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Articles in this series:
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Antenna & RF preamp
for weather satellites
Here’s the third article in our series on receiving and
decoding the VHF APT signals from weather satellites. It
describes an easy-to-build “turnstile/reflector” antenna
plus an RF preamp designed to mount up near the
antenna to improve the signal-to-noise ratio.
By JIM ROWE
A
S MENTIONED IN the first of
these articles, you don’t need a
high-gain tracking antenna to receive
the 137.5MHz or 137.62MHz APT
(automatic picture transmission) signals from the polar orbiting weather
satellites. A fixed antenna will do the
job but you do need one with an approximately hemispherical reception
pattern. It also needs to be capable of
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receiving circularly-polarised signals,
because the signals from the weather
satellites use this format.
There are three main antenna types
that meet these requirements but two
of them – the quadrifilar helical (QFH)
antenna and the Lindenblad – are not
at all easy to build. The antenna we’re
describing here is the third type which
is usually described as a “turnstile/re-
flector” (T/R) or “crossed dipoles with
reflector” antenna. In fact, we decided
to go with this type after building a
Lindenblad and getting quite disappointing results.
As a bonus, the T/R antenna is much
easier to build than the other two types
and is also less critical about the type
of roof it’s mounted above - although
it should still be mounted as high as
January 2004 33
Antenna Parts List
4 500mm lengths of 10 x 3mm
aluminium strip
1 82 x 80 x 55mm polycarbonate
sealed box
1 75 x 76 x 52mm PVC junction
box with one inlet
1 PVC conduit thread adaptor
1 73 x 75mm rectangle of 3mm
perspex sheet
4 25mm long untapped spacers
4 32mm long M3 machine
screws
8 M3 x 10mm machine screws
with nuts & lockwashers
4 M3 solder lugs
2 2470mm lengths of 10 x 3mm
aluminium strip
2 1300mm lengths of 16 x 3mm
aluminium strip
1 U-clamp and V-block assembly
1 2.4mm length of 32mm OD gal
mast pipe (optional)
6 M4 x 12mm machine screws
with nuts & lockwashers
1 360mm length of 75Ω coaxial
cable (phasing loop)
1 360mm length of 50Ω coaxial
cable (matching section)
1 75Ω coaxial ‘TV’ plug, line type
(Belling-Lee)
1 75Ω coaxial ‘TV’ socket, line
type (Belling-Lee)
1 length of 75Ω coaxial cable (to
suit)
possible, so that it has the largest possible unobstructed view of the sky in
your location.
As you can see from the photo, the
antenna is fairly simple. The “active”
elements consist of two horizontal
half-wave dipoles which are crossed
(ie, at right angles to each other), with
their feed points connected together
via an electrical quarter-wave length
of 75Ω coaxial cable. This introduces
a 90° phase shift at the reception
frequency and it’s this phase shift
that allows the antenna to receive
circularly-polarised signals.
The active elements are mounted
roughly 0.3 of a wavelength (0.3λ)
above a pair of matching crossed reflectors in a square frame. These reflectors
give the antenna a roughly hemispheri34 Silicon Chip
cal reception pattern, which can be
modified to some extent by varying the
spacing between the reflectors and the
active elements. Reducing the spacing gives more gain directly upwards
and poorer coverage at lower angles.
Conversely, increasing the spacing
reduces the vertical gain – eventually
to a null – and also gives other lobes
and nulls.
We used 10 x 3mm aluminium
strip to make the active elements and
also to make the frame that’s used to
secure the reflectors. The reflectors
themselves were made from slightly
stronger 16 x 3mm aluminium strip.
The construction details should be
fairly clear from the diagrams – see
Figs.1 & 2.
As shown on Fig.1, the active elements are all 500mm long. This gives
dipoles a whisker (1.5%) shorter
than they should theoretically be for
an end-corrected half-wavelength at
137.5MHz. However, it also means
that all four elements can be cut from
a standard 2m length of the aluminium
strip. The difference is not significant
in practice.
The inner ends of each dipole element are mounted on a 73 x 75mm
rectangular plate of 3mm perspex
sheet, which is cut into a “fat” cross
shape and drilled as shown. The
3.5mm holes are used for mounting the dipoles on the perspex plate
(using 12mm x M3 screws and nuts)
and also for mounting the complete
assembly inside an 82 x 80 x 55mm
polycarbonate box. The box specified
has a sealing groove and strip around
the lid for weatherproofing.
