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An LF-HF Up-Converter for VHF/UHF SDRs By JIM ROWE As we saw last month, DVB-T dongles can be used to turn almost any PC into an easy-to-use software defined radio or “SDR” for VHF and UHF frequencies above about 52MHz. But since the dongles don’t work below 52MHz, you can’t monitor all the interesting stuff below this frequency, such as the amateur bands and shortwave radio, CB radio and airport beacons and so on. This LF-HF UpConverter solves that problem. W E ASSUME THAT last month’s article on a software-defined radio (SDR) using a cheap digital TV dongle has you champing at the bit. For less than $30 you can have an SDR with most of the features of a fancy and expensive communications receiver with all sorts of reception modes. Now we are describing an UpConverter so you can monitor signals in the frequency bands below 52MHz. What’s an Up-Converter? It takes 26  Silicon Chip LF (low-frequency), MF (mediumfrequency) and HF (high-frequency) radio signals and shifts them up into the VHF (very high frequency) region where they can be received by a VHF radio receiver, specifically an SDR using a PC with a DVB-T dongle, as described last month. By the way, the terms LF (30300kHz), MF (300kHz-3MHz) and HF (3-30MHz) are all used to describe radio signals at frequencies below 30MHz, while VHF is used for signals from 30-300MHz, and UHF for signals above 300MHz. The Up-Converter described here uses the heterodyne principle to shift the LF-HF signals up by 125MHz, so that for example, a 500kHz signal moves up to 125.5MHz and a 1.25MHz signal moves up to 126.25MHz, and so on. Similarly, an HF signal of 29.75MHz moves up to 154.75MHz (ie, 29.75 + 125). As a result, any sigsiliconchip.com.au Par t s Lis t VHF/UHF ANTENNA MF/HF ANTENNA 9.500400 DVB-T DONGLE wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo wowowo USB CABLE (SHORT) SILICON CHIP +5V DC + HF INPUT . HF TO VHF UP-CONVERTER VHF/UHF INPUT PAL-TO-PAL PATCH LEAD VHF/UHF OUTPUT wowowo wowowo wowowo LAPTOP (OR DESKTOP) PC RUNNING SDR# OR SIMILAR SDR APPLICATION TO 5V DC POWER SOURCE UP-CONVERTER Fig.1: the Up-Converter is inserted between the two antennas and the DVB-T dongle. It up-converts the signal from the MF/HF antenna only when it is powered up. When it is not powered, an internal relay switches the VHF/UHF antenna straight through to the dongle, thereby bypassing the Up-Converter. nals down in the LF-HF region of the spectrum get moved up into the VHF region starting at 125MHz which can be tuned by the SDR. The Up-Converter is a little metal box with two antenna connectors, one for VHF & UHF signals and one for signals below 52MHz. It has a single output which connects to the antenna input on the USB DVB-T dongle which plugs into a USB port on your laptop or desktop PC. The general arrangement is depicted in Fig.1. The Up-Converter contains a relay which switches between the up-converted output and the VHF/UHF signal (ie, the “straight-through” or bypass condition). When the Up-Converter is powered up, it automatically feeds its up-converted signal through to the DVB-T dongle. When it is not powered, it is bypassed and the signals from the VHF/UHF antenna are fed directly to the dongle. This avoids any need for swapping over antenna cables. confusion when you’re using the UpConverter with your SDR but if you’re using an SDR software application like SDR# this isn’t so. That’s because SDR# has a feature which automatically compensates for frequency shift. This feature is enabled by clicking on the box next to the label “Shift”, in the Radio panel at upper left. All you have to do is type the conversion frequency into the text box to the right of the “Shift” label but note that it must be entered as a negative number, so that it will be subtracted from the up-converted frequency in order to display the correct signal frequency. When you have the Up-Converter turned on, all you have to do is click on SDR#’s “Shift” box to have it display the correct LF-HF input signal frequency. If you want to swing back to receiving in the VHF-UHF bands, turn off the power to the Up-Converter and click on the “Shift” box to disable it. SDR shift The full circuit diagram of the UpConverter is shown in Fig.2. Only two chips are involved: a mixer (IC1) and You might think that this “shifting up by 125MHz” would cause Circuit details Main Features & Specifications •  Up-converts (shifts) LF-HF radio signals up by 125MHz into the VHF spectrum, for reception via a PC-based SDR using virtually any DVB-T