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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 www.siliconchip.com.au 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 www.siliconchip.com.au 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 www.siliconchip.com.au 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. www.siliconchip.com.au 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 www.siliconchip.com.au 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 www.siliconchip.com.au 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