The Best Of 'In Practice'

Archives and 'extras' from GM3SEK's monthly RadCom column

Also see
Cumulative Index, 1993-2010
indexed by subject

CONTENTS Click the links for more details. 


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GM3SEK's Amateur Radio Technical Notebook


Attenuator design programs

Blowers: estimating performance from dimensions

Chokes: Toroidal and other chokes

Clamp-on RF current meter

The most useful tool for antenna and RFI investigations! Here are G0SNO's original article and some further construction ideas. 

Connectors: 'Good-enough' ways to assemble DIN and D connectors

'In Practice' for April and May 2004, now available as PDF downloads.
Part 1 (April, 6.5MB PDF)
Part 2 (May, 5.6MB PDF)

Connectors: Powerpole connectors

PowerWerx, Inc.

DC injector/extractor for coax feedlines

This simple "Bias-Tee" handles 1kW+, 1.8-50MHz.

Directional Wattmeters

Inside a Directional Wattmeter: the real inside story about SWR meters and directional wattmeters, from the classic QST article by Bruene.

Drilling templates

Driving earth rods

Filters:  Harmonic notch filters

The easiest way to remove harmonics from your VHF/UHF transmitter output... practical solutions from G4SWX.

FM and UHF-TV interference filters

RFI-proof your TV and FM receivers - and your neighbours' too! More practical, proven solutions from G4SWX.

High voltage power supplies

Essential information about safety, circuits and components

Microphone preamps for Icom transceivers

How to use Heil microphone inserts with Icom transceivers... without shouting!   Also an IC-746 mod requiring no preamp.

Noise levels - data from ITU

At most sites, there is at least 30-40dB difference in external noise level between Top Band and VHF.

Many ITU technical reports are now free to download in several languages (no more need to open an account).

PSU Designer software

Take the nasty surprises out of power supply design!

Recommended tools

Relay speed-up circuit

  • How to speed-up your antenna relays, to avoid RF arcs caused by 'hot switching'.
  • How to measure switching time with an ordinary non-storage 'scope.

Rotating Centre for Wire Dipoles

RS232 interfaces for transceiver control

Small Mains Power Supplies

Also see separate mini-series on High Voltage Power Supplies.

Switch-mode power supplies

How to modify computer and other switch-mode PSUs for amateur use
(not 'In Practice' articles, so not included in the Cumulative Index)

Tools and techniques for SMD soldering

Transistor PA bias circuits

Most 'bricks' are capable of much more linear operation if you upgrade the bias circuit. Here's how to do it. 



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Powerpole connectors (May, November 2001)

Anderson Powerpole connectors use a clever 'genderless' design that needs no separate 'male' or 'female' parts - they just click together in the correct polarity. The same basic idea works for a whole range of sizes and current capacities, all the way from 15A up to 320A and more.

The 30A size is ideal for amateur solid-state transmitters and amplifiers, and is now recommended by ARRL as a standard for 13.8V DC power connections in amateur radio.

The Powerpole wiring standard for 13.8V DC connectors

PowerWerx, Inc.

Viewed from the open end, contacts down: RED on the LEFT, BLACK on the RIGHT
Note: Powerpole connectors have no separate 'male' and 'female' versions. All Powerpole connectors must be wired exactly the same, as shown above. Any pair will then connect with the right polarity, 

Following ARRL's lead, 30A Powerpoles are becoming a world standard. They are used in the RIGrunner 13.8V  distribution strips supplied by West Mountain Radio (USA) and Waters & Stanton (UK).

For data and assembly details, see Torberry Connectors (the UK importers) and PowerWerx (USA).

In the UK, Powerpole connectors are available from amateur radio retailers, Farnell, RS or direct from UK importers Torberry Connectors (also trading on eBay as Racebolts).


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Microphone preamps for Icom transceivers (October 2001)

The preamp is the portion to the left of the dotted line - the other components are  part of the transceiver (component numbers in the transceiver vary according to the model).

