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Build this
By
LEO SIMPSON
5-Element
Yagi Antenna
for better FM reception
What’s your FM reception like? Pretty poor? Does the music sound distorted
or are there lots of sibilants on voice? If so, you need a proper FM antenna,
not a bit of wire hanging out the back of your tuner. But they’re not real easy
to buy these days! So why not build your own? This 5-element Yagi antenna
is designed specifically for the FM band. You’ll be amazed at how good those
stations can sound when they have a strong signal.
T
hese days, many people are listening to DAB+
broadcasts and we can well understand why. There
are more stations and provided the signal is OK, the
sound will be OK too. But it will only be “OK” to the extent
that the sampling rates used by virtually all DAB+ stations
are barely adequate to give good sound quality.
Yes, yes, we know that many people now listening to
internet streaming services and they have thousands of
music stations from around the world to choose from but
again, the sound quality is just average.
If you want really good broadcast sound quality, FM
stations cannot be bettered. Of course, if you live outside
the capital cities, there is no DAB+ and so it’s FM or nothing if you want to listen to music broadcasts with good
sound quality.
And if you are outside the major cities you are going to
need a good FM antenna to get the best sound quality –
nothing else will do. Of course, living in a large city, probably with hilly surrounds and tall buildings can still mean
that you will get weak signals and “multipath” reception.
This is the same effect as “ghosting” on TV. Multipath
causes bad distortion and you can only cure it with a good
antenna.
72 Silicon Chip
So what FM antenna should you buy? They are virtually
unavailable. This was the situation when we originally
presented this antenna design back in October 1988 and
repeated in March 1998. Things have not improved since
then and we still get asked for a good FM antenna. So we
are presenting it again, with very slight changes.
The antenna is a 5-element Yagi array. It has a folded
dipole, a single reflector and three directors. It has an esti
mated gain of between +8dB and +9dB with respect to a
dipole and an improved front-to-back ratio compared to a
3-element array, the most common FM antenna in the past.
By the way, if you have an old VHF TV log periodic array it could be pressed into service but it won’t perform
quite as well as this antenna which is cut to suit just the
FM band from 88 to 108MHz.
Narrow acceptance angle
As well as a good front-to-back ratio, this antenna is also
quite directional – or to put it another way, it has a narrow
acceptance angle. Both of these factors mean that reflected
signals coming in from the sides and rear of the antenna
will be suppressed. This is worthwhile because the more
suppression you can obtain for reflected signals, the less
siliconchip.com.au
5-ELEMENT FM BROADCAST ANTENNA
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
October 2015 73
This shot shows the fixing of the dipole top element to the
boom and plate. This is done with a 70mm long stainless
steel screw which passes through the top element, then the
boom and spacers below to the Lexan plate.
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.
distorted the resulting stereo sound will be.
We’re talking here about “multi-path” reception, which
used to plague analog TV reception and cause ghosts in the
picture. With FM broadcasts, multi-path reception causes
the sound to spit and sputter, especially if a low-flying
plane passes overhead.
Apart from reducing multipath problems, the big reason
to build this antenna is to obtain lots more signal than you
would get from a random piece of wire or the common
twin-lead dipole wire antenna that is supplied with many
tuners. Furthermore, because it will be installed outside
your home, the signal pickup will be even better. In fact,
our observations show that with a good FM antenna such
as this, it is possible to pick up stations (in stereo) which
may be more than 160km away.
Finally, by feeding more signal to your tuner, even from
your strong local stations, you will improve the reception
and it will be less susceptible to interference from equipment with switchmode power supplies. There will be less
distortion, better separation between channels and less hiss
in the background.
nas 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 machine 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 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.
Tools you will need
Most enthusiasts will have all the tools needed for this
project: a hacksaw, electric drill and a vice. It would also
help if you have a drill press but you can do without this.
You will also find that a tube cutter (as used by plumbers)
will be good for making clean cuts on aluminium tubing.
Apart from an antenna clamp (U-bolt and V-block bracket),
no special hardware or fittings are required.
Making this antenna is quite straightforward. If you
have all the materials available you can probably 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 the top is a plan view showing the length of all
five elements and their spacing along the boom.
