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Image source: https://unsplash.com/photos/aerial-photography-of-flowers-at-daytime-TRhGEGdw-YY Earrth Ra Ea Rad dio John Clarke’s Parrt 2 : w� Pa w�ispe isperrs of of the sk sky This ‘natural radio receiver’ lets you listen to the VLF and ELF emissions of solar and atmospheric disturbances, like storms or auroras. Having described how it works last month, let’s start building it. T he Earth Radio comprises a single-­PCB receiver that runs off a 12V DC supply or internal 9V battery, plus an external loop antenna on a timber frame measuring 690 × 690 × 98.5mm. That’s very compact for something that will pick up radio signals with wavelengths of many kilometres! Ideally, the whole thing should be kept away from sources of interference, including mains distribution wires. Because it’s battery-powered and portable, you can use it in the Fig.10: fit the components on the PCB as shown here. Take care with the orientations of the ICs, diodes, trimpots, electrolytic capacitors, transistors and LED. 80 Silicon Chip middle of a field or other open area, where it will have the best chance of picking up the very small signals that travel around the world through the Earth’s atmosphere. Construction The Earth Radio is constructed using a double-sided, plated-through PCB coded 06110251 that measures 96 × 69mm. The PCB is housed in a Ritec ABS translucent black instrument case (or equivalent) that measures 104 × 79 × 40mm (its dimensions may be rounded to 105 × 80 × 40mm). A separate loop antenna connects via screened cable and a jack plug. While assembling the board, refer to the overlay diagram, Fig.10, which shows what components go where. Begin by fitting the resistors and the three diodes. Verify the value of each resistor before installation by checking the colour code and measuring it with a multimeter. Ensure diodes D1, D2 and D3 are installed with the cathode stripes orientated as shown in Fig.10 and on the PCB screen printing. Diodes D1 and D2 are small, glass-­ encapsulated 1N4148 types while D3 is a larger, black 1N5819 schottky diode. Next, mount the sockets for the three ICs, taking care to orientate them as shown, with the notched ends towards pin 1 in each case. Then fit the 3.5mm jack sockets, CON1-CON3. Follow with trimpots VR1 to VR6 and VR8. The adjustment screws need to be orientated as shown for the resistance to change as expected. These come in several different values, so be sure to place the correct value in each position. They will be printed with a code indicating the value, although you can also check it by measuring resistance across the outer two leads. siliconchip.com.au The finished Earth Radio, with and without the 9V battery. The PCB attaches to the case using four self tapping screws. Install the capacitors next, starting with the smaller ones. The electrolytic types that come in cans need to be orientated with the correct polarity – the longer lead is positive, and this goes next to the pad marked with a + symbol. The stripe on the can is negative, so it will be opposite this. The smaller MKT and ceramic types can be installed either way around. Now you can fit the DC socket (CON4), volume pot (VR7) and the two switches, S1 & S2. Pass the 9V battery clip lead through the two holes provided near the terminals before soldering them to the pads. This is for strain relief, preventing the wires from breaking off. You can use PC stakes or just solder the wire ends into the PCB holes. The red wire is the positive and black is the negative lead. A 9V battery holder clip attaches to the PCB using a 6mm-long M3 machine screw, with a nut on the underside of the board. LED1 can be installed now after bending its leads by 90°. Position it so the top of the LED dome is 12mm in front of the PCB edge, with the centre of the LED lens located 5mm above the top face of the PCB. When bending the leads, make sure the anode (longer) and cathode (shorter) leads are orientated correctly for the PCB, as per the A (anode) and K (cathode) markings. Panel cutouts Before mounting the PCB in the case, you will need to make the cutouts siliconchip.com.au on the front panel as per Fig.11. It shows the hole positions required for the LED, switches, 3.5mm jack sockets, DC power input socket and volume potentiometer. Front panel labels are provided in Fig.12. You can print out these onto vinyl labels (or similar) ready to attach to the panels. The holes can be cut out with a sharp craft knife. For more information on making panel labels, see