Design notes
The inspiration for this simple preamp was an article by Michael Covington, N4TMI in QST for June 2000. I added a few extra components for passband shaping and RF immunity. In N4TMI's design the transistor was an MPS3904, which I changed for European semi-equivalents. In the conventional (through-hole) board, you could use either the MPS3904 or the BC109C as available. The BC849C is a surface-mount transistor (I don't know a US equivalent). The DC bias conditions depend slightly on the current gain of TR1. You may need to change R2 in order to get about 4V DC at the collector of TR1.

To make PCBs, right-click on a track image to save it, then print at actual size on UV-transparent tracing paper and follow instructions here.

Conventional board:  actual size is 0.75 x 0.80in Single-sided board, view from component side with 'x-ray' view of tracks.
Print track image at actual size and then expose with ink side contacting UV-sensitive PC board.


SMD board: actual size is 0.35in square!
SMD board, components and track are all on the same side. Note that track image is 'flipped' -
print at actual size, and then expose with ink side contacting UV-sensitive PC board.
Reverse side of SMD board can be either bare, or full copper for maximum RFI immunity
(countersink around input and output pins, but solder ground pin both sides).

  IC-746 mod requiring no preamp.


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PSU Designer software  (August 2001)

The PSU Designer main screen, simulating 3kV Bridge.psu
Cursor has just been double-clicked on the transformer to change its properties.


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Relay speed-up circuit (April 2002)

Correction: TR1 should be ZVN3306A and TR2 should be ZVN4306A
(and the same in the PDF file, Fig 3).

This circuit will help reduce RF arcing in most power amplifiers that use open-frame antenna relays. It should work with the fast (solid-state) PTT output of any transceiver. TR1 and TR2 could also be NPN bipolar switching transistors with appropriate voltage/current ratings.

Original references: K1KP, K6XX

Measuring relay switching times with an ordinary 'scope - how to become a "human storage 'scope" (May 2002)


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Inside a directional wattmeter (September 2002)

From the classic article by Warren Bruene, in QST, April 1959: Download.

Notes on the 'In Practice' article:

Thanks to Ron Barker, G4JNH, for pointing out that the equations for SWR and |rho| should both have been described in terms of EV and EI as vector quantities - in other words, it is necessary to take account of their phase relationship. This means that SWR is not given simply by (EV / EI) as I had claimed in the article, except at a voltage maximum where the vectors line up.

Ron has prepared an Excel spreadsheet which gives a superbly clear step-by-step analysis for any value of SWR and phase angle of reflection: Download.


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RS232-Transceiver interfaces

ARRL interfaces - from QST February 1993, or recent ARRL Handbooks.
   A PDF file is accessible online to ARRL members, but PCB layouts are public.
DF4OR's Icom CI-V pages - Ekki's CI-V Test program is an excellent link debugger
K6XX's IC-706 interface
DK7IN - opto-coupled interfaces
IK2BCP - opto-coupled interfaces - click Projects
How to get power from the RS-232 port
RS-232 connections


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Attenuator design programs

WINATT.EXE - GM4PMK's standalone Windows version of ATT. 
ATT.BAS (QBASIC source code, contains design equations)
ATT.EXE (Ready-to-run in a DOS box. You may need to experiment with screen sizes and fonts to display the ohm and pi symbols correctly.)



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Drilling templates (August 2007)

CAUTION - these are not precise engineering drawings!
They are only meant as templates for marking-out panels for drilling and filing.

Remember to print the PDF page at 100% size ("Page Scaling = None")

Download templates (only 40 KB)


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Recommended tools (August 2007)

  1. 300mm/12in ruler - you still need both metric and inch scales. The 'satin chrome' finish with black engraved markings has by far the best readability.

  2. Sliding set-square - the sliding type is the most adaptable.

  3. Box of drills, 1.09.0mm in 0.5mm steps - this should be the first set that you buy. Spend a bit of money on good-quality drills in a metal box.

  4. Main box of drills, 1.04.9mm in 0.1mm steps - this will be your most frequent choice for small jobs, but the narrower range of sizes means it shouldn't be the first one that you buy. Again, good drills and a metal box are worth the extra cost.

  5. Very sharp scriber - sharp enough to draw blood.

  6. Centre-punch - sharp enough to find the tiny pinpricks that the scriber makes. A plain centre-punch that you can hit with a hammer gives more control than the spring-loaded 'automatic' type, and it's cheaper too.