The complete assembly is held
inside the box using four M3 x 32mm
machine screws, which mate with
the threaded inserts moulded in the
bottom of the box. Untapped spacers
25mm long ensure that the assembly
sits so that the active elements leave
the box (via small slots cut in the centre of each side) with their top surfaces
very close to the top edge of the box
sides. Then just before the box lid is fitted, small strips of neoprene or rubber
are placed on the top of each element,
so the box sealing is preserved.
The larger 6.5mm holes in the perspex plate are to accept the two ends of
the cable phasing loop, along with the
end of the 50Ω matching cable section.
Because the phasing loop is a little too
long to be coiled up inside the box, it
loops out and back in again through a
pair of holes drilled in the bottom of
the box (about 40mm apart). The holes
should be made only just large enough
to accept the 75Ω phasing cable, so it
won’t be easy for moisture to find its
way in. You might also like to seal
around the cables with neutral-cure
silicone sealant when the antenna is
complete.
Like the phasing loop, the matching
cable section is 360mm long. This represents a quarter wave at 137.5MHz,
corrected for the cable’s velocity factor
(0.66). However, the matching section
is cut from 50Ω cable, which makes it
act as an impedance transformer. The
37.5Ω impedance of the two dipoles
in parallel is thus transformed into an
effective 75Ω impedance, so that the
signal can be fed down to the preamp
or receiver via standard 75Ω cable.
The 50Ω matching cable doesn’t exit
from the box through another external
hole but instead passes down through
a shortened PVC cable thread adaptor.
This adaptor is also used to couple the
top box to a 75 x 76 x 52mm single
inlet plastic junction box, used in turn
to mount the combination on the top
of the 32mm mast. It also provides an
“access hatch” to the 75Ω connectors
which couple the 50Ω matching cable
to the main 75Ω downlead, just down
inside the mast.
Initially, we were going to mount the
preamp inside this lower box as well
but this would have been a very tight
squeeze. It would also have meant trying to coil up the 50Ω matching cable
inside one or other of the two boxes,
which would be tricky as well.
Note that the PVC cable thread adaptor which is used to couple the two
boxes together is shortened by cutting
off most of the sleeve section which
is normally cemented over the end
of a conduit. By cutting this section
off, you’re left with a large-diameter
hollow PVC “bolt”, with a mating
PVC nut.
As shown in Fig.1, the reflector elements are bent up from two 1300mm
lengths of the 16 x 3mm aluminium
strip. Each piece is bent into an “L”
shape, with main arms 605mm and
645mm long and 50mm return arms
at each outer end. The two longer
arms are then overlapped in the centre
and both drilled with a pair of 6.5mm
holes, to take the threaded ends of a
standard U-clamp bolt.
This bolt and its matching V-block
are then used not only to hold both
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Fig.1: follow this diagram to manufacture the parts and assemble the turnstile/reflector antenna.
www.siliconchip.com.au
January 2004 35
In some cases, you might be able to
attach the mast of the weather satellite
antenna to the upper part of your TV
antenna’s mast, to get extra height.
This can be done using another pair
of U-clamp/V-block assemblies.
If your receiver isn’t going to be
too far away from the antenna, you
could now try running the main 75Ω
antenna downlead directly to the
receiver’s input. Provided the cable
losses aren’t too high, you just might
get quite acceptable results from this
direct connection.
On the other hand, the results might
be disappointing, in which case you’ll
want to build up the RF preamp and fit
it into another weatherproof box at the
base of the mast. That way, it can boost
the strength of the signals before they
go down the main downlead to the
receiver, thus improving the signal-tonoise ratio quite significantly.
OK, let’s now move on to describe
the RF preamp.
Fig.2: this diagram and the inset at right
show how the matching section and the
phasing loop are connected to the dipole
elements.
reflector sections together but also to
clamp the complete reflector assembly
to the 32mm mast at the desired spacing below the active elements.