dongle. •  Incorporates a signal-switching relay so that when power is not applied to the converter, the VHF signal output is switched directly to the VHF/UHF input from an antenna to avoid the need for cable swapping. LF-HF input impedance: 50Ω unbalanced, with overload protection diodes. VHF/UHF input/output impedance: 75Ω unbalanced. Conversion gain: approximately +10dB ±2dB over the input range 100kHz55MHz (corresponding output range 125.1–180MHz). Power supply: external 5V DC source (see text); current drain less than 70mA. siliconchip.com.au 1 diecast aluminium box, 110 x 60 x 30mm 1 PCB, code 07106131, 80 x 51mm 1 front-panel label (see text) 1 HCMOS 3.3V crystal oscillator module, 125MHz (Fox Elect­ ronics FXO-HC536R-125 or similar) (element14 2058072) 1 390nH SMD inductor (0805) 1 SPDT 5V mini DIP relay, JRC23F-05 or similar (Futurlec) 1 ferrite toroid, 18mm OD x 6mm deep (Jaycar LO-1230) 1 ferrite balun core, 14mm long (Jaycar LF-1220) 1 500mm length of 0.25mm enamelled copper wire 1 150mm length of 0.8mm enamelled copper wire 1 100mm length of 1mm-dia. tinned copper wire 3 small Nylon cable ties 1 BNC socket, single hole panel mounting (CON1) 2 75Ω PAL sockets, single hole mounting (CON2, CON3) 1 2.5mm concentric power socket, PCB-mount (CON4) 4 M3 x 10mm untapped spacers 4 M3 x 20mm machine screws 4 M3 hex nuts 4 self-adhesive rubber feet 1 5V DC plugpack or USB/DC adaptor cable Semiconductors 1 SA602AD/01 or SA612AD/01 double balanced mixer (IC1) (element14 2212077 or 2212081 respectively) 1 LP2950-3.3 or LM2936-3.3 LDO regulator (REG1) 2 1N5711 Schottky diodes (D1-D2) 1 1N5819 1A Schottky diode (D3) 1 1N4004 1A diode (D4) Capacitors 2 47μF 16V RB electrolytic 1 220nF multilayer monolithic ceramic 2 10nF multilayer monolithic ceramic 1 10nF COG-NP0 SMD ceramic (1206) 1 470pF disc ceramic 1 3.3pF COG-NP0 SMD ceramic (1206) Resistors (0.25W) 2 10kΩ SMD (0805) June 2013  27 D3 1N5819 REG1 LP2950-3.3 +3.3V OUT 10nF 4 Vdd 1 XO1 EN FXO-HC536R OUT -125 10nF 3 10k T1 CON1 LF/HF INPUT K D1* 1N5711 A 5T A K D2* 1N5711 1 InA 27T 2 K InB 6 OscB A Gnd 3 T2 11T OutB CON4 CON3 8 Vcc 4 OutA IC1 SA602AD OR SA612AD 5V DC INPUT RLY1 (JRC-23F-05 OR SIMILAR) D4 1N4004 10nF 470pF A 47 mF 220nF 125MHz 10k 3.3pF GND 2 GND 47 mF 390nH K IN VHF/UHF OUTPUT 2T 5 7 CON2 VHF/UHF INPUT T1: WOUND ON AN 18mm OD FERRITE TOROID T2: WOUND ON A FERRITE BALUN CORE, 14mm LONG * ONLY NEEDED WITH A LONG-WIRE HF ANTENNA SC Ó2013 LF-HF TO VHF UP-CONVERTER SA602AD, SA612AD D1–D4 A GND 8 K LP2950 4 1 IN OUT XO1 4 3 1 (TP) 2 Fig.2: the circuit is based on two ICs: an SA602AD/01 double-balanced mixer (IC1) and a 125MHz crystal oscillator (XO1). The balanced input signals at pins 1 & 2 of IC1 (fed in via transformer T1) are mixed with the 125MHz signal to produce sum and difference signals and these are fed via matching transformer T2 to the output via relay RLY1. a 125MHz crystal oscillator (XO1). IC1 is an SA602AD/01 (or SA612­ AD/01) double-balanced mixer designed spec­ifically for this kind of use. The LF-HF signals to be up-converted enter the circuit via CON1 and are fed through matching transformer T1 before being fed into the balanced inputs at pins 1 & 2 of IC1. The crystal oscillator module, XO1, is a very small HCMOS SMD device which produces a 125MHz clock sig- nal at its pin 3. Its output voltage is 2.65V peak-peak, which is rather too high for linear operation of the mixer. In addition, it’s essentially a square wave, rich in harmonics of 125MHz as well as the fundamental. So we feed it through a low-pass filter formed by the 390nH inductor and 3.3pF capacitor first of all, to filter out most of the harmonics (which would contribute to spurious signals themselves, via cross-modulation in Fig.3: this scope grab shows the 125MHz signal from the crystal oscillator. This was measured using a 400MHz probe and a 350MHz scope, so many of the upper harmonics have been heavily attenuated. Even so, it can be seen that the waveform is far from sinusoidal and that’s why it’s followed by an LC filter to clean it up and so reduce spurious responses. 