Wednesday, 5 October 2011

Grinding to a halt

Regular visitors may have noticed that posts to this blog have thinned-out over the last year. The situation will not get better; my life has changed, and I can no longer support more than one blog. Choosing which to keep was very difficult; but I felt that this one is composed of material which would be better presented in ebooks. It was always my intention to do just that; compile a 'best-of', and some of the posts alluded to it.

I will leave the blog 'live', and hope it will be of interest to someone. Goodbye for now, 73, Pete.

Sunday, 10 July 2011

Delivery of Unobtanium

Crystals for 10140kHz have become very difficult to obtain of late. A lot of hams have become interested in QRSS, and this sub-band of 10m is a prime focus for the specialisation. I had to order from a supplier in the US, and yesterday they arrived, complete with a complementary ballpoint pen!

I will be taking time in constructing the next manned experimental propagation transmitter (MEPT), and the improved stability afforded by the new crystals is just one intended improvement. I also want to take advantage of a reception technique called 'stacking', where successive transmissions are overlaid in the grabber (software receiver), building up a stronger image of the transmission. The technique requires the transmissions to begin exactly 10 minutes apart, preferably aligned with the hour. Non-stacking grabbers receive the signal as normal, but those which can stack grabs will show enhanced images in their stack archive. This will involve use of the timers in the microcontroller; I use a PIC16F690.

I will be simplifying the format of the transmission. I will no longer transmit my locator, as just about everyone has access to and that makes the locator not just redundant, but possibly a little irritating for most users.

Next post will be a report on the success (or otherwise) of the new MEPT. Work begins tomorrow morning.

Tuesday, 7 June 2011

30-metre Propagation Transmitter

Another new direction in radio for me; weak-signal work. Well, I've been involved with low-power radio (QRP) since my return to the amateur scene in 2005, but this is a new departure. I'm currently operating a manned experimental propagation transmitter (MEPT) in the 30m band. There are lots of people doing this now, and the body of knowledge we are corporately and severally building is impressive. The idea is to shift the energy density away from power, and expand the time instead. The net energy expended in communication is identical, but my 50mW signal takes many times longer to transmit than when I operate at 15WPM with a hand key at 5W. To put it into time terms, each dit takes three seconds to transmit.

So, did I make the MEPT, build it from a kit or buy it? There seemed so little to it, so I brewed my own. I've recently got up to speed on how to program and use Microchip PICs, and this was one project where a PIC is ideal. I use the PIC to control the keyline and frequency of the MEPT; four lines from one of the ports feed a simple R2R digital-to-analogue converter, which in turn drives a LED in the oscillator, pulling the crystal LF to give the 'waveform' structure of the signal. My signal is shown above, in the centre of the picture. It alternates between conventional dit-dah Morse code, giving my callsign and locator, and my callsign again in 'slant' CW. This form of encoding is very time-efficient; I can send my callsign in the time most stations take to send a zero in dit-dah Morse. The downward slanted lines are dahs, and the upward one are dits.

The picture above shows the oscillator, housed in a small steel tin. The crystal is a bit of a special; it was originally a 10.15MHz item, but with the aid of Hans Summers' excellent article on crystal penning (do a search for G0UPL), I was able to push it down ten whole kHz with a CD marker. It had to be 'aged', or dried-out after this treatment to drive off the volatiles in the ink but it eventually settled down. It is thermostatically controlled by heating it with a 47R resistor regulated by a simple linear FET / thermistor circuit, and the whole oscillator is enclosed in 20mm of closed-cell foam. The PIC controller, 74HC02 driver / keyer and the VN10KM PA are built up outside in the main enclosure.

I have found that two levels of voltage regulation are required for stability, and in spite of the thermostatic control holding the oscillator at 50 degrees centigrade, the MEPT still drifts slowly up and down during the day. An exercise for the coming weeks.