To strengthen the reflector assembly and also to partially enhance the
reflectors for lower reception angles,
the reflectors are enclosed in a 1210
x 1210mm square of 10 x 3mm aluminium strip. This is formed from two
2470mm lengths, each bent into an “L”
shape with the main arms 1210mm
long and a 50mm return at one end.
The two halves are then assembled
into a square using two 12mm x M4
screws plus nuts and lockwashers,
while four more 12mm x M4 screws
are used to bolt the ends of the reflector arms to the centre of each side of
The RF preamp
The main requirements for
this type of preamp are that
it should provide around 1520dB of stable amplification at
137.5/137.62MHz, with a low noise
figure. It should also be capable of
operating from a 12V DC supply which
is fed up the downlead cable from the
receiver.
This may all sound easy enough but
it’s trickier than you might think. In
fact, we tried out a number of different configurations in developing this
project but in most cases they gave
unsatisfactory results. Basically, they
either didn’t provide enough gain and/
or were too noisy, or they were too hard
to keep stable.
One simple design we tested used a
Mini Circuits MAR-6 microwave amplifier IC, as used in many masthead
amplifiers for TV. This was stable
the square. It’s all quite easy to build
and assemble.
Mounting the antenna
As mentioned before, the completed
antenna should be mounted as high
up off the ground as you can manage,
so that it gets the largest unobstructed
view of the sky. The 137.5MHz weather satellite signals are not particularly
strong and are attenuated even more if
they have to pass through heavy cloud,
tree canopies, etc.
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
No.
1
1
1
1
1
1
1
1
36 Silicon Chip
Value
150kΩ
110kΩ
100kΩ
47kΩ
33kΩ
1.8kΩ
360Ω
47Ω
4-Band Code (1%)
brown green yellow brown
brown brown yellow brown
brown black yellow brown
yellow violet orange brown
orange orange orange brown
brown grey red brown
orange blue brown brown
yellow violet black brown
5-Band Code (1%)
brown green black orange brown
brown brown black orange brown
brown black black orange brown
yellow violet black red brown
orange orange black red brown
brown grey black brown brown
orange blue black black brown
yellow violet black gold brown
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Fig.3: the VHF preamplifier circuit is based on a BF998 dual-gate MOSFET
and is powered from a 12V DC supply which is fed up via the down-lead.
enough but it didn’t provide enough
gain and for this type of application it
was relatively noisy as well.
We also tried a design based on a
BF998 dual-gate MOSFET that was
very similar to the RF stage in the
Weather Satellite Receiver described
last month. This gave enough gain
and was much quieter as well but
it was very difficult to “tame” – it
would oscillate at the drop of a hat,
despite all kinds of shielding and extra
bypassing.
Eventually, after much web research, experimenting, frustration and
tearing of hair (what little hair the author has left!), we finally arrived at the
configuration shown here. It still uses
a BF998 MOSFET but has a somewhat
different input coupling circuit which
allows the preamp to be peaked up for
quite acceptable gain and a low noise
figure (below 1dB), while at the same
time being much more stable.
As shown in Fig.3, the BF998 is used
as a cascode RF amplifier. The incoming RF signal (from the antenna) is fed
to gate 1 via a 220pF input coupling
capacitor and then via L1 and VC1,
which form an input tuning/matching
network. Gate 1 is also fed the correct
DC bias voltage via RF choke RFC1 and
a voltage divider consisting of 150kΩ
and 110kΩ resistors.
Fig.4: the PC board assembly. The red dots indicate leads
that must be soldered on both sides of the board.
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Gate 2 of Q1 is biased to achieve
maximum gain. Its bias voltage is
derived from a 33kΩ/100kΩ voltage
divider and this is fed to gate 2 via a
47kΩ decoupling resistor. Q1’s source
is also provided with the correct bias
voltage via a 360Ω self-bias resistor
and this is fed with some additional
current via a 1.8kΩ resistor.
Q1’s output is tuned by L2 and VC2
in the drain circuit. The RF output
from the preamp is then derived from
a tap near the “cold” (to RF) end of L2,
to provide an approximate match for
the 75Ω output cable to the receiver.
At the same time, the tap delivers the
+12V DC supply to run the preamp,
which is fed from the receiver via the
down-lead.
