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In Pt.1, we unveiled our new low-cost microwave Doppler Radar Speed Gun, designed for measuring the speed of cars, bikes, boats, horses and even human sprinters. This month, we show you how to build it and describe how it is used. Build Your Own Pt.2: By JIM ROWE Radar Speed Gun A S EXPLAINED in Pt.1, all the components in our new Doppler Radar Speed Gun are on two PC boards. The smaller DOPPLR1a board contains the microwave head circuitry and fits inside a small shield box attached to the underside of the coffee-can antenna barrel. By contrast, the larger DOPPLR2a board carries the counter/display unit circuitry and fits inside a standard UB1 plastic utility box. The two units are linked by a single cable that’s fitted with a Type A USB plug at each end. Although the larger DOPPLR2a board has more components on it than the smaller board, it’s a little easier to assemble because it’s only single44  Silicon Chip sided and the components are all of the familiar “leaded” type. This being the case, we’re going to assemble this board first. Counter/display board Fig.5 shows the assembly details for the counter/display board. Begin by fitting the 10 wire links. These can all be made using tinned copper wire or resistor lead offcuts, except for the one located just below transistor Q3. This link should be made from a short length of insulated hookup wire, because it runs quite close to a lead from the 1kW resistor just below it. Once all the links are in place, fit the six 1mm PC board terminal pins which are used for the three test points and their accompanying ground connections. Note that as supplied, the counter/ display board is configured to show readings in km/h. If you want the display to read in mph instead, then it’s just a matter of cutting the three tracks between the centre and top terminals of LK1, LK2 & LK3 and installing wire links between the centre and bottom terminals instead. Alternatively, you can fit 3-pin headers in the LK1-LK3 positions and use 2-pin jumpers to make the connections instead. However, you will still have to cut the tracks between the top two terminals of the headers. Now fit the IC sockets, taking care to orientate each one with its “notch” siliconchip.com.au Use headers or links – see text Fig.5 (right): install the parts on the counter/display board as shown here. Note that our prototype used 3-pin headers and 2-pin jumpers to program the timebase but wire links could also be used. end towards the left, as shown on the overlay diagram. This will help ensure that you later fit the ICs the correct way around. The resistors can go in next, followed by trimpot VR1. Note: you may wish to mount VR1 on the track side of the PC board, to allow for easy adjustment once the board has been fitted to the lid. Be sure to fit the correct value resistor in each location. Table 1 shows the resistor colour codes but we also recommend that you check them using a digital multimeter, as some of the colours can be difficult to decipher. The small ceramic, monolithic and metallised polyester capacitors can now all be mounted. These capacitors are all non-polarised, so they can siliconchip.com.au be fitted either way around. Follow them with the electrolytics which are of course polarised, so take care to fit them with the correct orientation. That done, fit the 38kHz crystal (X1). As shown on Fig.5, this mounts on its side, with both leads bent downwards about 2mm from the case so that they pass through the holes in the PC board. Solder its leads to the pads underneath, then fit a small U-shaped piece of tinned copper wire over the crystal’s case to secure it in position (the ends of the wire “U-loop” are soldered to matching pads on the the board). Now for the semiconductors. Begin with the two diodes, taking care to install them with the correct orientation. Also, be sure to use the 1N4004 power diode for D1 and the smaller 1N4148 diode for D2. Follow these with three 7-segment LED displays. These must all be orientated with their decimal point LEDs (which we don’t use here) at lower right. The four transistors can then go in – the PN100 device goes in the Q4 position (near the bottom of the board), while the three PN200 devices go in the Q1-Q3 positions below the displays. Once these parts in in, install the USB, power and headphone sockets. Finally, plug the eight ICs into their sockets, taking care to ground both yourself and the PC board earth before handling them. This is necessary because most of the ICs are CMOS devices and are vulnerable to damage December 2006  45 Fig.7: here’s how the leaded resistors are soldered to the PC board. Fig.6: this diagram shows how the various SMD parts are soldered to the PC board. Be sure to use a temperature-controlled iron fitted with a very fine chisel-shaped bit and take care not to overheat the tiny components. from electrostatic discharge. Your counter/display board is now finished and can be placed aside while you assemble the microwave head board. Note that we haven’t discussed the Hold switch (S1) at this stage, because it mounts on the box lid and is only connected to the display