www.siliconchip.com.au/Help/ FrontPanels Once the labels have been applied, attach the front and rear panels to the components on the edges of the PCB and secure them with the mounting nuts for the 3.5mm jack sockets and volume potentiometer. Next, secure the main PCB to the enclosure base with the screws supplied with the enclosure. Setting it up For a 50Hz notch (eg, for use in Australia and New Zealand), connect Fig.11: all the cut-outs on the front and rear panels are round holes that can be made with a drill. There are six holes required in the front panel and two in the rear panel. While some dimensions are relative, always measure from the edges. Australia's electronics magazine January 2026  81 Fig.12: these front and rear panel labels can be downloaded as a PDF from siliconchip.com.au/Shop/11/3561 then printed and attached to the panels. a DMM set to measure resistance between TP1 & TP2 and adjust VR1 for a reading of 68.1kW. Do the same for TP2, TP3 and VR2. Then connect the DMM between TP4 and TP5 and adjust VR3 to get a reading of 34kW. For a 60Hz notch, the procedure is the same, but adjust VR1 and VR2 for 56.2kW and VR3 for 28.1kW. Set VR4 and VR8 fully anti-clockwise, then adjust VR5 and VR6 fully clockwise. Connect a 9V battery or external 12V DC supply and check that LED1 lights with the power switch on. The circuit should draw around 13mA at 9V or 15mA at 12V. If that checks out, switch it off, wait for the capacitors to discharge, then insert IC1, IC2 and IC3 into their sockets. Make sure that their pin 1 dot/notch is at the same end as the notch on the socket and ensure the pins don’t fold up as you push them into the sockets. Remember that IC1 is the OP07. To check the quiescent current of the headphone amplifier, measure the voltage across each 1W resistor with the circuit powered back up. These should be less than 0.5mV each. If more than that, adjust VR8 clockwise to reduce the voltage and hence dissipation in the output transistors. If your 47nF capacitors for the Twin-T filter are all within 1% of 47nF, no further adjustments of VR1-VR3 are necessary. VR4 can be adjusted clockwise to deepen and narrow the notch. VR4’s resistance setting can be measured between pins 1 and 5 of IC2. Typically, 220W is a suitable compromise to ensure the notch is wide enough to cater for mains frequency variations and the slight errors in the values of the 47nF capacitors. If your 47nF capacitors are all more than 1% away from 47nF (ie, below 46.5nF or above 47.5nF), VR1 to VR3 will require trimming for best the nulling of mains hum. You can use a signal generator set at 50Hz (or 60Hz) and at a level of 200mV RMS, assuming a 600W output impedance. If you don’t have a suitable AC signal generator, a mains AC plugpack can be used with the voltage reduced using a resistive divider to achieve about 200mV RMS. Add a 560-680W resistor between the junction of the divider and the Earth Radio, and apply the signal between the tip and ring of CON1 via a 3.5mm stereo jack plug. You can use an oscilloscope to monitor the signal at the CON3 output or use headphones (or earbuds) to monitor this instead. Make sure the notch filter is enabled with S1 in the down position, and connect the oscilloscope probe to the tip or ring terminal or insert the earphone or headphone plug into CON3. Adjust VR1 and VR2 by small amounts each (either way) to minimise the loudness of the 50/60Hz output tone. Similarly, adjust VR3 to minimise it. Then adjust VR4 clockwise by a few turns and adjust VR1, VR2 and VR3 again. Keep adjusting VR1, VR2 and VR3 along with the depth trim pot VR4 until you achieve the best possible nulling. As mentioned, VR4 is best set at 220W or more, with its resistance measured between pins 1 and 5 of IC2. Loop antenna details Fig.13: a side view of the timber frame on which the antenna wire is wound. We made the antenna frame as shown in Figs.13-15. You could come up with your own design, provided Australia's electronics magazine siliconchip.com.au 82 Silicon Chip that the wire is wrapped around a square frame of similar dimensions. The wire loop comprises side-byside turns. The loop antenna we made uses 20 × 12mm DAR (dressed all round) timber and 8mm dowelling. We used pine, but hardwood should be used for a more permanent outdoor installation. There are two rectangular frames made from 690mm lengths each side, and a 960mm diagonal to triangulate the frame. The two frames are separated by 26.5mm using 8mm diameter dowelling in each corner of the frame. Extra dowels are used at the centre of each square frame