28  Silicon Chip the mixer). Then we reduce the filtered 125MHz signal down to a more suitable level for the mixer, via a voltage divider using two 10kΩ resistors. The signal is then fed into the oscillator input (pin 6) of IC1 via a 470pF coupling capacitor. Inside the mixer, the balanced input signals at pins 1 & 2 are mixed with the 125MHz signal and the mixing products appear in balanced form at the outputs (pins 4 & 5). Because IC1 is a double-balanced mixer based on a Gilbert cell, the outputs contain very little of the original input signal frequencies (Fin) and very little of the oscillator frequency (Fosc, 125MHz). Instead, they mainly they contain the “sum” and “difference” products, as follows: Sum product = (Fosc + Fin) Difference product = (Fosc – Fin) It’s the sum product that we’re really interested in, of course. Although the difference product is also present in the outputs, it is in a different tuning range and so it can be ignored. The balanced output signals from the mixer feed matching transformer, T2. As well as stepping them down in siliconchip.com.au SA602A IC1 D3 T2 CON4 11T 2T 1 CON3 TIE 27T 220nF 10k 470pF 10nF TIE T1 CON2 2 125MHz 5T +5V 5819 1 INPUT + + CABLE TIE XO1 LP2950 -3.3 390nH 3 4 47 mF REG1 10k MF-HF 47 mF 3.3pF 10nF 10nF D2* D1* VHF/UHF VHF/UHF NC COMMON RETREV N O CPU F H OUTPUT RLY1 NO COIL 13160170 3102 C D4 INPUT GND 4004 CON1 5711 GND 5711 Fig.4: install the parts on the PCB as shown on this layout diagram staring with the SMD components. Make sure that IC1 and XO1 (the crystal oscillator) are correctly orientated and note that you can remove any inadvertent solder bridges between IC1’s pins using solderwick. Diodes D1 & D2 are needed only if you intend using a long-wire HF antenna. GND JRC-23F-05 * D1 & D2 USED ONLY WITH A LONG WIRE ANTENNA – SEE TEXT impedance level (1500Ω/75Ω), T2 also converts them into unbalanced form to provide better matching to the input of the DVB-T dongle (or to any other VHF receiver, for that matter). The secondary winding of transformer T2 connects to the normally open (NO) contact of relay RLY1. This means that the converter’s output is only connected to CON3 when the converter is powered up. When +5V power is not applied, the moving contact of RLY1 connects to the normally closed (NC) contact and this connects directly to the converter’s VHF/UHF input connector (CON2). Most of the remaining circuitry in Fig.1 is to supply IC1, XO1 and RLY1 with power. IC1 and RLY1 operate from the nominal +5V rail, with diode D3 used to provide reverse polarity protection and D4 to absorb any backEMF spikes which may be generated by the coil of RLY1 when power is removed. Crystal oscillator module XO1 operates from +3.3V and this is derived by REG1, an LP2950-3.3 LDO device in a TO-92 package. The current drain of the Up-Converter is about 68mA, so it can be powered from a spare USB port on your PC, if you wish. In some cases though, the USB port on your PC may not be up to the job. That’s because because the bypass capacitor on the supply input of the Up-Converter is 47μF and the charging inrush current will exceed the maximum quoted in the USB specifications. Give it a go – if it doesn’t work, you will have to use a 5V DC plugpack. Diodes D1 & D2 protect IC1 from damage due to EMI spikes which may be induced into an external long-wire LF-HF antenna (if that is what’s being used). As shown, these two diodes are connected in reverse parallel and they limit the input voltage to around 500mV peak-peak, corresponding to around 2.7V peak-peak from the secondary of transformer T1 to pins 1 and 2 of IC1. Construction Apart from the three RF connectors CON1-CON3, all of the components are fitted on a PCB measuring 80 x 51mm and coded 07106131. The PCB and the three RF connectors are housed in a small diecast aluminium box, measuring 110 x 60 x 30mm. The component overlay is shown in Fig.4. There are seven SMD components in all, comprising IC1, XO1, the 390nH inductor, the 3.3pF capacitor, one 10nF capacitor (alongside XO1) and the two 10kΩ resistors. We suggest you solder them to the PCB before fitting anything else. This will make it easier, especially if you fit the five passive components first and then the two slightly larger active parts. Both the SA602AD/01 and the SA­ 612AD/01 mixer devices are in SOIC-8 packages and are pin compatible, so The completed PCB is installed in a metal diecast case, with the antenna input & output sockets mounted at either end. An on-board DC socket is shown here but an option is to use a panel-mount DC socket instead and include a power on-off switch between it & the PCB, so that the unit can be easily bypassed if you want to feed VHF/UHF signal straight through. siliconchip.com.au June 2013  29 (BOTTOM OF CASE) (LH END) C (RH END) C 14 A A 21 11.5 11.5 C L A C L 11.5 11.5 B 21 14 C C 57.5 HOLES A ARE 9.0mm IN DIAMETER, HOLE B IS 12.0mm IN DIAMETER, HOLES C ARE 3.0mm IN DIAMETER 23.5 20.5 (ALL DIMENSIONS IN MILLIMETRES) Fig.5: this diagram shows the drilling details for the metal