Tuesday, 4 May 2010

Simple passive filter for simple receivers

I've built one of these before, but it was impractically heavy. This one is much smaller; I can cheerfully let it dangle from the headphone socket of a radio without fear of it tearing the cable or damaging the socket. Because it is added inline with the headphones, it can be used on any radio, but it will only match the impedance of a pair of iPod or Walkman-style 32-ohm headphones. It's a constant-k T-section low-pass filter. It's design ed for a termination of 64 ohms, when the 4.7uF capacitor is used. The 1uF capacitor gives a miss-match, but this is not apparent in use.It's delightfully simple, and inexpensive. Most homebrewers will find the bits in their junk box. There are just eight components:
A box
An old headphone lead with plug
A headphone socket
Two 10mH power chokes
A 1uF capacitor
A 4.7uF capacitor
A small SPST switch (anything would do)
I used RS 233-5487 chokes, but any reasonably large choke will work. The main feature is the DC resistance; at under 4 ohms, these are ideal. Little wire-ended chokes will have a resistance of 80 ohms or more, and will prove too lossy. Current price is GBP1.49 each.
Electrolytic capacitors were used, but polyester caps would also be good. The switch and one of the caps could be omitted if you are happy to have just one cutoff frequency, or you could add another. I chose the values given because they give cutoffs of 1000Hz and 3000Hz, ideal for telegraphy and phone respectively.
The biggest benefit to users of simple receivers is that this filter will remove much of the hiss produced by an LM386 or similar simple (but noisy) AF amplifier. The box I used measures 50x50x35mm, but it could be squeezed into a smaller volume. It would be best to separate the two chokes as far as practicable, to reduce mutual coupling. It is light and small enough to be pocketed, and makes my homebrew receivers sound great.

Thursday, 12 November 2009

What is a goniometer?

Goniometer - The name means 'angle measurer'. In radio context, it is a set of coils arranged to determine the bearing (direction) of a transmitter. The scan at right is from the 1938 Admiralty Handbook of Wireless Telegraphy, a two-volume set which has spent more than half of its existence in my collection.

The name Goniometer was chosen for the blog because although its relevance to modern electronics is marginal, the word stands out eyegrabbingly in a page full of search results; this gets more traffic to the blog without resorting to the less ethical SEO methods.

A goniometer is useless without a set of source antennas, and at left I have another page from the Handbook, showing a set of Bellini-Tosi direction-finding antennas. The triangular loops at the top are the antennas, and the three two-turn coils at the bottom are simplified representation of the goniometer coils.

The operation of the system is straightforward. The sense coil ('S' in Fig.18) is rotated until a null (little or no output) is heard in the receiver. The sense coils are connected to a pointer, which indicates the direction of the source transmitter.

Tuesday, 10 November 2009

003 Fitting Part 1

Hacksawing, filing, finishing

Metal, and other materials from which we make the hardware of our projects, will need to be cut to shape to form the basic components of cabinets, panels, PCBs and other parts. Even when ready-made cases and enclosures are purchased, some cutting operations will be necessary to make the item fit our purpose.


Hacksaws are the basic cutting tool for hard materials. Two common types are the 12-inch (300mm) hand hacksaw, and the 6-inch (150mm) junior hacksaw. Junior hacksaws are an ideal first purchase for a home electronics constructor. The frame and a pack of blades can be bought very cheaply, and will last years. Full-size 12" hacksaws will cost more, and there is a choice of blades available for different materials and sizes of stock. Coarse blades (18 teeth per inch (TPI)) are suitable for soft material such as wood, plastic and aluminium, while finer-pitched blades (24 to 32 TPI) work better on hard stock like steel and will cut a much thinner material without the blade's teeth jamming over the work.

Blade fitting and tuning

Hacksaw blades are usually fitted with the teeth angled forward. There are two exceptions to this; when they are fitted to power hacksaws, where the blade is pulled back toward the case of the tool to perform the cut, and when a blind-hole saw is made. A description of how to make and use a blind-hole saw can be found at the end of this section.