  7. 6mm and 3mm centre-drills - the best way to start holes accurately in the right place, and also doubles as a countersink.

  8. 12mm hand reamer - Halls make lots of nice sharp tools of this kind, including a tiny version that is useful for PC boards.

  9. Step drills, - Halls again, though several other makes are now available.
    Useful sizes are 4-12mm in 1mm steps, and 6-24mm in 2mm steps. I prefer the stepped pattern to the plain conical drills, as they give parallel-sides holes in a range of definite diameters.

  10. Screw-down hole punches - you probably only need the 5/8in (16mm) size. Q-Max is the classic make, but once again several other makes are available.

  11. Vitrex 3mm round file. Disaster - after about 40 years, the manufacturers have have chosen this moment to stop making them! I'm afraid you'll have to manage with a coarse 3mm needle file.

  12. Tin-snips - less useful than they seem, because most of them distort the metal. The pair in the photograph are at least 70 years old, hand-made in Sheffield.

  13. Hacksaw - needs to keep the blade tensioned and accurately parallel with the frame. This Stanley model is a good one.

  14. Assorted flat and round files - over the years, you will acquire these like stray cats.

  15. Wood rasp - for quick-and-dirty removal of metal, wood and flesh.

  16. Not shown - vice, small electric drill, hand drill, 13mm pillar drill. Those are another story...


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Driving earth rods  (October 2007)

Before you begin, always check for buried cables and pipes! Read G0GRI's letter in November Radcom, page 93.

  • High-quality earth rods:

    • Most electrical trade counters stock the 14mm (5/8in) rods made by Furse.

    • TLC Direct supply the same rods and accessories by mail order. See item No TL ER58.

  • SDS Plus drills and accessories:

    For general DIY and amateur radio use, look for a model that has all of the following options and features.

    • Choose a mains-powered drill from the '2kg' range. Don't be tempted by the cheap '4kg' models - they are not as good for these applications.

    • Rotary-only option ('hammer stop') for starting holes in precise locations, and for occasional drilling of wood or metal (using appropriate drill bits, of course)

    • Smooth trigger-operated speed control - especially at low speeds, for starting holes in precise locations

    • Reverse rotary option

    • Hammer-only option ('rotary stop') for chiseling and light concrete breaking

    • Facility to lock the chisel at a selection of fixed angles

    • Safety clutch - absolutely essential.

    More information is here - part of the uk.d-i-y newsgroup FAQ, a treasure-house of good DIY information.

    Before you buy an SDS+ drill, check and read the small print of the specification very carefully. If you can't find a drill with all of those features at a reasonable price, keep looking.

    Recommended models:

    The links to suppliers are examples only, and I recommend that you shop around for special offers.

    • Makita HR2450 - I recommend this highly from personal experience... but expect to pay about 110.

    • Wickes own brand - well recommended by users, and probably best value, though I haven't tried it myself.

The same suppliers also sell drill  bits and accessories, eg the 'nut driver' adapter for 0.5in sq sockets.


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Estimating blower performance (September 2007)


Use this Excel spreadsheet to estimate blower performance from wheel dimensions and RPM (blue line) and compare with airflow requirements of tube (red line and dot).


Excel spreadsheet (540KB)


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Toroidal and other chokes (August 2007)

Suppliers for toroids


Radio noise levels (January 2008)

It makes no sense for any communication receiver to have the same sensitivity across the whole range from 1.8 to 30MHz - and certainly not to be the same at 50MHz as well.

For more detail, download ITU-R-P372(1), the International Telecommunication Union's definitive report on world-wide noise levels.

Many ITU technical reports are now free to download in several languages (no more need to open an account).



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Tools and Techniques for SMD Soldering (September 2008 and October 2008)

Zoom into photograph for extra detail.

Key requirements

If you don't have ALL of the following, SMD soldering will be much more difficult!

1. Magnifier

2. Good lighting

3. Work-holding for small boards

4. Small-tipped soldering iron

5. Tweezers

6. Thin flux-cored solder wire

7. Desolder braid

8. Flux pen or spray

9. Flux solvent for final cleanup.

The links provided here are only to show you what's available. Shop around for best prices.