case. The holes in the case ends should be made using a small pilot drill initially and then carefully enlarged to size using a tapered reamer. Performance Limitations While the combination of this Up-Converter and a DVB-T dongle can provide most of the operating features of a high-performance communications receiver, it’s unrealistic to expect the same performance.The high cost of communications receivers is the price you pay for superb sensitivity & selectivity, FM quieting, excellent image performance and so on. You are not going to get that sort of performance from a set-up costing a good deal less than $100. Apart from anything else, most DVB-T dongles are in a plastic case and that provides no shielding against the ingress of strong VHF signals like those from FM stations and DAB+ stations. So even though we have tried to make the Up-Converter’s output as clean as possible, you’re still likely to find spurious breakthrough signals in the part of the VHF spectrum into which the Up-Converter shifts the incoming HF signals. Another reason why spurious signals can appear is that the input circuitry of the Up-Converter is broadband rather than tuned; that keeps it simple and low in cost. In other words, the Up-Converter simply moves all signals in the 100kHz - 60MHz part of the spectrum up into the VHF spectrum, without favouring or discarding any particular signal frequency. By contrast, a true communications receiver has complex front-end tuning or pre-selection to keep strong unwanted signals at bay. In spite of that, it’s surprising what results you can get out of the DVB-T dongle/Up-Converter combination, particularly if you team them up with a long-wire HF antenna or an active indoor HF loop antenna with its own lowQ tuning circuit. you can use either as IC1. Both are made by NXP (formerly Philips) and are available from a number of suppliers including element14. Whichever one you use, just make sure you fit it with the orientation shown in Fig.4, ie, bevelled long edge downwards. XO1 is a tiny surface-mount module with a footprint of only 4 x 3mm. The modules used in the prototypes were Fox XPRESSO FXO-HC536-125 types 30  Silicon Chip (also available from element14). Its orientation is again critical, so make sure you position it with connection 1 (indicated by a tiny arrow or “foxhead” symbol etched into one corner of the top sealing plate) at lower left as viewed in Fig.4. You may need a good magnifying glass to locate the fox head symbol. After fitting the SMD devices, you can then mount DC input connector CON4, the leaded capacitors and relay RLY1. The relay is also very small, measuring only 12 x 7 x 10mm (L x W x H). We used a JRC-23F-05 relay from Futurlec in the prototype. Next you can add regulator REG1 and diodes D1-D4, making sure that you fit the correct diode in each position and with the orientation shown in Fig.3. Note, however, that diodes D1 & D2 need not be fitted if you use an inside loop antenna rather than an external long-wire antenna. In fact, with an inside loop antenna, it’s best to leave them out. Also, if they clip strong signals from an external antenna, they can produce distortion which may cause spurious signals to be generated, so again they may have to be omitted. Winding the transformers Transformers T1 and T2 have to be hand-wound, on an 18mm OD x 6mm-deep ferrite toroid in the case of T1 and a 14mm-long ferrite balun core in the case of T2. The primary winding of T1 consists of five turns of 0.8mm ECW (enamelled copper wire) wound fairly closely on one side of the toroid. The secondary winding has 27 turns of 0.25mm ECW, again wound fairly closely on the opposite side of the toroid. When both windings have been made, cut the free wire ends to about 10mm long and then strip off about 6mm of enamel from the end of each wire. That done, lower the toroid assembly onto the PCB, with each of the four wires passing down through their matching holes. Before you siliconchip.com.au Fig.6: the PCB is mounted inside the case on M3 x 10mm untapped spacers and secured using M3 x 20mm machine screws & nuts. It’s a good idea to fit four rubber feet to the underside of the case, so that the screw heads cannot scratch the resting surface. CON2 T1 IC1 X01 CON4 T2 CON3 RLY1 PCB M3 x 10mm UNTAPPED SPACERS M3 x 20mm MACHINE SCREWS solder them to the pads underneath, secure the toroid to the PCB using a pair of small Nylon cable ties – passing around the toroid and up and down via the 3mm holes provided. These cable ties will hold the toroid and its windings firmly in place