Screws & nuts
After a few years’ exposure to the elements, many anten74 Silicon Chip
Starting work
Before you start, make sure you have obtained all the
aluminium and hardware listed in the Bill of Materials.
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.
Cut the boom to length first. It is 2222mm long and made
of 19mm square aluminium tubing, which makes drilling
and assembly easier. If you are experienced in metalwork
and have access to a set of V-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 available from plumbing supply stores but it is
expensive and not easy to work.
While you’re at it, cut the folded dipole spacer which
also uses the 19mm square tubing. It is 50mm long. You
should have a piece of tubing about 120mm long left over
as scrap. Don’t throw it away. It will come in handy later.
(When reassembling this prototype antenna after years in
storage, we could not find the 50mm long spacer so we
substituted two 19mm lengths of 19mm plastic conduit
instead).
siliconchip.com.au
Note the two short sections of PVC conduit which act as
spacers between the underside of the boom and the dipole
insulator plate. Machine screws hold it all together. This is
an alternative square tube spacer shown in Fig.1.
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 70mm long
machine screws, nuts and split washers.
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 the three directors
are all the same length, ie, 1270mm.
Next, centre-punch the boom for all holes prior to drill
ing. Note that the boom is 2222mm long and the total of
the element spacings along the boom is 2182mm – see the
plan diagram on Fig.1. Mark the hole centre position for
the reflector element first, 20mm 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 diameter using a
tapered reamer.
Be careful when reaming holes out though because it is
quite easy to get carried away and then end up with holes
that are too big. 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 self-tapping 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.
shown in the accompanying photos.
The top and bottom pieces of the dipole are held at each
end with a 70mm long 3/16-inch Whitworth or M4 screw,
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 3/16-inch
Whitworth or M4 screw, nut and lock-washer.
Terminals for the dipole are provided with two 32mm
long 3/16-inch Whitworth or M4 screws, each fitted with
Making the dipole
The folded dipole is made from five pieces of 10mm
aluminium 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 using a file so that they key in with the
top and bottom elements of the dipole. Further detail is
siliconchip.com.au
What Is A Yagi Antenna?
The Yagi antenna design was developed by H. Yagi and S.
Uda at Tohoku Imperial University in Japan in 1926. In the VHF
(very high frequency) bands, of which the FM band (88-108MHz)
forms a small part, most antennas depend on electrically resonant elements, ie, elements which are a half-wavelength at the
frequency of interest.
In its simplest form, the Yagi consists of a dipole element and
an additional slightly longer parasitic element behind it, called the
reflector. More complex designs have shorter parasitic elements
in front of the dipole and these are called directors.
The reflector and directors are referred to as parasitic elements
because they also resonate over a frequency range similar to that
of the dipole. Part of the electromagnetic energy they capture is
re-radiated and picked up by the dipole. Hence the director and
reflectors add considerably to the signal which is picked up by
the dipole on its own.
By suitably dimensioning the reflector and directors, it is
possible to determine the overall frequency coverage of a Yagi
antenna, its gain and its directional characteristics. In general,
the more elements in a Yagi array, the higher will be its gain and
the smaller the forward acceptance angle.
There is a law of diminishing returns though. Above a cer
tain number of parasitic elements, no useful increase in gain is
obtainable. There is a definite trade-off between the practical
size of a Yagi and the amount of gain it provides.
October 2015 75
We strongly recommend stainless steel (or at worst hotdipped galvanised) fittings, as seen in this close-up shot
of the U-bolt and V-block. You can find stainless fittings at
automotive suppliers and ships’ chandlers.
Do you have trouble drilling round holes? You’ll do better
by drilling the holes slightly undersize and then reaming
them out to the exact size with a tapered reamer. Don’t
have a tapered reamer? Buy one – they’re handy!
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 section of square tubing, shown as a “folded
dipole spacer” in diagram F of Fig.1. The insulating plate
is secured to the spacer with two 32mm long 8-gauge selftapping screws which go through the spacer and into the
boom. The top piece of the dipole is then secured to the
boom with a 70mm long 3/16-inch 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 and you will need a 300Ω-to-75Ω balun to match
it to 75Ω coax cable.