To fit a blade to a junior hacksaw, simply apply one end of the blade in one of the mounting slots, pull the frame in to shorten it under tension and slide the far end of the blade into position. Take care not to slip, the blade will be very sharp when new.

Full-size hand hacksaws vary in the way they attach blades, so read the instructions supplied. In general, there will be a way of relaxing one of the blade-holders so that the blade's holes can be fitted into them. Full-size hacksaws need to have their blades tensioned. Again, follow the maker's instruction, but as a rule, if it is a screw-tension saw then two and a half turns from the point where the slack is taken-up is about right.


The work must be firmly clamped. Use a vice, or clamp the workpiece to a stout table with a G-clamp. Use packing pieces made of wood if the clamp or vice threatens to mark the face of the work. Make sure that the line of the cut is as close to the clamp or vice jaws as possible. Cutting too far from a solid mount will let the work vibrate. This creates noise, slows the cutting and the operation will be more difficult as the saw jumps around.

Remember that the saw has a 'kerf', a well-defined thickness of cut. Allow for this when you apply the blade to the work. Make the first stroke a light one, to get the blade started in the cut. Subsequent strokes can be more forceful. Do not apply undue pressure to the blade; for most work the weight of the saw will be enough. Too much force will make the blade wander around, as it squirms under the strain.

Stance is important. Relax the knee joints, put your left foot forward and your right foot back, angled out 45deg. Keep your back erect, and your torso facing slightly right of centre. Martial arts student will recognise this stance; it is common in many schools of Tai Chi Chuan. Use the full length of the blade, and keep the blade running parallel at all times. When the cut is nearly complete, the offcut part may need support; use your left hand reaching over the saw to do this. Left-handed workers should reverse all of the above instructions for their comfort.

Blind-hole saw; making and using

Blind-hole saws are used where access to both sides of the work is difficult or impossible. It's easy, and requires only a hacksaw blade of appropriate TPI and some duct tape. Wind the tape around the blade, at the end normally fitted to the front of the saw frame. Put plenty of tape on, it will act as a handle and protect the user's hand from the teeth. To use, simply pass the blade into a hole in the workpiece and pull the blade back while applying pressure downward. Be careful when pushing the blade back in for the next stroke; it's all to easy to bend and break it.

Other Saws

Some craft-knife kits (X-acto) contain a miniature tenon saw blade. This is very useful for fine work, such as cutting the copper cladding on circuit panels.


Files are used to smooth rough-cut surfaces, to remove small amounts of material and to take off 'burrs' created by other operations. There is a wide range of shapes and degrees of coarseness, but the home constructor needs just a couple to start with. A 150mm second-cut file and a 150mm round needle file. Needle files can be bought in sets from bargain stores, market traders and tool shops. In terms of quality, you get what you pay for. The second-cut file will be more expensive, but with care will last a long time. You can usefully add a 250mm bastard file (coarse-cut) and a 6mm diameter rat-tail (round) file to your collection for more aggressive work.

  1. 250mm bastard file
  2. 200mm half-round file
  3. 6mm rat-tail file
  4. 150mm second-cut file
  5. 100mm three-square file

Files, like hacksaws, only cut in the forward direction. However, lightly drawing the file back over the work can help to clear particulate matter from the teeth with some materials. Some metals, like copper and aluminium, are a real problem when being filed. These malleable materials clog the teeth readily, and picking the smears of metal out every so often can be a chore. A partial solution to this is to rub the file over a piece of chalk before work begins. This fills the teeth, and being soft it allows the work to proceed. The chalk lubricates the cut and prevents the soft metal from clogging the file.

Never use a file without a handle firmly attached. If you try to use a file by holding the bare tang, it would take only a moment's inattention for the file to slip and the tang to enter your wrist. Don't do it.


