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Decided to take the plunge and get into
digital TV? For most people, it’s
simple – just plug the new TV
or set-top-box into your
existing antenna. But for
many, the old TV antenna
and cable just won’t be up
to it. Here’s a new kit from
Oatley Electronics which
could solve your digital
woes . . .
Dead
Simple
Masthead
Amplifer
Design by
Branko Justic*
Article by
Ross Tester
90 Silicon Chip
www.siliconchip.com.au
siliconchip.com.au
A
bout now, with new HD Digital
“Freeview” channels starting
up, many people are investing
in digital TV technology.
Whether that means a new, youbeaut flat-screen TV with all the bells
and whistles or perhaps a digital TV
set-top-box used in conjunction with
your old faithful telly sitting in the
corner, you should be getting a host
of new TV channels, right?
Umm, no. At the time of writing
there are only ABC2, SBS2 and OneSport even worth mentioning. One or
two more are imminent.
But the Freeview ads promised
what, 20+ new channels? That may
happen in the future but so far, the
other (commercial) networks digital
offerings are nearly identical to their
analog offerings. And as we all know,
that isn’t much of an offering.
Incidentally, Freeview have recently
been ordered to change their “misleading” advertisements . . .
Programming aside, many viewers
have found another slight problem
with digital TV: they can’t view it!
This is usually limited to areas
where the analog TV signals have
been marginal anyway but the problem
certainly manifests itself more with
digital. With analog, your picture
might be noisy or snowy but at least
it’s there and sort-of watchable.
With digital, the dreaded “digital
cliff” means your signal is either there
or it isn’t. Unlike analog, with digital
there’s virtually no “grey area”.
Just as importantly, the digital cliff
can also rear its ugly head when the
signal levels are too strong.
We’ve all heard the stories about
people living close to the transmitters
who didn’t even need any antenna to
receive TV signals . . . well, many of
these are in for an unpleasant surprise
when they try to install digital.
The digital cliff is more like a digital
plateau: there is not enough signal, so
you get nothing; there is enough signal,
so you get reception; and there is too
much signal, so you get nothing. It’s
relatively simple to attenuate the signal if you get too much, so we’re more
concerned with not enough.
Do you need a digital antenna?
There is another reason why TV
signals might be inadequate for digital:
the antenna.
With few exceptions, the antennas
we have used for years have been
siliconchip.com.au
designed to suit the (analog) channels
being transmitted in your area.
Unless you were affected, you probably don’t remember the problems
when the 0-10 Network started – but
a raft of viewers at the time complained bitterly that their TV picture,
if it existed, was way down in quality
compared to the other channels.
That’s because their existing antennas were never designed for the higher
(channel 10) or lower (channel 0)
frequencies. TV antenna design has
always been a compromise between
performance and bandwidth.
Sydney and Melbourne VHF-only
antennas, for example, were originally
designed to cover specific and limited
frequency bands: Channel 2 (about
65MHz), Channel 7 (about 185MHz)
and Channel 9 (about 200MHz). Then
along came these new channels up to
20MHz outside the band and the antennas had to cope as best they could.
In some cases, they couldn’t!
That problem has become significantly worse now that Digital TV
has arrived. Of course, most modern
antennas are cut to cover Channel 10
(Channel 0 has long since gone by the
wayside).
But now we have digital channels
“slotted in” between (and above) the
existing analog channels – frequencies
most of the antennas were, once again,
never designed to cover.
UHF TV
So far, we’ve concentrated our discussion on VHF TV – the more recent
advent of UHF TV of course demands
a completely new antenna system.
Even many of the popular early
VHF/UHF “combo” antennas tend to
struggle somewhat as their UHF sections were only ever intended to cover
UHF Band 4 (eg, SBS TV on channel
28 – about 530MHz).
With many of the UHF TV translators spread around the country up in
the very high sections of UHF Band V
(for example channel 69, at 820MHz,
nudges the mobile phone frequencies)
so any antenna to be used for these frequencies has to be designed to receive
these frequencies.
Incidentally, we covered the subject
of digital TV and its pitfalls in much
more detail in the March and April
2008 editions of SILICON CHIP.