Note that there are quite a few 1nF
bypass capacitors throughout the
circuit. These ensure that points like
the “G2” and “S” leads of Q1 and
the “cold” ends of RFC1 and L2 are
held firmly at ground potential for
RF, which is necessary for stability.
These capacitors should be either
disc ceramic or multilayer monolithic
ceramic types and their leads should
be kept as short as possible.
Building the preamp
The preamp is built on a very
small double-sided PC board coded
06101041 and measuring 41 x 51mm.
All parts except the BF998 MOSFET
are mounted on the top of the board,
while the MOSFET mounts underneath because it’s a surface-mount
device. The location and orientation
of all parts is shown in Fig.4.
Note that some component leads
have to be soldered on both sides of
the PC board, as indicated by the red
dots on Fig.4.
Fig.5: check your PC board against these full-size etching
patterns before installing any of the parts.
January 2004 37
turns. The tap on L2 is spaced half a
turn from the end that is “cold” for RF
– ie, the end furthest from VC2.
The only other coil in the circuit
is RFC1 and this is wound on an F29
ferrite bead, using only a single full
turn of 0.25mm ECW.
To ensure stability, a shield plate
should be fitted across the top of the
board in the position shown. This plate
is cut from 0.3mm tinplate and measures
40 x 15mm. You’ll find that the PC board
has three 1mm diameter holes in this
location, to take 1mm PC board terminal
pins. Fit these first, then use the pins as
“posts” to support the shield plate when
it’s soldered to them.
The board also has holes for: (1) a
terminal pin at the preamp’s input,
(2) a pin for the tap wire for L2 and
(3) a pin for the preamp’s output. You
can use these pins for connecting
coaxial cables directly to the board,
if you wish.
However, as you can see from the
photos, it’s also possible to enclose the
four sides of the preamp with a simple
box made of tinplate, which provides
some shielding and also supports a
pair of panel-mounting 75Ω “TV”
sockets (ie, the type formerly known
as “Belling-Lee” sockets). These make
the input and output connections a
little more convenient.
Fig.6: here’s how to make the tinplate
enclosure and the shield plate for the
preamplifier. Both can be cut from
0.3mm-thick tinplate.
Wesat Receiver: Notes & Errata
VHF Weather Satellite Receiver,
December 2003: A 100Ω decoupling
resistor in the 6V supply line was
omitted from the circuit diagram. It
should be shown in series with the
+6V supply to VR2, VR4 and VR5.
The resistor is shown correctly in the
PC board overlay diagram but note
that the parts list should show two
100Ω resistors rather than one.
Also, RF choke RFC1, wound on
an F29 bead and located just behind
the RF input socket, should be wound
from two turns of 0.25mm ECW, not
three turns of 0.5mm ECW. The 2.2nF
ceramic bypass capacitor just to the
right of RFC1 on the board should
also be omitted. Both these changes
improve performance when an RF
preamp is being used.
38 Silicon Chip
Both L1 and L2 are air cored but are
wound on a 5mm drill shank or similar
5mm OD mandrel. L1 is wound using
0.8mm enamelled copper wire (ECW)
and has only two well spaced turns,
while L2 is wound using 0.8mm tinned
copper wire (TCW) and has five spaced
Tinplate enclosure
The dimensions of the tinplate enclosure are shown in Fig.6, along with
the hole locations and sizes for the two
sockets. Notice that both sockets are
mounted in the ends of the enclosure
by soldering their outer threaded sections directly to the box ends, on the
inside of the tinplate. This is done for
two reasons: (1) it gives a more reliable earth connection; and (2) there
isn’t room to fit the nuts inside the
enclosure anyway.
Note also that the nut for the output
socket is actually fitted to the socket
and tightened firmly before the socket
is soldered into the enclosure, to act as
a spacer. This ensures that this socket
doesn’t protrude inside the case by its
full threaded length.
The centre pin of both sockets is cut
short, to make sure they clear other
components. The input socket’s centre
pin is then soldered directly to the
PC board terminal pin marked “IN”,
while the output socket’s pin can be
connected directly to coil L2 via a
very short length of tinned copper
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These two views show the completed VHF preamplifier housed inside its
tinplate enclosure and fitted with 75Ω TV sockets for the input and output
connections. Note the short wire link connecting directly from the centre pin of
the righthand (output) socket to the tap on coil L2 (ie, the tap is not taken to a
terminal pin if the socket is is fitted).
wire, to make the tap connection (in
this case, the “OUT” pin is not fitted
to the board at all).