while you solder the winding wires. Transformer T2 is wound in much the same way, except that in this case the winding wires are passed up one hole in the balun core and then back down the other hole, and so on. The secondary has only two turns, wound with 0.8mm ECW, so it’s best to wind it first. Then you can wind the primary, which has 11 turns of 0.25mm ECW. When you have completed both windings, cut the free wire ends to about 10mm long and then strip off about 5mm of the enamel from each end. You should then be able to lower the complete assembly down onto the top of the PCB, with the wire ends passing down through the matching holes. The balun core should then be held down against the PCB using a single Nylon cable tie, passing through the two 3mm holes provided. Finally, solder all four wire ends to the pads underneath to complete the Up-Converter’s PCB assembly. Now you can prepare the box. Drilling the box The position and size of all eight holes to be drilled are shown in Fig.5. The four 3mm-diameter holes in the bottom of the case are for the PCB mounting screws, while the two larger holes at each end are for mounting the three coaxial connectors CON1-CON3 and also for providing access to the DC power input connector CON4. Mark the location of all eight holes first, then centre-pop them to prevent the drill from wandering. Then drill them all with a 3mm drill. The three holes marked “A” and the larger hole siliconchip.com.au The Up-Converter can be powered from a 5V DC plugpack or you can purchase (or make up) an adaptor cable to run it from a USB port (see text). marked “B” can then all be enlarged and carefully reamed to the specified diameters using a tapered reamer. That done, you should lightly countersink the 3mm holes on both sides to remove any burrs, and use a small half-round file to remove any burrs from the larger holes at each end. Mounting the PCB You will need to mount the PCB in the box before fitting the three coax connectors. As shown in Fig.6, the PCB assembly mounts inside the box on four M3 x 10mm untapped spacers and is secured using four M3 x 20mm machine screws and nuts. Once this has been done, it’s fairly straightforward to mount the three coaxial connectors in the ends of the box. In each case, you simply feed the connector’s body in through its matching hole, fit the spring washer/ earthing lug and then screw on the mounting nut. Tighten up the nuts using a 12.5mm spanner to secure the connectors firmly in place – ideally with the earthing lug roughly level with the centre spigot and on the outer side of it so that can be linked to the earthing pad on the PCB nearby using a short length of copper braid or 1mm tinned copper wire – see Fig.4. Make sure you use the BNC connector for CON1, and the two Belling-Lee/ PAL connectors for CON2 & CON3. With all three connectors now mounted on the box ends, solder the centre spigot of each one to its matching solder pad on the end of the board and connect the earthing lugs. Finally, attach the box lid using the four M4 screws provided and your Up-Converter is complete. Trying it out There are no setting-up adjustments to be made, so trying it out is just a matter of linking the converter’s output connector CON3 to the input of your SDR dongle using a PAL-to-PAL patch lead of suitable length and then swinging over the lead from your VHF/UHF antenna to connect to CON2 of the converter. The HF antenna connects June 2013  31 Worth a try: SinoRadios’ TG34 Indoor SW/MW Active Loop Antenna Considering its low price when purchased via ebay, the TG34 Indoor HF Active Loop Antenna kit offers surprising value for money. Here’s what you get: •  A diamond-shaped loop antenna formed from about 1.68m of flexible insulated wire, complete with a telescopic “stretcher” to hold the two centre “corners” apart by about 560mm, plus a tiny RF amplifier located at the bottom of the loop in a heart-shaped case measuring only 25 x 30 x 10mm (WxHxD). The amplifier case has a 3.5mm socket at the bottom, and a sub-miniature slider switch on the front to change between the MW and SW bands. •  A control box measuring 77 x 28 x 15mm, which holds the two AAA cells used to power the RF amplifier, plus an on/off slider switch, a power indicator LED and a small tuning wheel. The control box has a 3.5mm socket for connection to the amplifier case and a captive 400mm-long output lead terminated in a 3.5mm plug. •  A flexible lead 4.57m long with a 3.5mm plug at each end to connect the RF amplifier to the control box. •  A suction cup with a moulded