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 (but if you really need them, maybe I can help).

  • 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 resistors

  • From Bob Burns, G3OOU, using resistors from Farnell or Rapid Electronics.  Bob points out that series inductance can be reduced by changing the wire links to copper strip or braid.

• Another example, using two 100R 50W TO-220 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

• Do I really need an "RF earth"?

  There is no universal answer to this question!

  Here is some good advice from the ARRL Technical Department (in the USA they say "ground connection" where we in the UK would say "earth connection").

  • ARRL Lab Note Different Grounds for Different Shacks  - that title says it all.

  • ARRL: A Grounding Q&A. Earth connections fulfil three totally distinct functions - you need to be clear what you're trying to do.

• More good advice, from RSGB:

  • RSGB Leaflet EMC10: Avoiding Interference to Nearby Electronic Equipment

If you've read and understood all three of those references, you will understand why it's  wrong to claim that everyone needs an "RF earth".

• Components for the mains filter

  • Mains filter  - must be a 3-wire type with an earth line choke (notice the insulated earth input) and an AC current rating of 13A or more. The only filter I know of that meets this specification is the Belling Lee SF4240-16/01:

  

  Unfortunately the only UK distributor for this filter is Farnell, and when the article was published they quickly sold out.

  • Plastic box

  • Plastic cable glands

  • 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  Revised Issue 2.

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

• See all RSGB EMC Leaflets (free to download)

April 2009

DC Injector/Extractor

• Components

  • Murata 10nF 1kV capacitors for C1 (3 in parallel req'd), C2
    (minimum order multiple = 10, but well worth keeping some on hand)

  • RFC1, RFC2

  • V24ZA50P varistors (NB minimum order multiple = 5)

  • Diecast box - I'm not very happy with the quality of casting, but  they're just about adequate.

  • SO239 (2 req'd) - not critical, eg these.

• 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.

Go 'Over The Top' with a Rotating Dipole Centre

• Original version at G0IVZ (scroll about half-way down the page)

• Components

  • Fibreglass tube from Moonraker

  • Antenna Centre from Moonraker - remove all metal parts

  • Stainless steel hardware from Screwfix

  • Wire etc from Moonraker or Westlake.