If you elect to provide the preamp
with this simple tinplate enclosure/
socket support, fit the board into the
enclosure so that the top of the shield
plate is level with the top of the enclosure sides. That done, run a fillet of
solder along the edges of the board on
both the top and bottom, to bond the
tinplate to both of the board’s earthy
copper layers. This not only holds it
all together but also helps ensure stable
operation.
Checkup & tuning
When your preamp is complete,
connect its output to the input of the
receiver with a length of 75Ω coaxial
cable. That done, turn on the receiver
and quickly check a few voltages in
the preamp with your DMM, to make
sure it’s working correctly. You should
be able to measure about +11.8V at the
cold end of L2 and also at that end of
the 22Ω decoupling resistor.
You should also be able to measure
about +4.7V at the junction of the
150kΩ and 110kΩ bias resistors for G1,
and +4.9V or thereabouts at the top of
the 360Ω source resistor. Finally, you
should get about +8.8V at the junction
of the 100kΩ, 33kΩ and 47kΩ resistors
(ie, feeding G2 of Q1).
If all of these voltages are close to the
values given, your preamp should be
working correctly. Assuming that’s the
case, switch the receiver to one of the
two satellite reception channels (ie,
137.5MHz or 137.62MHz), then connect the preamp’s input to your signal
generator via a suitable cable and set
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the generator to the same frequency.
Now connect your DMM (set to the
5V DC range) across the 390kΩ RSSI
load resistor in the receiver, so you
can use it as a signal strength meter.
The signal generator can then be set
for about 2-3µV of output.
Next, turn up the receiver’s RF gain
control to about halfway. You may not
be aware of any signal at this stage
but try adjusting trimmer VC2 in the
preamp slowly using an insulated
alignment tool. Listen carefully for a
signal and also watch the DMM carefully to monitor the signal level.
Somewhere near midway in the
trimmer’s adjustment range, you
should find the signal and be able to
set VC2 for a peak in both the received
tone and the DMM reading. If the DMM
reading rises much above 2.5V, you
may need to reduce the signal generator’s output to bring it down below this
level again.
When the correct setting has been
found for VC2, leave it alone and turn
your attention to VC1. By adjusting
this carefully (again using an insulated alignment tool), you should be
able to find another signal peak and a
minimum for the accompanying noise.
Once you have set VC1 carefully
for this second peak, your preamp is
tuned up and ready to be connected
into the antenna downlead at the base
of the mast. We suggest that you fit the
preamp into another small polycarbonate box – ie, the same type as used
for the antenna’s active elements, so
it can be sealed to keep moisture out.
Both the input and output cables
should pass through close-fitting
holes drilled in the bottom of the box,
Preamp Parts List
1 PC board, code 06101041, 41
x 51mm (double sided, but not
plated through)
1 F29 ferrite bead (for RFC1)
1 short length 0.25mm enamelled copper wire for RFC1
1 short length 0.8mm enamelled
copper wire for L1
1 short length 0.8mm tinned
copper wire for L2
5 PC board terminal pins, 1mm
diameter
2 75Ω coaxial “TV” sockets
(Belling-Lee), panel mount
1 40 x 15mm piece of 0.3mm
tinplate for top shield
1 40 x 4mm piece of 0.3mm
tinplate for bottom shield
1 192 x 22mm piece of 0.3mm
tinplate for enclosure
Semiconductors
1 BF998 dual-gate MOSFET (Q1)
Capacitors
1 2.2µF 35V TAG tantalum
7 1nF disc ceramic
1 220pF disc ceramic
2 6-30pF trimcaps, small (VC1,
VC2)
Resistors (0.25W 1%)
1 150kΩ
1 33kΩ
1 110kΩ
1 1.8kΩ
1 100kΩ
1 360Ω
1 47kΩ
1 47Ω
to reduce the likelihood of moisture
finding its way inside. As before, it’s
a good idea to run some neutral-cure
silicone sealant around both cable
exits, to ensure that the moisture is
really kept out.
Happy weather satellite signal reSC
ception!
January 2004 39
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