Take a stance as described in 'Sawing', above. The main point to watch is that the file must be kept parallel to the work, or the surface will be curved instead of flat. If you want to form a curve, then start with the front tipped downward, and push it upward as the stroke proceeds. This is counter-intuitive, and you may feel it better to push the file 'over the top', but try it and see; it works better.

Burrs and sharp edges can be dressed-off with one or two light strokes at 45 degrees; sometimes a wider 45 degree 'chamfer' is more appropriate. Sharp corners can be knocked-off in a similar way.

Care of Files

The cutting teeth of files are glass-hard, and liable to chip off if abused. Keep files separate from each other in storage, and take care not to drop them, they may break. Files do wear out, so be prepared to replace them after some extended use. Worn-out files can be ground into other tools, such as scrapers and other edge-tools.

Other Cutting Tools

You may occasionally find use for a small, round gouge, as used by wood-carvers. You can modify PCB tracks with this.

An Abrafile (a thin, coarse round file mounted in a hacksaw frame) is great for piercing odd-shaped holes in panels, but they are becoming difficult to find.

Hobbyist drills (ie Dremel) are wonderful for fine work, and are ideal for occasional PCB drilling.

A small cabinet scraper will help when preparing wood, and it will be useful for removing burrs from copperclad board. Make one from a piece of old file; just remember to leave the burr on the edge when grinding it, this is the cutting edge.


To prepare a panel for painting, it needs to be keyed and cleaned. The best way of providing a key for paint is to use either a disc abrader (DA), or an orbital sander. Small parts can be prepared by hand using Scotchbrite or a similar nylon abrasive pad. Degrease with soap and water, or with a solvent cleaner.


Spray painting gives the smoothest finish, but other methods can be used. Hand-painting with a brush can be effective if applied with the panel horizontal, and time is taken to brush-out the paint evenly. Curing of spray paints is accelerated if the work is gently warmed with a hair drier first. Choose a colour which suits the purpose; matt black or olive green are trendy, but light aircraft grey is easy on the eye. Leave the paint to cure / dry, and don't be tempted to handle the work until the paint is hard.

Panel Marking

I was brought up to use rub-down lettering, but this method is slow, liable to damage and gives poor results except in the very best hands. Today, it is far easier to use a computer to lay-out and make self-adhesive panel covers. Use your favourite paint program, and with a little planning you can make a one-piece label to cover your front panel. Complex features and symbols are possible. If your home-made equipment needs a calibration chart, print one of those, too, on self-adhesive label, and stick on the case. If you can't find continuous A4 sheets, then choose sheets of labels which have the individual labels butted together, and simply reassemble them on your front panel. Test spray lacquers or brush varnishes to find one which doesn't make your printing run or bleed, and you have a complete system. Holes for control shafts and lamps are cut out with a craft knife.

Thursday, 5 November 2009

USB-Powered Direct-Conversion Receiver

I've built and blogged one of these before, but now I've developed the idea a little. The original suffered with a poor antenna and although it worked, it was more than a little deaf. The new USBrx has eighteen components, including the connectors, antenna and panel.
  1. Loop antenna wire (32feet, 9.75m) - any insulated wire will do.
  2. Loop antenna connectors male (2 required)
  3. Loop antenna connectors female (2 required)
  4. Tuning capacitor, 20-200pF, polyvaricon
  5. Knob to suit tuning capacitor
  6. Ferrite toroid, 9mm 4c65, FT37 type, any small HF ferrite
  7. SA602 mixer / oscillator
  8. Quartz crystal, frequency at band edge
  9. Emitter capacitor, 30pF
  10. Feedback capacitor, 30pF
  11. Bypass capacitor, 3n3 (3300pF)
  12. Coupling capacitor, 100nF
  13. Supply decoupling capacitor, 10uF
  14. USB extension cable
  15. Old earbud cable
  16. Copperclad panel (FR4), 55x55mm
You can save on some parts by permanently mounting the loop wire, without using the connectors. I use my 32-foot loop on other radios, so I need to be able to disconnect it. If you plunder an old AM radio for the tuning capacitor, you may have a knob with it already.