Having said all that, in the majority
of cases, most older VHF antennas
WILL receive digital VHF TV channels
AUSTRALIAN TELEVISION
CHANNEL FREQUENCIES
Channel
Video
Number Carrier
(MHz)
0
46.25
1
57.25
2
64.25
3
86.25
4
95.25
5
102.25
5A
138.25
6
175.25
7
182.25
8
189.25
9
196.25
10
209.25
11
216.25
28
527.25
29
534.25
30
541.25
31
548.25
32
555.25
33
562.25
34
569.25
35
576.25
39
604.25
40
611.25
41
618.25
42
625.25
43
632.25
44
639.25
45
646.25
46
653.25
47
660.25
48
667.25
49
674.25
50
681.25
51
688.25
52
695.25
53
702.25
54
709.25
55
716.25
56
723.25
57
730.25
58
737.25
59
744.25
60
751.25
61
758.25
62
765.25
63
772.25
64
779.25
65
786.25
66
793.25
67
800.25
68
807.25
69
814.25
Audio
Carrier
(MHz)
51.75
62.75
69.75
91.75
100.75
107.75
143.75
180.75
187.75
194.75
201.75
214.75
221.75
532.75
539.75
546.75
553.75
560.75
567.75
574.75
581.75
609.75
616.75
623.75
630.75
637.75
644.75
651.75
658.75
665.75
672.75
679.75
686.75
693.75
700.75
707.75
714.75
721.75
728.75
735.75
742.75
749.75
756.75
763.75
770.75
777.75
784.75
791.75
798.75
805.75
812.75
819.75
November 2009 91
quite happily – maybe more by good
luck than good measurement (actually,
it’s more a factor of plenty of signal
level in the first place!).
But if they don’t, something needs
to be done. It’s either a new digitalband TV antenna, or . . .
Enter the masthead amplifier
The role of the masthead amplifier is
often misunderstood. Most imagine it
is some form of miracle worker which
can find TV signals which aren’t there
in the first place. That is simply not so.
If an antenna cannot receive signals,
no amount of amplifying is going to
fix that!
Its job is not so much to increase the
level of a received signal but more so
that other losses in the system (eg, the
coax cable downlead, splitters, etc) are
compensated for.
For instance, if you’re in a relatively
low signal area, the amount of signal
received at the antenna may be barely
adequate.
In fact, if you hook up a TV set
right at the antenna you might get a
relatively good or even quite good
picture. But by the time the signal gets
to the receiver you might have lost a
significant proportion over the length
of cable – and it may be completely
inadequate for the TV to tune, process
and produce a picture.
The masthead amplifier’s job is to
overcome this by keeping the signal
level up at the bottom of the cable.
The situation is worse at UHF than
at VHF and indeed is much worse for
high UHF channels than low, simply
because cables become progressively
more lossy as frequency increases.
There are cables... and cables
There are many types of 75 “TV
Coax” ranging from very cheap (and
usually very nasty and high loss!)
through to some very esoteric (quite
expensive!) types offering much better
performance.
For example, the (unfortunately)
popular 3C2V coax cable used extensively for imported pre-assembled
patch cords, etc, has a whopping 31dB
attenuation per 100m at 700MHz (~
channel 52).
So if you have, say, a run of 10m
from your antenna to your TV set (quite
a common length) you’re going to lose
over 3dB – more than half of any signal
your antenna has been able to sniff out.
And that’s before any losses from plugs
and sockets, splitters or anything else.
Many people in poor signal areas
have a TV antenna mounted very high,
maybe on a mast on a high point some
tens (or even more) of metres from
the home.
I’m speaking from experience here:
while my home is only 15km from the
Sydney TV transmitters at Artarmon/
Gore Hill, it is nicely shielded by a
50m-high cliff about 100m away.
Back in the VHF-only days, to get
any reception at all, I had to mount
my antenna nearly 30m off the ground
(with, of course, a masthead amplifier).
Even then, reception was ho-hum.
If the antenna was lowered by even
a metre, the picture deteriorated to
virtually nothing.
Today, with UHF translators on the
NSW Central Coast, I still need about
10m of height to get a decent picture.
That brings us to another use for a
“masthead” amplifier – maintaining
levels for signal distribution. We’ve
already mentioned signal splitters – a
typical two-way splitter can lose anywhere from 3-6dB; a three or four-way
double that and more.
If your signal is marginal to begin
with, this can easily mean the difference between picture and no picture
with digital. A masthead amp can
help a lot.
The other application is for longdistance coax paths. You may want to
send TV signals to the opposite end of
the home or maybe down to the back
shed! (We know someone who sends
TV from his house way down to his
boatshed on the river – probably 100m
or more away).
Even using the very best grade of
commercial coax, signal losses can
be intolerable. In all these cases, a
masthead amplifier can be used to
overcome cable losses in the system.
In fact, the gentleman with the boatshed uses exactly the system we are
describing here.