plastic hook, plus a small plastic spring clip with a loop of strong thread, to support the top of the loop antenna when it is mounted on a window or glass door. •  Two different output adaptor leads, to couple the output of the control box into MW/SW receivers which don’t have a 3.5mm external SW antenna socket. One of these leads is about 350mm long with a 3.5mm line socket at one end and a pair of plastic alligator clips at the other, for clipping to a receiver’s rod antenna and earthing screw. The other lead is only about 80mm long but with a 50mm-long ferrite rod antenna at the far end to magnetically Fig.7: SDR# spectrum & waterfall displays for Radio Australia on 11.9143MHz (AM). Note the -125,000,000 (ie, 125MHz) entry in the Shift dialog, so that the correct tuned frequency is displayed. 32  Silicon Chip couple into a receiver’s own ferrite rod antenna when all else fails. •  A small (160 x 90mm) suedefinish drawstring carry bag, into which all of the TG34 bits and pieces can be placed for users who want to take it on their travels. •  Finally, there’s an 80mm CD-ROM with user manuals for the TG34 and other SinoRadios’ products, in PDF format. By the way, the claimed frequency overage of the TG34 on the SW band is from 3.9MHz-22MHz, while on the MW band it’s 520kHz-1710kHz. As you can see, the TG34 seems to be intended primarily for mounting on the inside of a glass window or door using the suction cup and hook arrangement. However, this is by no means the ideal way of mounting a loop antenna, because this type of antenna has a doughnut-shaped response pattern with nulls in either direction along the axis of the loop (ie, at right angles to the plane of the loop itself). So mounting the loop parallel to a window or door results in minimum sensitivity in the direction perpendicular to the window or door. In any case whether you’re using a loop antenna like this on your travels or in a fixed location like your home, you really need to be able to rotate it in the horizontal plane so that you can find the position which gives best reception of the signals you want to receive. When I first received my TG34 kit, I tried it out suspended via the suction cup and hook arrangement against a window. The results were actually quite respectable on both to CON1 of the converter – see Fig.1. That done, connect CON4 to a source of 5V DC; eg, a spare USB port on your PC or a low-power 5V plugpack. When you switch on the 5V power, there should be a tiny “click” as signal switching relay RLY1 activates and connects CON3 and your SDR siliconchip.com.au the MW and SW bands. The gain provided by the TG34’s RF amplifier was very worthwhile and it was quite easy to tune for a peak using the little tuning knob on its control box, using SDR#’s spectrum display as a guide. All the same, and aware of the shortcomings of the fixed “against the window” location of the loop, I decided to see how much better it might be when mounted on a simple rotatable stand like that shown in the photo. I made the stand from a 750mm length of 27mm OD PVC pipe (cut from a 1m length), with a base made from a piece of 16mm Melaminecovered chipboard. The base was cut into an octagon shape measuring 240 x 240mm, with a 27mm-diameter hole cut (using a hole-saw) in the centre to take the bottom end of the PVC pipe. Then at the top of the pipe, I sawed and filed two slots diametrically apart, about 3mm wide and 20mm deep. These allowed the top of the loop to be dropped into both slots, before a 27mm ID PVC cap was pushed onto the top of the pipe to hold the loop in place. The centre of the supplied telescopic spreader was then attached to the PVC pipe using a small patch of double-sided adhesive foam, with another small patch of foam used to attach the little RF amplifier case to the PVC pipe down near the base. It’s very simple but it allows the complete free-standing loop antenna set-up to be placed on a bookshelf, just inside a window, and rotated as desired to optimise reception. I even found a $9 “Lazy Susan” turntable at one of the local bargain stores, which could be placed under the base to allow the antenna to be rotated even more easily. The results were quite impressive, too. The rotating stand allows you to find the best loop orientation very easily, again using SDR#’s spectrum display as a guide. So although an indoor HF antenna will never be as good as an outside long-wire antenna mounted well off the ground, I can report that an active indoor loop antenna like the SinoRadios TG34 can give quite acceptable results, especially