The antenna is a loop, 32 feet long, and works as a quarter-wave magnetic loop for 40 meters. It was first described by Ben Edginton G0CWT, to whom I'm indebted for the idea. It's basically an opened-out version of the 'original' USBrx antenna, itself a copy of the Poundshop antenna, first used by me in 2005 as part of the Poundshop Radio. These antennas are directional, and very compact. An eight-foot square for forty metres is something which can be occasionally strung across a room (I use lengths of nylon string and small bulldog clips), and taken down when business is done. The loop is resonated with the polyvaricon variable capacitor, which can have one side connected to the panel. I made a 64-foot version in 2007, and with extra windings on the transformer I used it on Topband.

The antenna has a very low impedance, in the region of 2.5 ohms, so a transformer is used to match the loop antenna to the 1500 ohms of the SA602. One turn (a single pass through the toroid's hole) for the antenna and 25 turns of 30SWG enamelled copper wire for the SA602 input.

The output, using the tip and body contacts of the 'earbud' cable, will drive the microphone input of a PC soundcard. You can listen to it with headphones directly from the soundcard output, but you won't like it. Everything from the crystal's frequency to 25kHz in both directions will be coming at you, and it's quite a cacophony. To make sense of it, you need a little help from one of my favourite pieces of software, the SAQrx. You can download this from here. This is intended for use with LF antennas for monitoring the famous Alexanderson Alternator transmissions, which use the callsign SAQ, but it has a host of other uses, including rudimentary digital signal processing. Because the SAQrx software can filter out any narrow band from near DC to over 20kHz, you can use it to choose a spot frequency to listen to. You can choose narrow, medium and wide bandwidths, and the gain can be adjusted to suit. The program works well under Wine in a Linux machine, as shown in the photograph. Comment if you need any help with this one!

Thursday, 29 October 2009

002 Hand Tools

Some tools are absolutely necessary for building radio equipment. The following short list should be shopped for, before attempting any practical work. These are the bare minimum required to assemble the projects detailed in this book, and may be all you ever need...

Soldering iron

We'll discuss each tool in turn, and suggest what to buy to get the best value and utility.

Soldering Irons

We use soldering irons to melt solder, which is an alloy designed to melt at relatively low temperatures. The solder is allowed to flow over the joint area where we wish to connect one component to another. The actual art of soldering is described in section 007 of this book.

Soldering irons can be bought very cheaply from discount shops; the quality of these will vary from tolerable to unpleasant, but they will be cheap, and will get you started. The power rating to look for is in the range 20 to 40 watts. Anything more powerful will be too large physically, and if the temperature is unregulated (highly likely in a cheap iron), then it will be difficult too achieve a clean soldered joint. Big irons are useful for heavy work, such as antenna work or soldering pieces of PCB material together. Besides the iron, you will need a damp cloth to wipe the tip of the iron on to remove the accumulated flux residue. Better-quality irons have a piece of special sponge material set into a small tray in the stand; keep this damp and wipe your iron's tip regularly.

I use three irons for work at home; a temperature-controlled soldering station, a 100W high-power iron and a small butane-powered portable. The soldering station is good for general work, and mine has a cleaning feature with rotating sponges inside the housing, which retains the moisture the sponges need to work. The 100W iron is useful for soldering pieces of copper-clad together to make cases or complex modules. Larger screening cans are best soldered with a big iron. The gas iron has a very small tip, and I use it for surface-mount devices. It has an integral flint wheel in the cap.


You will need at least one pair of pliers. A pair of snipe-nosed pliers: get miniature 'electronic' types, as what you are working with will be small, and the leads may not be gripped by larger types. A pair of large, engineer's pliers will also prove useful for gripping larger items. One special pair to get if you decide to take your wiring skill to the next stage is the round-nosed plier. These, as their name suggests, have a pair of long, thin conical tips; these are used to curl wire into a ring shape for making 'mechanical' joints. I modify mine to be as versatile as possible by filing one of the tips smaller than the other, to make a wider variety of joint sizes.