But keep in mind that one problem
with a masthead amplifier is that it amplifies everything it receives – wanted
signal as well as noise – so it’s very
The two sides of Oatley’s K-274 Masthead amplifier. One
end contains the input terminals – it suits 300 ribbon or 75
coax – while at the opposite end is the cable clamp to suit
75 coax output (to the TV set). The photo at right shows
the balun transformer mounted on the upper side of the PC
board which allows the two different types of cable to be
used. The board is fully populated and soldered with SMD
devices.
92 Silicon Chip
siliconchip.com.au
COAX CABLE
OUTPUT
(TO TV, ETC)
ALL COMPONENTS
ARE SMD TYPE
ALREADY MOUNTED
ON PC BOARD
TV COAX
PLUG
K
300
75
2200 F
16V
2N5819
A
6-12V
230V
AC
PLUGPACK
SUPPLY
The circuit diagram of the complete masthead amp. You only have to wire
the power supply and connect the TV coax plug (shown here in green boxes).
important that it introduces as little
of its own noise as possible.
Ready to build?
This amplifier, from Oatley Electronics, covers both the VHF and UHF
bands – in fact, significantly more than
the TV band. Actual range is from
50MHz to more than 1000MHz (1GHz).
It doesn’t discriminate between analog
and digital signals – if they are there,
they will be amplified.
The performance of this amplifier is
something to write home about. Based
on a couple of low-noise BFP67 transistors, the two-stage preamp has the
excellent noise figure of just 1.75dB.
This compares very well indeed
to most “out-of-the-box” commercial
masthead amplifiers. It wasn’t so long
ago that manufacturers used to be delighted with a noise figure of 3dB. The
better ones got into the 2-3dB range but
this one does even better.
The gain is also more than adequate.
Over the whole VHF/UHF bands, from
TV channels 1-68, it’s no worse than
26dB and is as high as 33dB. Again,
these are very good figures – much less
and you don’t have enough gain; too
much more and you risk not having
an amplifier but an oscillator!
The case has four half-moon “clamps”
to hold cables securely when closed.
We’d be inclined to plug the unused
holes with silicone sealant to prevent
insects and spiders getting in.
Pre-assembled PC board
You might have noticed from the
photographs that the PC board is
populated with surface-mount devices
(SMDs).
Don’t panic: the good news is that
the PC board comes ready built and
tested – all you need do is supply suitable power (10-15V DC) and connect
coax for your antenna and output.
The PC board can be fed from a
300or 75 TV antenna (most these
days use 75 coax cable) or even from
A worm-drive hose clamp fits nicely
into the mouldings on the back of
the case to hold it firmly to a mast. A
large cable tie could also be used.
a distribution amplifier or other source
of RF signal.
We won’t mention other signal
sources which could benefit from a
masthead amp – or in this case a distribution amp – because they might
not like you connecting an amplifier
to their box (we’re too FOXy for that).
Besides, you would have to fit an Ftype socket on the amplifier.
Bunny ears
The masthead amp mounted inside one of Oatley’s HB4 cases which were
obviously intended to be used for . . . masthead amps!
siliconchip.com.au
It is obvious that the original design
of the PC board was to act as an amplifier for “rabbit ear” antennas and you
could do the same. Performance is
often so good that you can get away
without connecting an outside antenna. (See how we modified a rabbit
ears antenna overleaf).
Once again that depends on your
location – topography and distance
from the transmitter play a major role
November 2009 93
75 (COAX CABLE)
CONNECTION
+ --
FIGURE-8 FROM
POWER SUPPLY
75 COAX CABLE
in determining whether you’ll have
enough signal.
There are two large holes in the
PC board which are intended for the
mounting screws for a pair of rabbits
hears. If you are in a strong signal area
(eg, <10km or so from the transmitter
and no intermediate hills or buildings
to block signal) try rabbit’s ears – you
could be surprised.
Remember that rabbit ears are simply a dipole antenna and, as such,
require broadside-on orientation to the
transmitter direction and horizontal or
vertical polarisation to suit the polarisation of the local transmitter.
The other thing to keep in mind is
that as frequency increases the length
of the dipole decreases – theoretically,
at the highest TV channel frequencies the telescopic dipoles should be
pretty-much collapsed. Experimentation is the key here.
And if you cannot easily obtain
telescopic rabbit ears?
Simple: use a pair of stiff wires –
even the proverbial Aussie fencing
wire will do fine or maybe that special
“cotanger” wire which every wardrobe
is full of . . .