piece to give extra stiffness. The 26.5mm spacing provides for 40 turns of 0.63mm enamelled copper wire side-by-side, allowing for a 16μm thickness of enamel around the wire. The enamel adds up to 1.3mm over 40 turns, while the 0.63mm diameter copper wire accounts for 25.2mm of the overall 26.5mm spacing required. The wire loop is wound over the corner dowels. The overall size of the loop is a 660 × 660mm square with a slight radius at each corner as the wire curves over the outer-most quarter segment of each dowel. The 960mm diagonal braces strengthen the frame, keeping it square by preventing it from collapsing into a rhombus shape. The two diagonals are interconnected across the centre of the frame by gluing a short piece of 20 × 12mm pine to add strength. The frames, diagonals and wire loop are 690mm, 960mm and 660mm long, respectively. These convenient but similar values are due to the decision to use a 660mm square loop and have the dowel holes be 19mm in from each end of the lengths. We started the design with the goal of a 660mm square wire loop. This provides for a loop antenna that can fully use standard wire reel lengths while providing a reasonable signal capture area. For the wire loop, the 8mm dowel corner pieces need their centres to be spaced apart by 8mm less than 660mm (that’s half a dowel diameter each end). So that’s 652mm. Then these 8mm holes are located 19mm in from each end of the frame pieces. This means the overall frame side pieces need to be 652mm + 19mm + 19mm for an overall length of 690mm. siliconchip.com.au Fig.14 (left): an end-on view of the antenna frame, showing how the sideby-side wire windings are held on dowels between the two sides of the timber frame. Fig.15 (right): the various lengths of timber needed to make the antenna frame. Australia's electronics magazine January 2026  83 For the diagonal braces, the centre-­ to-centre spacing of the dowel holes need to be calculated using Pythagoras’s Theorem. With two sides at 652mm, we calculate the hypotenuse length as 922.07mm, rounded to 922mm. Adding the 19mm distances on each side of the hole positions, we get 960mm. Building the antenna Construction is straightforward and requires just a few basic hand tools such as a tape measure, square, saw, drill and sanding paper. Mark out the lengths on the timber pieces. We cut our lengths using a fine-toothed blade saw to provide neat cuts. Drill the 8mm diameter holes in each piece, then cut the dowel pieces: two 98.5mm long, four 74.5mm long and two 50.5mm long. We filed down a series of flats on the dowel along the sections at each end where they enter the 8mm holes in the frame. This provides clearance for glue within the hole around the dowel. A fully round dowel in the same-sized round hole will push the glue out of the hole. Alternatively, use fluted dowel, if available. PVA glue can be used to adhere the pieces together. Assemble the frame pieces and apply glue to the dowels to attach the frame pieces. Wipe off excess glue with a damp cloth. When the glue is dry, you can glue in the bracing spacer that goes in between the braces. Clamp it in place until glue dries. Finally, sand off the frame to a smooth finish and coat it with paint or clear varnish. Winding the coil The finished Earth Radio shown from various angles (not to scale); note that the front panel is an older revision (see Fig.12). A kit is available for $50 (SC7582) and includes all required parts, except for the case, battery, timber and wire. 84 Silicon Chip Australia's electronics magazine Three reels of 0.63mm diameter enamelled copper wire are used. As a reel finishes, we join the end to the next reel to provide the 105m total length required for the antenna loop. Start by wrapping a 100mm length of the 0.63mm diameter wire around the frame near one corner dowel, ready to wind on turns. This holds the wire start in position. Place each winding neatly side-by-side. The wire will need to be joined every 13 turns or so, since each wire reel only contains about 36m of wire. For the wire joins, strip about 10mm of the enamel from the two ends using a sharp hobby knife or emery paper, then place a 20mm length of 1mm heatshrink tubing over the wire end siliconchip.com.au on the new reel, moving it well away from the end so it won’t receive any heat as the two ends are soldered together. It is best to have joins positioned along one of the sides rather than over a corner bend; cut the wire shorter if the join would occur on a corner bend. Once the join is made, slide the heatshrink over the join