Cutters are available in a bewildering array of shapes and sizes. The ones to buy first are electronic side-cutters. These are for cutting thin copper leads on components, but can also cope with light-gauge copper connecting wire. Never use them on steel wire; they'll lose their edge or break. For very fine work, a pair of 'flush-cut' cutters can be added to the kit. If you anticipate cutting heavy cable, such as hard-drawn antenna wire, then a stronger pair will be needed. A farmer's fencing tool is great for this, as it includes a range of functions in the head, including a hammer and a nail-extractor.


Buy just two screwdrivers initially, and add to the range as you feel you need them. The ones you need are a small Pozidrive, and a 3mm flat-blade. Philips screwdrivers are different to Pozidrive; the angle is steeper and they only have four large flutes. Pozidrive tips have an additional four smaller flutes. They look similar, but you feel the difference when you try turning a tight screw. Extra screwdrivers can be bought as needed, a 6mm flat-blade and a larger Pozidrive are sensible additions. If you decide to buy a screwdiver kit which includes a handle and a range of interchangeable tips, then you have lots of options, including hex bits and Torx bits. Be careful about quality; poor alloy choice means that the tips will wear quickly and chew-up your screw heads.


Use whatever knife style you feel comfortable with. Here at G1INF, the favourite is a Stanley blade in a home-made handle. The choice is between fine blades with good access for detailed work, and heavy blades with fat handles for meatier cutting.


The most likely spanner sizes you will need are 5.5mm (M3), 7mm (M4), 8mm (M5), 10mm (M6) and it would be wise to get a selection of larger sizes, or a small adjustable spanner, for working with control spindle nuts. Small kits of combination spanners (ring one end, open the other) are to be found in hardware stores, bargain shops and market stalls. These are ideal for light home use, but be sure you get that essential 5.5mm spanner. Tip - A 5BA spanner works on M3.


Unless you have worked with radio or electronics before, you are unlikely to have seen a trim-tool. They are essential for setting-up trimmer capacitors. It is tempting for a beginner to use a small screwdriver to adjust a trimmer, but you'll soon learn by your mistake. Screwdrivers have a considerable metal mass, and this adds to the capacitance of the trimmer (and the circuit you're trying to tune). The process will be very frustrating, and although possible if you have the time to cut-and-try the over- or under-adjustment, it's far easier and quicker to use the real thing. Cheaply available from electronic suppliers, trim-tools are a tough plastic rod with a tiny piece of hard metal in each end. They do not appreciably change the capacitance, and you can adjust the tuning in 'real-time'. One word of warning - Trim-tools are not tough enough to be used as a screwdriver, so don't be tempted.


A vital first piece of test equipment is a small, cheap, digital multimeter. At the time of writing (2009), these could be bought for around £5 in bargain stores. There are cheaper models, but they may not have the ranges you require. Cheap analogue (moving-needle) meters are rarely worth buying; they are usually too insensitive. Digital meters have a high-impedance buffer amplifier before the ADC, and are very sensitive. A better-quality analogue meter has its uses. You can see trends in the measured value, and for some measurements it simply feels more comfortable to watch a needle swinging.

Minimum specification for a multimeter:

DC Volts ranges 0.5V to 500V
DC Current ranges 100mA to 10A
Resistance ranges 100 ohms to 2M ohms
Sensitivity - 20,000 ohms per volt (20K ohm / V)

Direct current (DC) voltage and DC current are the main parameters measured when testing a circuit. It is useful to check resistance in a number of situations. If you are unsure of a resistor's value, then measuring it with a meter will confirm it. Circuits can be checked (with the power off) with a resistance meter, and items like fuses and connectors can be analysed for faults.