Of course rigid wire won’t be quite
as adjustable as rabbit ears but replacement wire is pretty cheap!
So what’s left to do?
Not much! If you want to use it
as a true masthead amplifier, you
need to connect the amplifier to your
antenna. Provision is made for both
94 Silicon Chip
and second/third or more TVs down
the track.
OR
Mount it up high
300
(RIBBON OR
RABBIT EARS)
CONNECTION
Here’s how to
connect the
antenna (coax or
ribbon), coax to
the TV plug and
power to the TV
plug.
300twinlead and 75 coax cables
with a balun transformer already
mounted on the PC board.
Unless you have very good reason for not doing so (eg, you own a
twinlead factory), we would strongly
recommend you use coax rather than
twinlead.
While it has higher attenuation,
coax offers several other advantages
– eg, it doesn’t break down so easily
in the weather; it offers better protection from ghosting and interference
(twinlead has no shield so often picks
up unwanted signals along its length)
and finally, the best reason – twinlead
is becoming a lot harder to obtain!
The PC board is not clearly marked
but follow our diagrams and you won’t
go wrong. One warning: make sure you
use 75 coax, not 50!
A cheapie might save a few bob now
but it could give you grief later on,
especially if you start adding splitters
The opposite end of the PC board
has a traditional cable clamp arrangement for the coax feed to your TV.
Ideally, the masthead amp should be
mounted at the top of the mast (ie,
as close to the antenna as possible)
so it doesn’t amplify any noise in the
downlead.
If so, it should be mounted in a
weatherproof box of some description. Oatley Electronics have available a suitable box (Cat HB4, $4.00)
which was designed for masthead
and distribution amplifiers. The box
is self-locking and, as you do so, it
clamps both the input and output coax
in place. So you don’t even have to
mount the PC board – it can simply
sit inside the box.
No mounting hardware is supplied
with the box but a large cable tie or
(preferably) a worm-drive hose clamp
can pass through the bracket at the
back of the case and secure it firmly
to the TV antenna mast.
Once the box is mounted with the
coax cables coming out the bottom, it is
relatively weatherproof – but we’d be
inclined to run some silicone sealant
into the three other coax access holes
to prevent moisture getting in and also
to deter spiders and other small insects
calling your masthead amplifier home.
There may be situations where you
cannot mount the masthead amp at
the masthead. Don’t be concerned, it
should still work “down low” (ie, close
This was how Oatley Electronics
showed the power supply
construction, with the rectifier diode
and capacitor glued to the outside
of the plugpack case. We are not
enthusiastic about this method: we’d
much rather open up the case and
place the components inside, as
described in the text.
siliconchip.com.au
Improving “Rabbit Ears” Reception
We mentioned in the article that the PC board appeared
to be designed for direct use with a 300antenna. Having
recently seen first hand how patchy the performance was
from a set of Rabbit Ear antennas (with a relative in hospital),
we thought “why not?”
The Rabbit Ears antenna shown here are typical of those
available from a variety of sources for $10 or less. We show
how we modified this particular antenna; obviously the one
you get may be different but we imagine in most cases, the
principles will be the same.
We opened our antenna (four screws in the base underneath the four rubber bumper feet) and found quite a deal
of empty space inside.
Apart from a baluin (which we don’t need as there is one
on the amplifier PC board), there is also a large lead weight,
as shown below (which keeps the unit stable).
There was plenty of room to mount the PC board sideways
but we wanted to mount it end-on, so that the coax cable
could emerge from the original place.
Only a minor modification was required to the case to
achieve this – there is a moulded plastic pillar right in the
middle which was obviously intended to support some type
of board (it doesn’t support the existing balun PC board).
After unsoldering the balun PC board and discarding it, we
clipped the pillar off and smoothed the plastic out with a
hot soldering iron.
To mount the PC board, we needed to shorten it. We
did this by moving the balun back a little, drilling three new
holes in the same pads/tracks and soldering it back in place.
We then cut the PC board across the large antenna holes,
as shown in the centre photo below. A small amount of pad
must be left to allow soldering.
The board, with coax cable attached, then fits nicely with
the coax emerging in the right place.
Two short lengths of tinned copper wire connect the original solder lugs to what remain of the antenna mounting pads.
We ran some hot-melt glue over the end of the PC board
Opening the case (four screws) reveals
a balun and a lead weight. We don’t
need this balun so we discarded it.
siliconchip.com.au
not so much to hold it in place (even though it does to some
degree) but more provide insulation between the board and
the lead weight which mounts above the board. While the
weight sits on pillars giving clearance, we thought it better
to be sure, to be sure. . .
