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Cost-effective Ferrite Chokes and Baluns

Cost-effective Ferrite Chokes and Baluns. Web version Click your way through the presentation. Ferrite Chokes and Baluns. Topics: Why might I need one? What makes a good choke? Three recommended designs. Ferrite Chokes and Baluns. Topics: Why might I need one? What makes a good choke?

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Cost-effective Ferrite Chokes and Baluns

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  1. Cost-effective Ferrite Chokes and Baluns Web versionClick your way through the presentation

  2. Ferrite Chokes and Baluns Topics: Why might I need one? What makes a good choke? Three recommended designs

  3. Ferrite Chokes and Baluns Topics: Why might I need one? What makes a good choke? Three recommended designs

  4. Chokes and baluns: Why might I need one? Q1: What are RF chokes for? A: To stop RF currents from flowing where they aren’t wanted …and so, to help solve EMC problems.

  5. Wrong! Chokes and baluns: Why might I need one? Q1: What are RF chokes for? A: To stop RF currents from flowing where they aren’t wanted. Q2: What are baluns for? A: Er…to make my antenna balanced?

  6. Chokes and baluns: Why might I need one? Q1: What are RF chokes for? A: To stop RF currents from flowing where they aren’t wanted. Q2: What are baluns for? A: Exactly the same as for Q1: to stop RF currents from flowing where they aren’t wanted.

  7. Feedline could be either coax or parallel line Feedline doesn’t radiate Same as saying…No “common-mode” RF currenton feedline Chokes and baluns: Why might I need one? Fields around a balanced dipole: the textbook picture WRONG AGAIN Real life isn’t like this! Only the antenna radiates

  8. Common-modeRF current on feedline Feedline radiates Induces RF current on TV downlead Chokes and baluns: Why might I need one? Reality looks like this… High risk of T V I … or many other kinds of R F I (RF Interference)

  9. Antenna feedpoint Common-mode current flows on outside surface • The inside of a coaxial cable is private – completely screened from the outside world. • At RF, current flows only on surfaces of conductors. • The outside surface of the coax shield is a completely separate conductor from the inside surface. • The outside surface of the shield is part of the outside world. This is where the common-mode currents flow. Coax basics:

  10. Chokes and baluns: Why might I need one? Another example… Coax-fed dipole Common-mode RF current Feedline is radiating “RF in the Shack” RF earth INJECTING RF into the mains Mains earth

  11. Chokes and baluns: Why might I need one? Answer: To block unwanted common-mode currents Feedline is radiating “RF in the Shack” INJECTING RF into the mains

  12. Chokes and baluns: Why might I need one? It all starts here: the antenna feedpoint

  13. Common mode choke here Common-mode RF current Feedline is radiating “RF in the Shack” problems RF into the mains Chokes and baluns: Why might I need one?

  14. Common mode choke here Another choke here Mains choke Chokes and baluns: Why might I need one? More realistically, you may need two or three chokes:

  15. Common mode choke here = “balun” The same chokes, doing the same job “Feedline choke” “Mains choke” Chokes and baluns: Why might I need one? Different names for (almost)the same chokes

  16. Chokes and baluns: Why might I need one? The same chokes –same ferrite cores,same number of turns,same diameter. Only the cable isdifferent, dependingon the application. Because they’re all doing the same job –blocking unwanted common-mode RF currenton that particular cable.

  17. Chokes and baluns: Why might I need one? Summary: To solve EMC problems Specifically… to stop RF currents flowing in the wrong places Chokes may be needed in several different places: at the antenna feedpoint (“a balun”) at other places on coax feedlines on other wiring in the shack (eg computer cables) on mains wiring. The same choke designs will work for a range of applications (only the cable and connectors need to change).

  18. Ferrite Chokes and Baluns Why might I need one? Next: What makes a good choke?

  19. Chokes and baluns: What makes a good choke? Remember, this is an EMC problem. The Curse of EMCis that every situation is different. • Some EMC problems are ‘soft’ and easy to solve – • for these, almost any choke will give good results • But some problems are much harder – • these need chokes with much higher performance • The curse of emc is, you never know which it will be… So always aim for overkill– high-performance chokes are far more likely to do the job.

  20. Why? Chokes and baluns: What makes a good choke? What does “high performance” mean? In any RF choke,high performance = high impedance • For hard EMC problems, that means… • Impedance of several thousand ohms • Maintained across a wide bandwidth • Impedance must be mainly resistive See the Radcom article.

  21. Chokes and baluns: What makes a good choke? Chokes that don’t work well …or may only work for ‘soft’ EMC problems.

  22. Chokes and baluns: What makes a good choke? Chokes that don’t work well • Air-wound chokes • What’s good: • Cheap and easy to make (only needs extra cable) • Lightweight • What’s bad: • Very narrow-band • Can easily be detuned in real-life situations.

  23. Chokes and baluns: What makes a good choke? Chokes that don’t work well • Strings of ferrite beads • What’s good: • Broadband, mainly resistive impedance • What’s bad: • Usually not enough impedance (needs a large number of large beads of the right type) • Very expensive to do properly • Very long and heavy.

  24. Ferrite Chokes and Baluns Why might I need one? What makes a good choke? Next: Three recommended designs

  25. Chokes and baluns: Three recommended designs Chokes that do work well • Ferrite cores and multiple turns • But you MUST: • Use a good design • Use the specified type of core. • Unknown surplus ferrite coresWILL NOT WORK! • For good results, use the right ingredients and follow the recipe.

  26. Chokes and baluns: Three recommended designs The problem is the cost! • Best ferrite cores are made in the USA (eg Fair-Rite 31 and 43 grade materials) • Not cheap, and large mark-ups by US distributors • Importing to UK adds extra shipping costs, currency exchange costs, VAT and “handling charges” = double the price • Best performance comes from large cores which are very expensive • Some US designs may use 4-5 large cores per choke – way too expensive! Searching forcost-effective solutions…

  27. Chokes and baluns: Three recommended designs Three recommended designs for HF covering the low, middle and high HF bands (Developed from an idea by W2VJN, in the 2010 ARRL Handbook) • What’s good: • Cost-effective: all use the same type of core, available in the UK (about £2.70 each) • Very easy to make, can be applied to different types of cable, different frequency ranges • Very high performance – broadband, with a high resistive impedance. • Dependable EMC solution – can tackle all but the very worst common-mode problems.

  28. Chokes and baluns: Three recommended designs Three recommended designs for HF • What’s bad: • You have to use those cores – no substitutes. • You have to make your own. • Not the ultimate best performance. Even better chokes are available… but will cost you a lot more for the ferrite! • (Remember, the original aim was cost-effectiveness –best performance obtainable at reasonable cost.)

  29. Chokes and baluns: Three recommended designs Three recommended designs for HF • Remembering what’s good about these chokes… • All use the same type of core, by Fair-Rite • Cores are in stock in the UK (about £2.70 each) • Very easy to make, can be applied to different types of cable, different frequency ranges • The most cost-effective strategy: • Keep a small stock of bare cores. Make up the exact kinds of chokes you need,when you need them.

  30. Ferrite Chokes and Baluns Why might I need one? What makes a good choke? Three recommended designs More informationwww.tinyurl.com/inpractice

  31. Thanks for reading!73 from Ian GM3SEK

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