board later. Microwave head board This second board is considerably smaller than the first but is more challenging because about half of the components on it are small surface-mount devices. It’s also double sided but this shouldn’t cause you any problems because the board supplied in the kits will have plated-through holes and solder masking on both sides. Only one component mounts on the underside of the board – the Type A USB socket. Everything else mounts on the top of the board, because virtually all of the underside copper is used as 46  Silicon Chip an earthed ground plane and shield. Before you begin fitting any components to this board, examine the overlay diagram of Fig.8 to familiarise yourself as to how it all goes together. That done, begin the assembly by fitting the surface-mount (SMD) parts. To do this, you’ll need a soldering iron with a very fine chisel-shaped tip, which you need to keep particularly clean. Ideally, it should also be a temperature-controlled iron, so it doesn’t get too hot and damage the tiny components. In addition, you’ll need a small pair of crossover tweezers to handle the SMD parts and a wooden toothpick to hold each part down while you solder it in position. You’ll also find an illuminated magnifier a big help – especially if it’s on the end of a spring-loaded arm, so you can place it in just the right position above the PC board. By the way, manually soldering SMD parts in place isn’t all that dif- ficult if you tackle them carefully and one at a time. Fig.6 shows how to solder both passive and active SMD parts to a PC board. You should fit the SMD parts to the head-end board in the following order: (1) the 100W 0805 resistor at upper right; (2) the 1nF 1206 ceramic capacitor near the top of the board, just to the left of centre; (3) the five 10nF 1206 ceramic capacitors; (4) oscillator transistor Q1 (this must be orientated with its “fatter” collector lead at upper right); (5) mixer diode D1, orientated with its “two-lead” side towards the antenna microstrip line on its right; (6) the ERA2-SM microwave amplifier chip (IC1), orientated with its locating dot and diagonal-cut end (pin 1) towards the bottom of the board; (7) RFC1, the UHF choke, which is the largest of all the SMD devices (orientated with its pin 1 identification dot at lower right). That completes the trickiest part of the board assembly and you should now be ready to fit the rest of the parts. Begin this second phase by fitting the USB connector, which mounts on the underside of the board. It’s fitted in siliconchip.com.au Fig.8: the microwave head board carries quite a few surface mount devices (SMDs) and these should be mounted first, as described in the text. The USB connector mounts on the underside of the board. the normal way by carefully pushing all its connection leads and mounting clips through the matching board holes, then soldering them to the pads on the top of the board. Next, fit the leaded resistors but note that most of these are mounted in a slightly unorthodox way – either with both end leads cranked down and cut short for “surface mounting” or with only one lead dressed this way and the other bent down in the usual way to pass through a board hole. Fig.7 shows how the leads are prepared and the resistor fitted to the PC board in each case. Start with the resistors that are fitted with one end passing down through the board hole. These are: (1) the 100W resistor which connects the emitter of Q1 to ground; (2) the 1kW load resistor for mixer diode D1; and (3) the 470W DC return resistor between the antenna microstrip line and ground. In all three cases, it’s the lead at the “earthy” end of the resistor which passes down through the board hole. These leads are then soldered to the copper pads on both sides of the board. In contrast the “cranked down” leads at the other ends of these resistors are soldered only to the pad on the top layer. The remaining “leaded” resistors can now be installed. Three of these have both leads cranked down as at the top of Fig.7 – ie, the 10kW collector load resistor for Q2, the 1.5MW bias resistor for Q2 and the 100W collector resistor for Q1. The last leaded resistor to fit is the 10kW bias resistor for Q1, which is fitted in a different way again. As shown in Fig.8, this resistor is fitted alongside the 100W collector resis- Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o siliconchip.com.au No. 1 1 1 1 6 4 2 6 1 2 4 7 2 3 2 Value 2.2MW 1.5MW 1MW 330kW 100kW 47kW 22kW 10kW 6.8kW 4.7kW 1kW 680W 470W 100W 47W 4-Band Code (1%) red red green brown brown green green brown brown black green brown orange orange yellow brown brown black yellow brown yellow violet orange brown red red orange brown brown black orange brown blue grey red brown yellow violet red brown brown black red brown blue grey brown brown yellow violet brown brown brown black brown brown yellow violet black brown 5-Band Code (1%) red red black yellow brown brown green black yellow brown brown black black yellow brown orange orange black orange brown brown black black orange brown yellow violet black red brown red red black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown brown black black black brown yellow violet black gold brown December 2006  47 Fig.9: the shield box which encloses the microwave head board is made from a rectangular piece of 0.3mm or 0.25mm-thick brass sheet. Cut it out as shown in this diagram and fold down the sides to form the box. tor. One lead is bent down and over before cutting it short, so that it can be soldered to the same pad on Q1’s collector line as the 100W resistor. The other end is then bent around in a hairpin shape and then down, so that it can be soldered to the copper pad just below the base lead for Q1, where the two 10nF SMD bypass capacitors are also connected. Be sure to cut this lead to length before you solder it, as it’s not easy to cut off the excess afterwards. The four 10nF leaded monolithic capacitors are next on the list. These all use the same arrangement used for some of the resistors – ie, one cranked lead and one lead bent down for through-hole mounting. It’s just a matter of carefully dressing their leads and cutting them to length before fitting them. The leads that pass through the board holes are again soldered on both top and bottom sides of the board. Diode D2 goes in next and this is fitted in the same way as the resistor shown at the bottom of Fig.7. Make sure that it’s the anode lead that passes down through the earthing hole. Transistor Q2, the PN100 leaded transistor, can now be installed. This Fig.10: the antenna barrel for the microwave head unit is made from two coffee cans. It’s made by first drilling a hole for the antenna in can “A”, then soldering the two coffee cans together to form the barrel (see text). 48  Silicon Chip siliconchip.com.au is again fitted in an unusual way: its emitter lead passes down through a board hole in the normal way, while the other two leads are bent at right angles about 4mm down from the transistor body, so that they “sit” on the pads on the top of the board in surface mount fashion. After bending them, cut these leads off about 2mm from the bends before soldering them to their respective pads. The emitter lead is soldered to the pads on both sides of the board. The 220mF electrolytic capacitor mounts on its side in the lower left­hand corner of the board. Before mounting, its leads need to be bent outwards a little, then down through 90°. Its negative lead then passes through a board hole in the usual way (and is soldered at both top and bottom), while its positive lead is bent horizontally again and cut short for “surface mounting” to its pad. A U-shaped loop of tinned copper wire is then installed over the electro’s body, to hold it securely in position. The two 1mF tantalum capacitors are also installed with their bodies flat against the PC board. In both cases, their leads are cranked downwards, for “surface mounting” on the pads below. Be sure to fit them with the correct polarity. Once they’re in place, mix up a small amount of quick-setting epoxy cement to hold them securely in place – see Fig.8. Your microwave head board is now just about complete. All that remains is to attach the antenna wire at top centre. This is made from a 35mm length of 1.3mm enamelled copper wire, with about 4mm of enamel insulation scraped off one end. This “scraped end” is then soldered to the rectangular pad at the top of the antenna feed line, as close to “on-axis” as you can make it. Finally, check the free length of the wire with a steel rule or vernier calliper and if necessary, trim the far end to bring the free length to exactly 28mm. Functional check-out Now that your boards have both been wired up, it’s time to give them a quick functional check-out. This is easily done by connecting them together via the USB cable, plugging a pair of stereo headphones into the 3.5mm jack on the counter/display siliconchip.com.au This is the view down the antenna barrel. The 30mmlong antenna wire can be seen right at the back and sits exactly 50mm from the rear – see Fig.10. Make sure the antenna wire goes through the middle of the hole in the can and doesn’t short against the metal. board and connecting a 12V DC supply (positive to the centre pin). The latter can be a 12V bench supply or a 12V battery pack of some kind. As soon as power is applied, the 7-segment LED displays should immediately begin showing a random count. Shortly after this, you’ll also begin to hear hum in the headphones and possibly some other noises. If all is well so far, try moving your hand back and forth near the antenna wire on the microwave head board. You should hear a buzzing sound when you do this, with a pitch that depends on your hand’s speed. It will be higher in pitch when your hand is moving faster and lower when it’s moving more slowly. And if you watch the LED displays at the same time, they should give a higher reading for fast hand movements too. If your results are as we’ve just described, your Radar Speed Gun boards are probably working as they should. However, if there seems to be some kind of problem or you want to make sure, you’ll probably want to do some troubleshooting. Here are the things you can try: (1) With your multimeter set to DC volts, measure the voltage at the cathode (banded) end