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By LEO SIMPSON & BOB FLYNN
Build this 5-element
FM antenna
for better 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. Why not build this
5-element Yagi antenna which is designed
specially for the FM band? You’ll be amazed
at how good those stations can sound.
34 Silicon Chip
It’s amazing isn’t it. People spend
thousands of dollars on their hifi
equipment and then just hang a bit of
wire out the back of the gear to pick
up the FM stations. Well, the music
might sound first class from CDs but
it will definitely not be up to par from
the FM stations.
This doesn’t make sense really.
Most people know that they need a
good antenna for TV reception; the
same applies to FM. The same recep
tion problems that plague TV also
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.
March 1998 35
Bill of Materials
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 thick Perspex, 120 x
40 x 3mm
1 U-bolt and clamp to suit mast
4 8G x 13mm screws
2 8G x 32mm screws
2 3/16-inch Whitworth or M4
roundhead screws 70mm long
1 3/16-inch Whitworth or M4
roundhead screw 60mm 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 stainless steel
Miscellaneous
Mast and wall mounts or bargeboard mount (hockey stick style),
300Ω-to-75Ω in-line balun (for
outdoor use), semi-air spaced
coax cable (Hills SSC32 or
equivalent), plastic cable ties,
silicone sealant.
affect FM stations. You wouldn’t put
up with just a bit of wire hanging out
of your TV so why compromise with
FM signals?
If you do, you are bound to get
weak signals and multipath reception
which is the same effect as “ghosting”
on TV. Multipath causes bad distor
tion and you can only cure it with a
good antenna.
Sure, if you live in a strong signal
area, you might get away with just a
tap off your existing TV antenna but a
separate FM antenna will always give
better results. And if you changed over
to a UHF TV antenna or pay TV, you
36 Silicon Chip
What Is A Yagi Antenna?
The Yagi is not a new antenna
design by any means. It 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 (88108MHz) 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
don’t have that option. So what FM
antenna to buy? There are only a few
available and of those that are, few
are suitable for fringe areas.
To be specific, the most common
FM antenna available is a 3-element
Yagi which gives, at best, about 6dB
gain with respect to a simple dipole.
These are OK in strong signal areas but
if you want more than just a couple of
stations at reasonable signal strength,
you need more gain; ie, you need a
“fringe-area” antenna. You also need
more directivity to cope with poor
reception conditions where multipath
is a real problem.
After looking at what’s available,
we decided to design and build our
own. Actually, we ended up building
a number of variations before settling
on the design presented here.
The new antenna is a 5-element
Yagi array. It has a folded dipole, a
single reflector and three directors. It
has an estimated gain of between +8dB
and +9dB with respect to a dipole
and an improved front-to-back ratio
compared to a 3-element array.
Narrow acceptance angle
As well as an improved front-toback ratio, this antenna is also more
directional. To put it another way, it
has a narrower acceptance angle. This
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.
means that reflected signals coming
in from the side of the antenna will
be suppressed. This is worth
while
because the more suppression you
can obtain for reflected signals, the
less distorted the resulting stereo
sound will be.
In other words, if you have prob
lems with distorted sound from FM
stations (due to reflected signals or
“multipath”), you should get a notice
able improvement with this antenna.
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 twinlead
dipole wire antenna that is supplied
with many tuners. Furthermore, be
cause 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 re
ception. There will be less distortion,
better separation between channels
and less hiss in the background.
In fact, with a good FM tuner
combined with a good antenna, it
can be difficult to pick the difference
between a CD player and the same
piece of music “off air”. Does that
sound outrageous? Well, it’s not, as
far as average CDs are concerned even
though CD players have far superior
noise and distortion compared to
signals broadcast on FM.
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
(U-bolt and V-block bracket), no spe
cial hardware or fittings are required.
Making this antenna is very straight
forward. If you have all the materials
available you can probably do it in a
single afternoon.
Fig.1 shows all the details of the
5-element antenna. It shows the di
mensions 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.
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.
Screws & nuts
After a few years’ exposure to the
elements, many antennas are in a poor
state. Aluminium may not “rust” but
it does corrode, particularly in seaside
areas or in metropolitan areas where
there is a lot of industrial fallout. This
corrosion can be a lot worse if you
don’t use the right screws and nuts.
We recommend the use of stain
less steel screws, nuts and washers
throughout, whether for machine
screws or self-tappers. They do cost
more but they last indefinitely. 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
recom
mend galvanised, bright zinc
or cadmium plated steel screws. In
seaside areas these can be visibly cor
roded with just a few days’ exposure.
In rural areas, well 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
Before you start, make sure you
have obtained all the aluminium and
hardware listed in the Bill of Materi
This topside view of the antenna shows how the folded dipole is attached to the
square boom. Note the short section of aluminium tubing 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.
als. After all, 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. It is 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 substi
tute 25mm diameter tubing for the
boom. In fact, you could use 25mm
stain
less steel round tubing which
is readily available from plumbing
supply stores.
While you’re at it, cut the folded
dipole spacer which also uses the
19mm square tubing. It is 50mm long.
March 1998 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 a 70mm long machine screw, nut and split washer.
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) undersize and then ream
them out to the correct size 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 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
self-tapping screw, as shown in dia
gram 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 assem
bled and mounted on the boom first.
Making the dipole
The mast clamp and V-block assembly can be purchased from electronic parts
or automobile accessory retailers. We strongly recommend hot-dip galvanised
types if possible. Avoid cadmium plated or zinc plated clamps which can rust
quite quickly, particularly in seaside areas.
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.
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 drilling. Note that the
38 Silicon Chip
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
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. Two
short tubes, shown as diagram E on
Fig.1, are cut and shaped so that they
key in with the top and bottom ele
ments 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
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 dia
gram 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 lockwasher.
Terminals for the dipole are pro
vided with two 32mm long 3/16-inch
Whitworth or M4 screws, each fitted
with a nut and lockwasher 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 8-gauge 32mm
Do you have trouble drilling round holes? You’ll do better
by drilling the holes undersize and then reaming them out
to exact size with a tapered reamer.
long self-tapping screws which go
through the spacer and into the boom.
The top piece of the dipole is then se
cured to the boom with a 60mm long
3/16-inch Whitworth or M4 screw, nut
and lockwasher.
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 Per
spex, you could use a piece of clear
material 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 at
tached to the mast and you will need a
300Ω-to-75Ω balun to match it to 75Ω
coax cable. You can use 300Ω ribbon
if you wish and omit the balun but
to obtain the most interference-free
signal, we recommend coax cable for
your installation.
Unfortunately, many antenna
clamps are sold with a cadmium plat
ed 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 few
hundred metres from the seaside we
are very aware of just how quickly
metal hardware can rust and corrode.
If you can, buy antenna clamps that
are hot dip galvanised. These last a
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.
lot longer than the cad-plated jobs.
U-bolts and clamps intended for auto
exhaust systems are generally quite
good in this respect. But 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.
If you really want to gild the lily, go
to a ship’s chandlers and buy stainless
steel U-bolts and clamps. They’ll last
forever; well just about.
We 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.
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 dete
riorate 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 such as British Paints “Silvar”.
Installation
When you have finished your an
tenna 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 insu
lation 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 way you will minimise
ignition noise from passing traffic.
Finally, install the antenna as high
as possible above the roof and gutter
ing. 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 an
tenna. 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 main FM stations of
interest. If you are really keen, you
could consider installing a rotator, to
obtain the very best reception from
SC
all stations.
March 1998 39
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