  • Soldering iron: thermostatically controlled irons are best, but the Antex CS18 isn't bad for under 20. I'm suggesting this model because a sharp pointed tip is available.

    • A sharp pointed tip is an absolute necessity. This short conical shape gives the best heat storage, right behind the point.

Don't use the thin pencil-style tips - they chill too easily.

  • Solder wire:

    • Use tin/lead alloy, no larger than 0.5mm diameter, for example any of the 0.5mm or thinner solders here.

    • Avoid unleaded solder - it's more difficult to use, and it is not required for home construction.

    • Rapid Electronics have an SMD rework kit which includes a smaller quantity of 0.4mm (28swg) tin/lead/silver cored solder and jelly flux in a syringe applicator. This very thin solder is easier to use in a traditional way (heat the joint, then apply cored solder) without melting too much solder in one go.

    • Another source of 0.35mm tin/lead/silver cored solder. This 100g reel will go a very long way!

  • Flux pen or syringe - another essential.

    • Some users prefer a free-flowing liquid flux, while others prefer a jelly flux which is more sticky and helps to hold the components in place.

    • Either kind will do - the important thing is to always have a supply of extra flux. Don't rely entirely on the flux in the cored solder.

  • Desolder braid - no wider than 3mm. It often helps to swipe with fresh flux before use.

  • Flux cleaning solvent - once again, essential. The brush dispenser is expensive, but the brush head can be re-used with other less expensive aerosol cans.

  • Magnifier:

    • How to choose a magnifier - a good description of the range available, from a reliable supplier.

    • Also see tools and craft suppliers, along with various eBay shops.

    • Strong 'half-eye' reading glasses - try them on at the drugstore. Boots currently have a nice frameless style.

  • Work light - here is a very good buy from IKEA.

  • Tweezers - drugstore again, or many of the UK Components and Tool Suppliers.

  • ESD mat and wristband - sorry, no room to write about those, but you really do need them.

  • YouTube video of JBC soldering stations. There are more soldering videos on the same page.

SMD Soldering Techniques (October)

  • Solder paste - a different technique. Here is a nice presentation including more links.
    Solder paste available here for example.

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Building Small Mains Power Supplies (November 2008)

  • In Europe, most important standard for the safety of electrical  products is the Low Voltage Directive 2006/95/EC, which covers the vast majority of electrical products in everyday use.

The Low Voltage Directive only applies to products that are "placed on the market" within the narrow legal definition of that term. Equipment constructed for our own use is excused the need for compliance testing and paperwork, but that is no excuse for incompetent and dangerous home construction! We must still meet the applicable standards.

The Radcom article shows how this can be done using a combination of approved components and good construction techniques.

You can buy these components on the websites of distributors such as Farnell, Rapid Electronics or CPC.

  • Another viewpoint on home-constructed mains supplies is Elektor magazine. This page doesn't have illustrations, but you can see that it follows the same principles as my Radcom article.

  • PSU Designer software by Duncan Munro, M0KGK - highly recommended!

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High Voltage Power Supplies (January, February and March 2009)

You can download the three-part Radcom article from the links above.

PowerPoint presentation from the RSGB Convention, October 2009
No sound track, but you can click through the slides to follow the presentation.
(to download 2MB file, right-click 'Save Link As...'  )

High Voltage Power Supplies


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DC Injector/Extractor for Coax Feedlines (April 2009)

Handles 1kW+, 1.8-50MHz - but only if you use the specified components for C1 and RFC1!

  • Components


  • Construction notes

    To mark out the diecast box for the two SO239 sockets, you can download this template (updated from an earlier column). Print at 100% size and cut out a single piece of paper containing the SO239 outlines A and C. Wrap this template around the underside of the box and use the guide lines to adjust it accurately to position, and then tape it in place and mark through the paper using a sharp scriber.

    There is just enough space to fit M3 bolts and nuts, although self-tapping screws are quite adequate.

    To make the lid fit, you will need to file away the flanges on the underside.


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Go 'Over The Top' with a Rotating Dipole Centre (April 2009)

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GM3SEK's Amateur Radio Technical Notebook

Updated 14 June 2018
Page 2003-2018 IFWtech.
Some graphics and downloads RSGB; PowerWerx, Inc.