of diode D1 on the counter/display board. It should Use This Device In A Responsible Manner Be sure to use this device in a responsible manner. In particular, DO NOT use this device to measure the speed of vehicles on a public road. The main reason for this is that drivers will not know what is being aimed at them, particularly as you will not be in police uniform. That in turn could cause alarm and could even cause some drivers to brake heavily or take evasive action. And if there was an accident, you might be held legally responsible in some way. Similarly, DO NOT let anyone use the Radar Speed Gun in your car when travelling on public roads. This would not only prove distracting for the driver but the microwave radiation from the unit could cause interference to other spectrum users – including the radar speed units used by traffic police. In any case, the police will probably be able to detect the radiation from your unit and could apprehend and charge you with trying to disturb the operation of their equipment. In short, to avoid trouble with other motorists and the “boys in blue”, use your Radar Speed Gun only on the racetrack, drag strip or in some other private area. December 2006  49 This is the completed barrel unit with the microwave head unit (arrowed) attached. The inset below shows how the microwave head PC board is fitted to the shield box, after the box has been soldered to the barrel. Above: it’s a good idea to protect the microwave head assembly using heatshrink. be very close to +11.4V relative to ground. (2) Check the voltage at pin 14 of IC6 – it should be very close to 11.4V (3) Measure the voltage across the 220mF electrolytic capacitor on the microwave head board – it should measure approximately +7.5V. If these voltages all check out correctly, most of the circuitry is probably working correctly. If you have a oscilloscope, you can check that the crystal oscillator on the counter/display board is working properly by looking at the waveform on test point TP1. You should find a slightly rounded square-wave with a frequency of 38kHz. Alternatively, if you have a frequency counter, it should show the same frequency. Now transfer your scope probe to TP2. Here you should find a train of fairly narrow positive-going pulses, with a peak-to-peak amplitude of about 11.5V and a frequency of 9.0778Hz if links LK1-LK3 are set for km/h readout. Alternatively, this frequency should be 14.6103Hz if you have cut the tracks and fitted the three links for mph readout. These frequencies can also be checked with a frequency counter if you have one. 50  Silicon Chip If all is well so far, transfer your scope probe to TP3 and again move your hand back and forth near the microwave antenna. You should see a train of narrow negative-going pulses, again about 11.5V peak-to-peak. These pulses will only be about 300ms wide and the frequency will depend on the speed that your hand is moving. If your unit passes these tests, you’re ready for the next stage in the assembly – making the head-end shield box. in Fig.9. Make sure that the ends of the sides meet cleanly at each corner. This forms the basic shield box, with the head-end board itself forming the “top” when it’s fitted. To finish the box off, use a highpower soldering iron to run a small fillet of solder down inside each corner. This will ensure that the corners are properly sealed, for both physical strength and shielding. The box can then be placed aside while you make up the radar gun’s antenna barrel. Making the shield box Making the antenna barrel The kit for the Doppler Radar Speed Gun will include a rectangular piece of 0.3mm or 0.25mm-thick brass sheet. This is used to make the shield box which encloses the microwave head board – see Fig.9. The brass sheet is first cut to a size of 89 x 76mm, after which a 12.5 x 12.5mm square cutout is made in each corner. A 6 x 6mm square is then cut from the centre of one of the narrow ends, as shown. This is the clearance hole for the antenna, when it’s all assembled. When the cutouts have all been made and any burrs filed off, the four sides are then bent down by 90°, corresponding to the dashed lines shown To make the antenna barrel you’ll need two clean tin cans, each measuring 127mm in diameter and 173mm long. These don’t come with the kit but they’re easy to obtain from your local supermarket because they are the kind used for 500g cans of low-cost instant coffee. That means that you’ll end up with one kilogram of instant coffee as well as the two cans. If you transfer the coffee into some jars, you’ll have plenty of instant coffee for quite a while! Note that if the cans come complete with clip-on plastic lids, then be sure to keep at least one of these lids to use as a dust cover over the open end of the finished barrel. Alternatively, you siliconchip.com.au can use the plastic top from a bulk CD container as a dust cap. Once the two cans are emptied, washed and dried, you can proceed to turn them into your antenna barrel. Both need to have their inner top flange removed and this is easily done using a can opener of the type which cuts around the inside of the rim using a sharp wheel. The