The sensitivity is where many very cheap meters will fall by the wayside. The lower the sensitivity, the more current is taken from the circuit under test. If you apply a low-sensitivity meter to a high-resistance circuit, you will get a false reading, as much of the circuit's voltage will be drained by the meter. [graphic of meter loading]

One aspect of cheaper multimeters is the quality of the test leads. They will break long before the meter gives trouble, so note that both the pictured meters are fitted with home-made test leads. These are easy to make. Red 'bullet' crimp terminals are compatible with standard 4mm test connectors, and are cheap and easy to buy. A crimp tool for applying them should cost less than £3-.

Other Tools

Tools are seen by some as an object of desire in themselves. People collect tools, not because they need them, but because they look interesting, or they think they may one day have a use for them. If you want to amass a toolkit of your dreams, then go ahead, It's part of human nature. However, if you are fastidious about utility, then the tools above may be all you ever need. There are some extra tools worth considering, and they are described next.

Desoldering Pump

If you plan to make printed circuit boards, then a desoldering pump will be needed if you need to repair them. When a device is removed from a PCB, the hole invariably remains filled with solder, making it impossible to fit a replacement device. A desoldering pump is used with a soldering iron to remove the solder, leaving the hole open. They work like a back-to-front bicycle pump; you push the piston all the way into the cylinder until it latches, heat the joint to melt the solder and then apply the tip of the pump to the joint. Pushing the release button causes the piston to fly back under spring pressure, sucking the molten solder up the tip. The solder is collected in the cylinder, and will need to be emptied from time to time. Unscrew the tip carrier, and shake out the solder droplets. Any which remains stuck to the face of the piston can be released with a small flat-bladed screwdriver. Always wash your hands after this operation, as the finely-divided solder powder is easily ingested, and is toxic. There may be local regulations for the disposal of heavy metals, so either take advice from your local authority about disposal, or consider recycling the solder.

Wire Strippers

I have deliberately left wire stripper out of the list of essentials, because they are unnecessary for low-volume non-commercial wiring. I only have one wire stripper in my radio toolbox, and that is a special one for a particular fine, single-core wire I sometimes use when prototyping digital equipment. I keep wire strippers in my 'commercial' wirer's toolbox, because my clients expect me to use them, and I do. But at home, I have other ways.

There are at least three ways to take the insulation from the end of a wire without using wire strippers. The first, and least recommended, is to place the wire between your teeth and pull. The chances of chipping a tooth are high, and you run the risk of ingesting slivers of wire. Don't do it! Another way is to slice around the insulation with a knife, and pull off the resulting tube of insulation. This works well, but care must be taken not to cut the wire, or your fingers.

The most frequently-used method is to use your side-cutters. Grip the wire with your cutters, with the flat side of the cutters toward the end of the wire, and pull. It takes practice to remove the insulation cleanly, but once the knack is acquired, you won't look back. This method works well with multi-stranded wire, which is the commonest type. It is flexible, due to its rope-like structure, and the strands deform under the pressure of the cutters, so that more of the cut is applied to the insulation. When stripping single-core wire, make a rotating cut with the cutters before applying the pull.

Don't use this method if you are working in industry, as it is not approved by any employer I've met. You will inevitably mark or even cut through strands of wire when you're learning, but practice makes perfect.

In my experience, a cheap pair of strippers will cause more damage than the use of side-cutters, especially in inexperienced hands. Try the side-cutter method, and by all means invest in a pair of strippers if you feel the need. The type popular with installation electricians, which have a pair of jaws with a dozen or so tiny teeth, are ideal for everyday uses. For high-quality work, a pair of separation-jaw strippers will give first-class results. These are approved for use on aircraft and military equipment and are expensive.

As I mentioned in the first paragraph of this section, I keep a special stripper for miniature solid wire. It is specially designed to cut the insulation, and leave the core conductor intact. These, like the separation strippers described above, are not cheap. The one in the photograph has lasted the author for over twenty-five years.