and shrink it down with a hot air gun. Continue winding to complete the 40 turns. End the loop by wrapping the wire around the corner dowel. If using 0.5mm diameter wire, the procedure is the same but you only need two reels and one join. There will be a few more turns, but because the wire is slightly thinner, it should still fit in the space available. Now the two wire ends need to be soldered to twin-core shielded cable. Just connect the two shielded wires in the cable to the loop wire ends. The shield at the antenna end is left unconnected – cut it back to the end of the insulation so it can’t short to anything. The wire connections need both to be insulated with heatshrink tubing. Next, secure the cable to the frame with a clamp. We used a TO-220 transistor clamp (Jaycar HH8600) and screw, although a clamp fashioned out of a small piece of 1mm thick aluminium, a small P-clamp or cable ties would be suitable as well. The far end of the twin core shielded cable is terminated to a stereo 3.5mm jack plug. The twin cores connect to the tip and ring connections, while the shield attaches to the sleeve of the jack plug. Testing Testing can be performed by holding the antenna frame by hand and listening using headphones or earbuds and keep the volume to a minimum with VR7 to avoid hearing damage. VR5 sets the overall gain and volume of the receiver at IC3a’s output, while VR6 sets the recording level output following this amplifier. In use, while holding the antenna above your head, rotate the frame for minimum noise and hum. It is quite sensitive to detecting artificial electromagnetic-induced noise, so it is best to use it well away from any mains supply and overhead wiring. It may be that you will need to move to a large park or country area to prevent such noises encroaching on the siliconchip.com.au A clear shot of the loop antenna that we built. Figs.13-15 only show the antenna frame, but you can attach a rod to keep it upright with length and material to suit your needs. sounds you are listening for. For more permanent use, the frame can be supported about 4m above ground level. This can be done using a length of 25mm timber dowel, which can attach to the loop antenna frame with screws or cable ties. The dowel can be supported using a metal pole or star post that’s hammered into the ground. Whispers of the sky Catching the tweeks, choruses and whistlers can be elusive, especially if you intend to be listening at the time. Instead of listening all night and morning, you can record the signals and check them later. You may choose only to record when the conditions are best, such as during solar events. You can get information about space weather and solar events from the Australian Bureau of Meteorology at www. sws.bom.gov.au One thing to watch for is that if you are recording its output, the recorder can possibly create electrical noise that the Earth Radio will pick up. Typically, a recorder that operates from a battery supply will produce less noise than one operating from a mains supply. In some instances, there may be less noise when the Earth Radio’s ground is connected to an Earth stake. The recorder can be digital or analog, but a digital version makes it easier to search the recording for interesting noises later. Australia's electronics magazine You can also import an analog recording (or the signal directly from the Earth Radio) into a computer with software such as Audacity (which is free). Using Audacity is an ideal way to process the signal. It can amplify it, run filters and remove noise using the Effect → Volume and Compression or Noise Removal or EQ and Filters menu option. This can clean up the recorded signal so you just hear the desired waveforms. After processing, export it as a .mp3 or .wav file suitable for loading into Raven Lite 2. This is the spectrograph software we used. It is very intuitive to use for loading an audio waveform and showing the spectrogram. Audacity software is free, open source software for recording and editing sounds and is available from www. audacityteam.org/download Raven Lite 2 is available from www. ravensoundsoftware.com/raven-­litedownloads/ Order the Raven Lite 2 version and ‘purchase’ the licence, which is free. Both Audacity and Raven Lite are available for Windows, Mac and Linux systems. For more information on some of the atmospheric phenomena this radio can pick up, see our article titled “Atmospheric Electricity: Nature’s Spectacular Fireworks” in the May 2016 issue (siliconchip.au/Article/9922). SC Happy listening! January 2026  85