same opener is then used to remove the bottom of one of the cans, which subsequently becomes the front half of the barrel – ie, can “B” in Fig.9. Don’t remove the bottom from the other can though (can “A”). Next drill a 4mm hole in the side seam of the “A” can, with its centre as close as you can make it to a point 50mm up from the inside bottom of this can. The easiest way to do this is to first measure the distance inside the can from bottom to top. That done, move your rule to the outside and mark a point on the side seam that is down from the top rim by the total distance less 50mm. Finally, centre-punch this point and drill the 4mm hole. After the hole is drilled, carefully enlarge it to 6mm diameter using a tapered reamer. You should then remove any remaining burrs using a jeweller’s needle file or similar. Next you should remove the lacquer from the outside of this can around this 6mm hole by rubbing it with steel wool soaked in methylated spirit. You should remove the lacquer from a rectangular area about 30mm up and down from the hole (along the seam) and about 12mm on either side, giving a cleaned area about 60 x 24mm. This is where the shield box will later be soldered to the can. You can now use a heavy-duty soldering iron to solder the bottom rim of can “B” to the top rim of can “A”. This simply involves butting them together and running a smooth solder fillet right around the mating joint. Note that it’s also a good idea to line up their side seams as well, as this gives a neater end result. Once you’re happy that the two cans are cleanly and securely joined together to form the barrel, the next step is to solder the shield box (empty at this stage) to the side seam “underside” of can “A”. Be sure to align the 6 x 6mm end cutout in the shield with the 6mm barrel hole. Again the idea is to run a neat but strong solder fillet around all three outer edges of the box siliconchip.com.au The counter/display unit fits inside a standard plastic case. This case will be supplied pre-drilled and with a screened panel as part of the Jaycar kit. sides. Another solder fillet can then be run along the edge on either side of the 6 x 6mm cutout. Your barrel and shield box assembly are now be ready for the final and most delicate stage – that of soldering the head-end PC board assembly into the shield box. Just before you do this, make sure that the top inside edges of the shield box sides and outer end are clean and free from oil or grease. If you wish, you can tin around these edges but don’t leave more than a very thin layer of solder, otherwise you won’t be able to slip the PC board into the box for final soldering. Now take the head-end PC board assembly and turn it over so that the component side is underneath and with the antenna wire at the top. That done, angle the board downwards and pass the antenna wire through the 6mm hole and into the barrel, until the top end of the board meets the end of the shield box. Once it’s there, lower the complete board assembly into the shield box, so that its copper groundplane is just below the box lip. It should now stay in this position while you attach it securely inside the box by running a small fillet of solder around the edges. Here’s a useful tip: you’ll find this job a lot easier if you position straight lengths of 0.5mm-diameter tinned copper wire inside each edge before you begin soldering. This wire “encourages” the solder to bond across between the PC board copper and the brass inner sides of the box. It won’t be easy to cover the top of the antenna wire hole in the barrel using solder alone. The answer to this is to place a small piece of copper shim over the remaining hole, bent by about 80° in the centre so it forms a patch to seal the hole (it overlaps both the can metal and the PC board’s ground plane copper). Solder the edges of this copper patch to both the barrel and the PC board, to complete the shielding around the antenna. This will ensure that all of the micro­w ave energy passes into the antenna, to be radiated from the barrel. The antenna barrel assembly should now be complete, although you might want to give it a coat of paint to hide December 2006  51 Watch Out For Spurious Readings Because the Doppler audio signals produced in the Radar Gun’s microwave head are quite low in level, they need a great deal of amplification (between 2000 and 22,000 times) in the counter/display unit to bring them up to a level which can be converted into pulses for reliable counting. This large amount of amplification makes the Radar Speed Gun susceptible to interference from electrical noise and AC hum, which tend to cause spurious readings when it is not aimed at a moving object. For example, if the amplifier picks up 50Hz hum, this will give a spurious reading of 11km/h. Similarly, 100Hz hum will give a spurious reading of 22km/h, while impulse noise from electric motors, etc, will give different spurious readings. You’ll also find that if you aim the Radar Speed Gun at fluorescent lamps, this too will give spurious readings – but for another reason. The discharge plasma in fluorescent tubes pulses on and off at double the mains frequency – ie, at 