Test Equipment

You will be able to do a lot of testing with just your multimeter. Its capabilities can be extended by making add-on modules, some of which are described in the projects section of this book. They will allow you to measure capacitance and radio-frequency power. There are other projects there which don't involve the multimeter; these include the noise generator, audio amplifier and gate dip oscillator. Other items of test equipment which will be useful to a radio experimenter are outlined in section 017, but as they are expensive and complex, they will not be discussed at length. The two main equipments in this category are the oscilloscope and the spectrum analyser; get an oscilloscope if you can, but check article 017 'Radio Frequency Testing' before you buy.

Care of Tools

I possess tools which date back to my apprenticeship, over thirty years ago. They've stayed with me because I've looked after them, and they've looked after me. Storage of tools is important, to keep them from damage (and from damaging people); an old adage - 'A place for every tool, and every tool in its place'. Keep your tools in a dedicated box, or set aside one or two drawers of your desk for them. Keep them clean; wipe them down with a dry (or slightly oily) cloth, and lubricate moving parts when needed. Pliers and cutters need their hinge joints oiled at least twice a year, more frequently if you use them full-time.

A useful way of storing test leads is to coil them individually, and pop them into small ziplock bags. Squeeze the air out before the seal shuts, and they snug down into a drawer or box very neatly without sprawling everywhere and tangling. If you have wall-space, hang them on hooks for easy access.

Instruments need special care. Check the battery occasionally. Some instruments use a tiny current, and the battery can degrade and leak before it is exhausted. Protect meters and other sensitive equipment from mechanical shock; moving-coils meters are particularly delicate, and a little attention to handling and storage will keep them safe. Protect them when storing, tarnsporting or even in use by means of polythene foam cushioning.

Other Workshop Facilities

A good stout bench, desk or table is needed to work on. I know amatuer constructors who cheerfully work on a bureau, wooden tray on the lap and even on the kitchen work surface. There have been designs published in the old paper magazines for special electronic hobbyist's work-stations.

A vice can be very handy; make sure it's well secured to the bench, and that the rear (fixed) jaw is overhanging the edge of the bench. Cupboards for storing equipment and components, small boxes for keeping things tidy, drawer-cabinets for small items. Take time to think how best to arrange your working space, and ask the rest of the household for their opinion!


Think carefully before setting up shop in a shed or other outside structure. Is it dry? Does it freeze? Some devices (LCDs are an example) can be damaged by very low temperatures. Inside the home is best, but an outside shack can be a rewarding retreat if you can keep the temperature up and the damp down in winter. My mentor, Eric G3HTP (SK), kept his station under the stairs; he was quite comfortable, and far enough from the sitting-room not to disturb the family. I work in a lean-to extension at the back of our house, but I have used a metal shed, which made an excellent ground for my antennas.

Monday, 26 October 2009

24MHz HF Theremin

I've been wanting to blog this for a few months, but life got in the way of preparing enough material for release. I've updated the static site at g1inf4u with enough information for an experimenter to try their luck with one. The full link is here, and do please comment if you'd like more information.

The theremin is a 'mature' project; it's been around for a couple of years. The original still works, and I find it a great asset when I'm teaching the art of the Boatswain's Call. The two instruments are very close in character, and the way they are played. They share the distinction of being the only instruments whose pitch is controlled without the player's fingers touching it; only the distance from the tone-determining antenna or hole is relevant.

Theremins are relatively rare. I've just had to add the word to my netbook's dictionary, and few people recognise the word, either. The sound is unmistakeable, however, and while similar to a bowed saw, the range of frequencies is quite large. My theremin can make any tone from a low ten-hertz growl to a ten-kilohertz shrill.

I've seen pocket-sized theremins cropping up here and there, but none offer level control. I've used an ORP12 photocell with a white LED, and this gives a useful dynamic range. It would need a window amplifier to cut off the signal entirely, but then the circuit begins to get complex. Currently, it's simple and elegant.