In Practice



Thanks to everyone for reading and contributing to this column since 1993.
There will be no more monthly 'In Practice' columns,
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All 207 monthly columns, indexed by subject

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The more recent information below will be moved into this archive.

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Further information about recent articles

For older information, check the separate Cumulative Index and 'Best Of' pages.

Cost-effective Ferrite Chokes and Baluns (May 2010)

Original design concept for these chokes, published in the 2010 ARRL Handbook, was by George Cutsogeorge, W2VJN.

Other Ferrite Cores

If you are ordering other types of cores,  Fair-Rite 31 material is probably the best grade for general purpose HF and low-band chokes. See Jim Brown, K9YC's references below.

  • THE PROBLEM SOLVER - Fair-Rite 31 clamp-on core, Part No 0431177081

    This is a serious ferrite core for RFI problem-solving - not only because it can make a highly effective choke at HF, but also because it clamps onto an existing run of cable without needing to remove connectors. (Remember the 13th Commandment: "Thou shalt not cut off thy neighbour's mains plug".)

    But also remember that an HF ferrite choke will typically need 6-8 turns passing through the centre hole. Regardless of the type of core, one pass through the centre will never be effective at HF!

    About 8 turns of 3-core mains cable on this specific core is now my best recommendation for a whole-shack mains filter (the old Belling-Lee three-wire filter is no longer available).

    Mouser is currently the best deal for this large clamp-on core, because FedEx shipping to UK for orders over £50 is FREE! ( is like a normal UK-based order page - you'll be charged VAT in the usual way, and then the parcel arrives in about 3 days with nothing more to pay.)
    This link takes you directly to the correct core.

  • The same Fair-Rite 31 material is also available in a "FT240" 2.4-inch toroid format
    Fair-Rite Part No 2631803802. 10-12 turns of coax on a FT240-31 makes a nice low-bands choke or balun.

    • Again, Mouser's free FedEx offer is currently the best deal.
      This link takes you directly to the correct part number (ignore the photograph).

Other references

June 2010

Care and Feeding of VHF/UHF Long Yagis

Websites about Yagi design

For more links, references and construction ideas, see the VHF/UHF Long Yagi Workshop pages here on this site.

'1/12th wave' Impedance transformer
using alternating coax impedances (Figure 3b)

April 2010


  • Loctite Threadlocking User's Guide

  • Loctite 243 - search Google for lots of UK suppliers, eg Farnell.

  • Halford's stock the semi-solid Loctite 248 in the 'lipstick' package. (I haven't tried this, but prefer the liquids because they run into the threads so easily.)

Adhesive Heatsinks

March 2010

Power Attenuators

February 2010

Decoupling Capacitors

It was a real squeeze to fit the basic story into the two pages, so there's quite a lot of web-only content this month.

  • Additional notes and references

    There is a better picture of all the capacitors on the Radcom Contents page.

    [1] Link to the October 2009 column (on the RSGB Members-Only site)... coming when the RSGB webmaster gets round to it.

    [2] Although Z1 and ZC1 are both complex (vector) impedances, the important criterion is that |Z1| needs to be much greater than |ZC1|. If this is achieved, a detailed vector calculation is unnecessary.

    [3] The models in Figure 3 (page 80) work quite well with simple fixed values of C and Ls, at least for values of C up to about 100nF. However, Rs should be modelled as a function of frequency, particularly near the series and parallel resonances which are quite sensitive to the values of Rs.

        Also, do you see a resemblance between the green traces in Figure 6 and Figure 4? It suggests that the 100nF 1kV capacitor (top right in the photo on the Contents page) may actually contain two different capacitors in parallel.


  • Correction

    Page 81, bottom of column 1: should refer to Figure 4.


  • Additional plots and discussion

    These plots show what can happen if you attempt to damp out an unwanted parallel resonance that doesn't actually exist.

    Both plots below are for  the 10uF electrolytic and 10nF ceramic capacitor in parallel. Figure 6 on page 81 shows no sign of parallel resonance. (It is barely visible as a wobble around 8MHz, but is thoroughly damped by the internal resistances of these particular components.)

    Both of these so-called 'cures' have been recommended in various books, and they only make things worse!


    10Ω resistor in series with the electrolytic capacitor:

    Comment: this has no benefit at all - it only increases the value of Rs at low frequencies.

Ferrite bead on one lead of the electrolytic capacitor:


Comment: a very bad idea - it creates exactly the kind of resonance it was supposed to prevent!

Bottom line:

If you don't have the test equipment to see what could be going wrong,
it's best to keep it simple.


January 2010

Errors in VSWR Meters

December 2009

Current versus Voltage baluns

  • Articles by W7EL:

  • Articles by K9YC:

    • Transmitting Chokes

    • RFI, Ferrites and Common Mode Chokes for Hams - lots more detail.

    • Fair-Rite ("Amidon") #61 grade ferrite is generally recommended for current baluns and common-mode chokes on the higher HF bands, and the newer #31 grade for the lower bands.

    • Don't use multiple beads - use large toroids (FT240-31 or FT240-61) with multiple turns of coax. It's less expensive and more effective.

    • Do read K9YC's recommendations in detail - hard work, but well worth it!

  • Article by W9CF:


    (*)  These two references were part of a specific discussion about the merits of connecting the balun at the output of an unbalanced tuner or alternatively at the input. However, they both include some detailed circuit analysis of how baluns work.

November 2009

'Low Noise' Yagis

August 2009

RSGBtech mailing list

Messages and attachments are public. To contribute and use other facilities, you need to join the group.

Metal Film Resistors

  • Another example, using two  resistors:


I built this to demonstrate 'zero lead length' construction for improved VHF performance. The VSWR is excellent up to 50MHz, limited mostly by the quality of the coax connector termination. The effects of shunt capacitance in the TO220 packages (about 2pF) become more noticeable above 50MHz but this load is good for most purposes up to 144MHz.

July 2009

ESD Protection

June 2009

Complex Impedance, VSWR and Reflection Coefficient

  • There are many sites and programs that convert between VSWR, reflection coefficient and return loss, but most of them don't allow complex impedance. Here are a few that do:


    Can you explain the small difference in VSWR between the two programs?
    (Hint: both programs have calculated correctly.)

May 2009

A Filtered Mains Supply for Your Shack

  • Components for the mains filter

    • Mains filter  - the recommended 3-wire mains filter is no longer available. I would now recommend using a ferrite choke with all 3 wires of the mains cable.
      The best ferrite core for general purposes is probably the biggest Fair-rite 31 clamp-on core. Remove the outer jacket of the cable, twist the three wires together and then wind as many turns as you can onto the core. Make sure that the two halves of the core are clamped absolutely tight together - use superglue.
      After the ferrite choke you can also use a conventional 2-wire mains filter (mains earth tag connected to the metal case at both ends). This filter should be rated at 16-20A. Remember that, in this application, the mains filter will not work on its own without the ferrite choke as well.

    • For safety you must enclose the whole choke and filter assembly in a plastic box.

    • Distribution board - my 8-way board came from a Wickes DIY store. Similar products are also available from CPC (search the list for the multi-way extensions) and from Screwfix.

  • PME

  • See all RSGB EMC Info (free to download)

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