when mounted on a simple rotatable stand like the one shown. By the way, the TG34 kit can be ordered online via ebay from SinoRadios (http://stores.ebay.com/SinoRadios), who are currently offering it for US$11.99 plus airmail postage of A$11.55 to Australia. So the total cost is about A$22.50, which seems to make it a very good match for our low-cost SDR using a DVB-T dongle and our new Up-Converter. dongle input to the Up-Converter’s output. If you then start up SDR# (or whatever SDR application you’re using), you should start to see signals coming in from the LF-HF part of the spectrum; shifted up into the VHF region, of course. If you are using SDR#, this won’t be a problem provided you have already set the “Shift” frequency (in the Radio section of the control panel at upper left) to a figure of -125,000,000. If you then click in the small square box just to the left of the “Shift” label, the displayed frequency will swing down to indicate the correct frequency of the tuned HF signal. You’re now free to explore the LF, MF and HF bands in the same way as you’ve been exploring the VHF and UHF bands. And, of course, you can return to looking around on the VHF and UHF bands at any time simply by siliconchip.com.au This view shows the author’s SinoRadios TG34 active loop antenna mounted on a simple stand. Mounting it on a lowcost “Lazy Susan” tuntable allows it to be easily rotated for best reception. June 2013  33 Up-Converter to explore the LF-HF bands will depend very much on the antenna you use with it. Probably the best type of antenna to use is a longwire antenna mounted outside your house or apartment, as long as possible and mounted up as high as possible. The input earth to the Up-Converter should also be connected to a good RF earth, eg, by connecting its metal case to the metal chassis of an earthed piece of equiment. Alternatively, you can connect it to a galvanised metal stake driven into moist ground. In a lot of cases, this kind of antenna set-up won’t be practical though. You may have no way to mount a large outside antenna of any kind and in this situation you’ll probably be forced to go for a loop antenna mounted in the nearest window, or a helical-wound “vertical broomstick” antenna mounted as near to the window as possible. Note that if you go for the helical antenna you’ll still need a good earth but this won’t be needed if you go for the loop antenna. Active loop antenna Figs.8 & 9: SDR# spectrum and waterfall displays for a 26.675MHz narrow-band FM signal (top) and a 702kHz AM signal (bottom). As in Fig.7, a frequency shift of 125MHz has been entered so that the correct tuned frequency is displayed. switching off the 5V power to the UpConverter and clicking again on the box just to the left of SDR#’s “Shift” label, to de-activate the 125MHz downward shift in the display frequency. By the way, if you want to frequently switch between the two antennas, then it’s a good idea to fit an on/off power switch to the Up-Converter. One small piece of advice: be sure to use a good-quality double-screened 34  Silicon Chip PAL-to-PAL patch lead to connect your Up-Converter to the input of your DVB-T dongle. This will allow less ingress of VHF signals (and therefore lower the incidence of spurious signals), than with an “el cheapo” singlescreened patch lead. I found this out the hard way! Which HF antenna to use? The results you’ll achieve using the Even with a plain loop antenna, the results you get will depend on your location and altitude. If these conditions are not very favourable, you might like to try using an active indoor loop antenna, ie, one with an inbuilt RF amplifier and possibly some sort of tuned preselector circuit. I considered the possibility of developing an active HF antenna of this type but then I looked around on the web (and ebay in particular). There I found a number of complete HF active indoor antenna kits, made in China and available for between $12 and $15 plus postage. Since you wouldn’t be able to build one for such low prices, I quickly decided to forget the idea of a DIY antenna and simply buy one of these Chinese bargains. The loop I bought was the TG34 from SinoRadios (http://stores.ebay. com/SinoRadios), who currently offer it on ebay for US$11.99 plus airmail postage of A$11.55 to Australia. It arrived in my mailbox within one week, for a grand total of A$22.50. Since then I have been putting it through its paces with my prototype Up-Converters and DVB-T based SDR set-up. The panel on the two preceding SC pages has the details. siliconchip.com.au