You could use 300Ω ribbon if you wish (and also if
you can get it!) and omit the balun but to obtain the most
interference-free signal, we recommend coax cable for your
installation. Also, 300Ω ribbon deteriorates in the weather
a lot faster than coax.
Unfortunately, many antenna clamps are sold with a
cadmium-plated and passivated finish. These have 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.
If you can, buy U-bolts and clamps that are hot-dip galvanised or stainless steel, as used for car exhaust systems (or
boat fittings), as these will last a lot longer. 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.
boom be stopped up with silicone sealant. This will stop
them from whistling in the wind. Better still, you can buy
Delrin plugs to suit the square aluminium tubing. These
look neater.
It is also a good idea to paint your antenna, if you live
in an area where corrosion is a problem. 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.
Whistling in the wind!
We also suggest that the ends of all the elements and the
76 Silicon Chip
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, solar panels (photovoltaic or hot water), metal
wall siding, nearby metal guttering, electric cabling, metal
roofing or sarking (ie, reflective insulation such as Sisalation) can all 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 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 1.5 metres 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 its directors (the shorter
elements) are aimed at the main FM station(s) of interest. You may have to call the station to check where their
transmitter is located because they are seldom at the same
location as the studios.
siliconchip.com.au
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, use a magnet.
Unfortunately, the transmitters are often widely separated
around the cities so you might have to compromise – aim
at the one you most want or between them if there are two
or more. But if you are really keen, you could consider
installing a rotator, to obtain the very best reception from
all stations.
You should find the stations come in loud and clear
without too much fiddling around. If all you get is silence,
or bad reception, check that your coax cable is connected
properly and/or that it doesn’t have a short somewhere
(usually at the beginning or end).
Remember that coax cable is lossy so you want as short
a length as you can install. And speaking of the coax, it
should be firmly fixed to both the antenna boom and the
mast. Otherwise it will flap around in the wind – annoying to start with but eventually leading to the coax failure.
Use either black cable ties (don’t use white – they don’t
last) or black self-annealing insulation tape.
SC
Because the impedance of the folded dipole antenna is
300Ω and the coax cable downlead is 75Ω, an inline balun
(balanced-to-unbalanced transformer) must be used to
connect the coax cable to the driven element. It is essential
that this be made waterproof with either a matching boot
(normally supplied with balun) or, failing that, some
coaxial sealing tape. Note also how the coax is firmly
attached to both the antenna boom and to the mast pipe to
stop it flapping in the breeze.
siliconchip.com.au
A tube-cutting tool makes easy work of cutting the
aluminium rods to length, with nice square cuts. They’re
also handy if you have to cut PVC conduit. And they’re
cheap, too (we bought ours at Bunnings for less than $20).
Bill of Materials
– 5 Element FM Antenna
Aluminium
2.3 metres of 19mm square aluminium tubing with
1.8mm wall thickness
8.5 metres of 10mm diameter aluminium tubing with
1mm wall thickness
Hardware
1 piece of white Perspex, Lexan, etc, 120 x 40 x 3mm
1 galvanised or stainless steel U-bolt and clamp to
suit mast
4 8G x 13mm screws
2 8G x 32mm screws
3 3/16-inch Whitworth or M4 roundhead screws
70mm long
2 3/16-inch Whitworth or M4 roundhead screws
32mm long
2 3/16-inch Whitworth or M4 roundhead screws
19mm long
7 3/16-inch or 4mm ID split or lockwashers
7 3/16-inch Whitworth or M4 nuts
2 3/16-inch Whitworth or M4 wing nuts
2 3/16-inch or 4mm ID flat washers
Note: all screws, washers and nuts should be
AS316-grade stainless steel
Miscellaneous
Mast and wall mounts or barge-board mount (hockey
stick style),
300Ω to-75Ω in-line balun (Jaycar Cat LT-3028 plus
matching boot and F-connector),
Appropriate length semi-air spaced coax cable (Jaycar
WB-2004, WB-2006; Hills SSC32 or equivalent),
Plastic cable ties,
Silicone sealant.
October 2015 77
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