May 2010

Cost-effective Ferrite Chokes and Baluns

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

• Extended 18-page version of the In Practice article with more background and detailed winding instructions.

  PowerPoint show from RSGB Convention, October 2010

  • Requires PowerPoint (or free PowerPoint viewer)

  • To download, right-click the link above and choose a Save As... option

  • To view online, your browser must be configured to automatically open .pps files using PowerPoint.
     

• The cores

  • Fair-Rite part number 2643167851 is available in the UK from Farnell. Currently this is the cheapest way to buy, especially if a few people or a club group together to get over the small order limits.

  • Can I use a different core?  NO - not for these designs. You cannot expect the same performance using different cores!

Other useful cores

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

• Biggest Fair-Rite 31 clamp-on core, Part No 0431177081

  Very useful for trouble-shooting because it clamps onto an existing run of cable - but
  it always needs several turns - once through won't get you anywhere!
   

  • This core is now my best recommendation for a whole-shack mains filter.
     

  • Mouser is currently the best deal because FedEx shipping to UK for orders over £50 is FREE! (uk.mouser.com 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.

  • I don't know of any UK dealer for this one.
     

• FT240-31, Part No 2631803802
  Large toroid. 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

• Steve Hunt, G3TXQ:

  • Common Mode Chokes – neat graphical presentation of impedance measurements on a wide range of chokes, clearly showing the impedances and bandwidths achieved.

  • Why reactive (air core) chokes are undesirable – they can sometimes make common mode currents worse.

• Jim Brown, K9YC:

  • RFI, Ferrites, and Common Mode Chokes For Hams – a growing compendium of theory, measurements, choke designs and good practical advice.

  • Transmitting Chokes – a shorter PowerPoint presentation.

  • Understanding and Eliminating RF Interference – another very good PowerPoint presentation, covering much the same material from a different viewpoint.

  • http://audiosystemsgroup.com/publish.htm – Jim’s extensive publications page.

• Chuck Counselman, W1HIS:

  • Common Mode Chokes – how one man transformed his ‘noisy’ location to a quiet one, using heroic quantities of ferrite.

• Tom Rauch, W8JI:

  • http://www.w8ji.com – use the keyword choke to search the whole site. It’s all worth reading.

• Owen Duffy, VK1OD:

  • A Method for Estimating the Impedance of a Ferrite Cored Toroidal Inductor at RF – analysis using frequency-dependent complex permeability (K9YC makes some very similar points).

  • http://www.vk1od.net – again, don’t limit yourself to just one page. Owen’s entire site is well worth reading.

June 2010

Care and Feeding of VHF/UHF Long Yagis

Websites about Yagi design

• Yagi designs by Martin Schreyer DK7ZB
• Yagi designs by Justin Johnson, G0KSC
• 'Optimized Wideband Array' (OWA) Yagis with 50 ohm feed
  Also see G0KSC's OWA designs, and Google for OWA Yagi.

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)

• QST article by Darrel Emerson, G3SYS/AA7FV
• Excel spreadsheet to calculate cable lengths.

April 2010

Loctite

• 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

• Heatsinks with adhesive pads

• Heat conductive adhesives are also available, but it's cheaper to buy the complete heatsink ready-made.

March 2010

Power Attenuators

• Book - Managing Interstation Interference by George Cutsogeorge, W2VJN.

  • Thresholds of receiver damage and receiver overload are explained here... but then read the whole book.

• Attenuator design program for Windows: WinATT by GM4PMK.

• W7ZOI 40dB 100W attenuator:

  • Original article, QST June 2001

  • Updates on W7ZOI's website

  • Application of the attenuator in a simple SDR spectrum analyser by G4AON.

• 400W+ version:

  • 5k 5W metal film resistors

  • 5k 10W metal film resistors no longer seem to be stocked in the UK (I have a few spares, enough to help the first 4 people who e-mail me).

  • Diecast box

  • Drilling templates - download PDF and print at 100% (1/1) scale.

• Another QRO attenuator (30dB) by G4AXX.

• Constructing dummy loads, August 2009.

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 Z_C1 are both complex (vector) impedances, the important criterion is that |Z1| needs to be much greater than |Z_C1|. 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

• Inside a Directional Wattmeter:

  • September 2002 In Practice.

  • Directional Wattmeters - the classic article by Warren Bruene
    (W0TTK/W5OLY, Collins Radio Company)

  • Directivity errors in VSWR sensors - see February 2007 In Practice.

    Links to larger diagrams: Figure 2  Figure 3  Figure 4.

December 2009

Current versus Voltage baluns

• Articles by W7EL:

  • Baluns: What They Do and How They Do It - a ham radio classic.

  • The 1:1 Current Balun (*)

• 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:

  • Putting a Balun and a Tuner Together (*)

   

  (*)  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

• Noise temperature:

  • Basic concepts

  • Sky noise temperature (page 17 onwards)

• Antenna noise temperature

  • Several References - especially in Skolnik's classic Radar Handbook and in Radio System Design for Telecommunications by Freeman.

  • 144MHz Yagi performance tables by VE7BQH. Note that the noise temperatures are calculated for an array of 4 Yagis under simplified assumed conditions related to moonbounce operation. These assumptions may not apply exactly to terrestrial VHF/UHF DXing and contesting.

• Much more about modern VHF/UHF Yagi design and construction.

August 2009

RSGBtech mailing list

• Join here

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

Metal Film Resistors

• For example:

  • Rapid Electronics

  • Farnell

• Dummy loads using two 100R 50W TO-220 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

• ESD mats and wrist straps
  Search eBay.co.uk for esd mat, then sort through the list for suitable items.
  This link should take you there: http://tinyurl.com/mdenmc 
  Look for mats of about 500x600mm that include a wrist strap.

• ESD horror stories from Doug Smith's website - a mine of useful information about RF measurements, EMC and ESD:

  • 'EMP' effects from nearby static discharges

  • ESD from shaking coins in a plastic bag.

• ESD protection inside ICs - an interesting general review in the opening chapters.

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:

  • http://www.emtalk.com/vswr.php - uses "gamma" to mean reflection coefficient

  

  • NETCALC - my ancient DOS calculator that does it all!

  • NETCALC for Windows and MAC by Harry Whitfield G6AUC:

  

   

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

  • Nice animation of standing wave envelope (but doesn't have complex impedance input).

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

  • RSGB leaflet EMC07: Protective Multiple Earthing

  • RSGB leaflet EMC12: Part P and the Radio Amateur (NB England and Wales only).

• See all RSGB EMC Info (free to download)