ground plane and the
antenna wire braid (green wire). We used ordinary
hookup wire to make the connections.
siliconchip.com.au
Finally, here is what the antenna socket looks from the
outside. We used a standard Belling-Lee connector, just
as you would find used on the antenna socket on most
TV sets. A matching plug was wired to the end of the
coax cable coming from the new DAB+ Antenna.
November 2015 43
the antenna so that it is at least a half
wavelength away from the nearest metallic object such as guttering or roofing. This means a distance of about
750mm away from guttering.
Take care when installing the antenna. Safe working with ladders is
particularly important. Take your time
and don’t take risks. You don’t want
to end up in hospital.
Line up the antenna so that it is
aimed at the closest DAB+ transmitter. As you can see from the table on
page 40, they can be fairly widely separated but all contain the same stations
(in a geographic area) so simply aim
at the one that is either closest to you,
or the one that gives you best performance (eg, minimal errors shown on
the DAB+ station readout).
More often than not, they will be
the same transmitter.
Choosing coax cable
You probably know that there is a
wide range of prices for coaxial cable,
ranging from cents per metre to dollars
per metre. What’s the difference and
why is it important? When it comes to
coax quality, price is usually a pretty
good guide.
Apart from the coax impedance (you
want 75Ω), the main criteria you look
for is attenuation, or loss. Unfortunately, all coax is lossy – this means that
even if you get the last microvolt of
signal from your antenna, depending
on the quality of the coax lead, some
of it/a lot of it/most of it can be lost on
the trip to your receiver.
As a matter of interest, when we
were checking out the performance
of this antenna, we were getting diabolically bad results. It turned out that
the length of coax cable was crook –
really crook!
Swapping to a new length of cable
gave us the results we were expecting
(actually better – but we would say
that, wouldn’t we!).
Cheaper coax has a solid plastic
dielectric, mid-range has an extruded pattern which is mostly air (hence
“air-core”); the best domestic coax
has “foam core” dielectric, (which has
minimum loss) and the outer conductor is not only pretty tightly woven (for
minimum loss) it also has one or more
levels of aluminium foil surrounding
the copper mesh (for minimum loss
AND to minimise interference!)
Attenuation is expressed in dB/100m
and increases as frequency increases.
Because we’re not talking super high
frequencies (~200MHz) you can be a
bit less fussy in selecting coax.
But really, if you’re making this antenna because you need all the signal
you can get to avoid the digital cliff
– and especially if the length of coax
lead-in needs to be relatively long
– buy the best coax you can afford,
within reason.
When the antenna is mounted on
its mast, make sure the coax is firmly
secured to that mast (and to the antenna boom) with cable ties, otherwise
the cable can flap around in the wind.
There is nothing more annoying than
lying in bed late at night and listening
to the cable slapping against the mast!
Cable ties are cheap: use them!
Performance
Before conducting any measurements, we hooked the Yagi up to a
“generic” DAB+ radio in a known very
poor signal area – Narrabeen, in Sydney’s northern beaches area.
In fact, until recently there was
no DAB+ in Narrabeen. The “new”
Collaroy Plateau transmitter has alleviated this to some extent but our test
location is deep under the plateau escarpment – anything but line-of-sight.
What a difference!
The Yagi pulled in quite a few stations which were simply not there before; of those that were, signal levels
were much improved.
Then, to prove the performance of the
DAB+ Yagi, we took a number of spectra using the Signal Hound Spectrum
Analyser (as reviewed in the October
2014 [siliconchip.com.au/Issue/2014/
October/Signal+Hound+USBSA44B+Spectrum+Analyser] and
June 2015 [siliconchip.com.au/Issue
/2015/June/SPIKE%3A+Improved+S
oftware+For+The+Signal+Hound]issues).
Two of these graphs are shown on
page 38 – they confirmed our first tests
and demonstrate that the new antenna
has a gain of around 12dB with respect
to a whip antenna – that’s pretty good
and well worth the investment!
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