100Hz in the case of tubes running from 240V 50Hz. Because some of the Radar Speed Gun’s microwave energy is reflected back from the plasma in bursts modulated at this rate, the unit’s mixer produces a “false” Doppler frequency of 100Hz. As a result, you’ll not only hear a loud 100Hz hum in the headphones but also get a spurious speed reading of 22km/h. In practice, these spurious signals are not really a problem, since they are swamped by the much stronger return signals received when you aim the unit at a real moving target. Just don’t be alarmed if your unit displays 11km/h or 22km/h (or some other figure) while indoors or near a source of electrical interference – that’s perfectly normal. its coffee can heritage. If you decide to do this, carefully place some layers of masking tape (or gaffer tape) all around the USB socket on the back of the shield box, to stop paint from entering the socket. You can then apply the paint to the outside of both the barrel and the shield box, using either a spray can or brush. Our prototype was sprayed with black automotive lacquer. You may also wish to protect the microwave head board assembly with some heat­ shrink – see photo on page 7. Final assembly Before mounting the board, you need to fit the small rectangle of red Perspex supplied with the kit behind the cutout in the lid, to form the viewing window. You can secure it by using a drop or two of super glue or contact adhesive around the edges. That done, the PC board can now be mounted on the inside of the lid on four M3 x 25mm tapped metal spacers. Secure it using M3 x 6mm countersink head screws at the lid ends and roundhead M3 x 6mm screws at the board ends. You also need to fit the Hold/Run switch S1 into its rectangular cutout in the centre of the front panel. It 52  Silicon Chip pushes through from the front – but make sure that you orientate it so that the ‘1’ on its rocker actuator is towards the left. Note that you may need to “square up” the switch cutout if it has rounded edges. That done, turn the panel over and attach a short length of tinned copper wire to each of the connection lugs on the back of the switch. Attach them securely, by looping the end of each wire through the hole in its lug and then compressing the loop with your pliers, before soldering. This will ensure that the joints don’t come apart when the wires are soldered to the PC board pads. Alternatively, you can use a short length of figure-8 wire to connect the switch to the PC board. Next, set VR1 (which adjusts the gain from 20-220) to mid-range. The board can then be lowered into position on the spacers, with the switch wires passing through their respective holes in the centre. Secure it using the roundhead M3 x 6mm machine screws, then solder the switch wires and fit the lid in position. Your Radar Speed Gun is now ready for its final check-out. Final check-out The Doppler Radar Speed Gun is simple to use. All you need to do is connect the two parts of the system together using the USB cable, connect a 12V battery pack (or some other source of 12V DC) and plug in a pair of stereo headphones (if you have them). Within about 20 seconds of power being applied, you should begin hearing sounds in the headphones, indicating that the Doppler signal processing circuitry has sprung into life and stabilised. After that, it’s simply a matter of pointing the antenna barrel at a suitable moving target and holding it steady for a few seconds so that the frequency counter’s readout can stabilise with the speed reading. You’ll also find that the sound in the headphones helps a lot in directing the beam at the vehicle and holding it in the right position. You’ll soon get used to identifying the “whooshing” sound produced by the Doppler signals. Once the speed of the vehicle is being displayed on the counter, you can operate the Hold switch to freeze the reading. Remember that for the highest reading accuracy, the axis of the Radar Speed Gun’s barrel should be aligned as closely as possible with the path of the moving target. Of course, this won’t always be possible because you can’t stand directly in the vehicle’s path! However, if the vehicle is on a racetrack, you might be able to position yourself at the end of the straight, so that you can aim directly at the vehicles coming towards you. If you can’t do this and have to make your measurements at an angle of 25° or 30° to the vehicle’s path, you can still work out its speed fairly accurately simply by dividing the readings by a correction factor. This correction factor is simply the cosine of the measuring angle. For example, if you’re making measurements at an angle of 25°, the correction factor will be cos(25°) = 0.906. So if you get a speed reading of 110km/h, the vehicle’s true speed will be (110/0.906), or very close to 121km/h. Get the idea? Another thing that can effect the accuracy is movement of the microwave head itself. For the most accurate readings, the antenna barrel should be held as steady as possible. If you find that too difficult, you may want to fit the antenna barrel with a U-shaped metal bracket, so it can be mounted SC on a photographic tripod. siliconchip.com.au