And that’s it! Where we had a fairly watchable picture in
the past, it’s now a rock-solid picture much less prone to
interference and flutter as people walk past.
One aside: the black loop in the picture above originally
contained a single length of wire connected to the same
terminals. Now we might be breaking some unwritten law
of advanced Rabbit Ear technology but we couldn’t see any
sensible reason for this loop. So when we re-assembled our
antenna, we left it out. Trust us: it works without it!
We shortened the masthead amp board
a little to help it fit better, drilling new
holes for the three balun wires.
Short lengths of wire connect the
two “ears” to what’s left of the 300
terminals – and that’s about it!
November 2009 95
clamp inside.
DC power is also connected via this
plug, which has the blocking components to make sure power doesn’t get
back into the TV set but allows power
to reach the masthead amplifier via the
coax cable. Therefore it is important to
use this plug and not simply substitute
a standard TV antenna plug.
Use the diagram and accompanying photo to show you where the DC
power is connected relative to the coax
connector.
So that’s it: nice and simple and
capable of excellent performance. SC
Where from, how much?
When you “crack” open the plugpack as described in the text, you’ll find there
is plenty of room for the diode and electrolytic capacitor. That 0.68 resistor is
a bit of a mystery – we removed it from our rebuilt supply. After testing, make
sure you re-glue the two halves of the plugpack together and change the label to
indicate it is now a 10V DC supply!
to your TV set) but performance might
not be quite as good; adequate, perhaps
even an unnoticeable difference – but
theoretically, not as good.
Power supply
Finally, we need to add a power
supply. In the Oatley kit, a small 6V
AC plugpack is included along with
a Schottky rectifier diode and a small
electrolytic for a simple half-wave
rectified supply.
Given the poor regulation of small
plugpacks and the low voltage drop
across the Schottky diode, this gives
around 9.5V DC on light loads (which
the masthead amp is – just 25mA) so
will power the masthead amplifier.
The specifications say 10-15V DC
so if you want to run a higher supply voltage you would possibly end
up with slightly better noise figures
and perhaps more gain. A half-waverectified 10V AC supply would result
in about 14V DC.
Oatley’s method of power supply
construction is, shall we say, unique:
they hot-melt glued both the diode and
electrolytic to the power supply case
and soldered the leads on.
We would much prefer a method
we’ve used in the past, when there is
room, and that is to put the rectifier
components inside the plugpack.
When the plastic welds on the power supply case are squeezed in a vyce
and tapped until the weld breaks, you
can separate both halves of the case.
96 Silicon Chip
There is plenty of room for both
the diode and capacitor at the top of
the case and all the “bitey” bits are
safely covered by the transformer and
Presspahn insulation.
When we opened the supplied plugpack in this way we were surprised to
find a 0.68 1W resistor in series with
the secondary. No, we don’t know why
either – so we removed it.
Fortunately, its mounting lug provided a perfect position for soldering
the diode in series with the secondary
(anode to the secondary). The electrolytic capacitor connects between the
diode cathode and the other end of
the secondary. A short extension to
the positive wire from the capacitor
was necessary to do so.
If you use this method, when you’ve
checked it out and it works, simply
glue the case lid back on (super glue,
contact adhesive, etc) while clamping
the two halves together. Don’t forget to
change the label to read 9V DC instead
of 6V AC (a thin-pointed spirit marker
is ideal).
Whether you choose Oatley’s easy
way or our harder but more professional way, you need to end up with
a DC supply around 9-10V or so, connected with the polarity shown in the
diagram earlier.
Connecting the supply
Also included in the kit is a special
PAL-type TV antenna socket (male
plug) connector which has a coax cable
The K274 Masthead Amplifier
kit is available from Oatley Electronics (www.oatleyelectronics.
com) for $14.90. This includes the
pre-assembled amplifier board,
power supply components and TV
antenna plug.
The masthead/distribution case
sells for $4.00 (cat HB4).
This coax TV plug, supplied with the
kit, doesn’t simply connect your amplifier to the TV set. It also contains the
components which allow you to send
DC power back up the coax cable (seen
here in grey) while stopping the power
getting into the TV set. Don’t be tempted
to substitute a standard coax connector
– your TV set might object to it. You
also need to connect DC power (from
the modified plugpack) – the positive
wire goes to the arrowed (+) side. You
might also need to cut a small notch
in the back of the plug (alongside the
coax cable cutout) to accommodate the
power wires.
siliconchip.com.au
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