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!