Costeffective Ferrite Chokes and Baluns Web version Click

Cost-effective Ferrite Chokes and Baluns Web version Click your way through the presentation

Ferrite Chokes and Baluns Topics: 1. Why might I need one? 2. What makes a good choke? 3. Three recommended designs

Chokes and baluns: Why might I need one? Q 1: What are RF chokes for? A: To stop RF currents from flowing where they aren’t wanted …and so, to help solve EMC problems.

Chokes and baluns: Why might I need one? Q 1: What are RF chokes for? A: To stop RF currents from flowing where they aren’t wanted. Q 2: What are baluns for? A: Er…to make my antenna balanced? Wrong!

Chokes and baluns: Why might I need one? Q 1: What are RF chokes for? A: To stop RF currents from flowing where they aren’t wanted. Q 2: What are baluns for? A: Exactly the same as for Q 1: to stop RF currents from flowing where they aren’t wanted.

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

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

Coax basics: 1. The inside of a coaxial cable is private – completely screened from the outside world. 2. At RF, current flows only on surfaces of conductors. Antenna 3. The outside surface of the coax shield is a feedpoint separate conductor from the completely inside surface. 4. The outside surface of the shield is part of the outside world. This is where the Common-mode currents flow. current flows on outside surface

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

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

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

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

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

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

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

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).

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

Chokes and baluns: What makes a good choke? Thethis Curse of EMCproblem. Remember, is an is 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.

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 Why? • Impedance must be mainly resistive See the Radcom article.

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

Chokes and baluns: What makes a good choke? Chokes that don’t work well û Air-wound chokes What’s good: 1. Cheap and easy to make (only needs extra cable) 2. Lightweight What’s bad: 1. Very narrow-band 2. Can easily be detuned in real-life situations.

Chokes and baluns: What makes a good choke? Chokes that don’t work well û Strings of ferrite beads What’s good: 1. Broadband, mainly resistive impedance What’s bad: 1. Usually not enough impedance (needs a large number of large beads of the right type) 2. Very expensive to do properly 3. Very long and heavy.

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

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

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) Searching for • cost-effective Importing to UK adds extra shipping costs, solutions… currency exchange costs, VAT and “handling • Not cheap, and large mark-ups by US distributors 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!

Chokes and baluns: Three recommended designs for HF covering What’s good: the low, middle and high HF bands (Developed from anthe idea by W 2 VJN, in the 1. Cost-effective: all use same type of core, 2010 available in the UKARRL (about. Handbook) £ 2. 70 each) 2. Very easy to make, can be applied to different types of cable, different frequency ranges 3. Very high performance – broadband, with a high resistive impedance. 4. Dependable EMC solution – can tackle all but the very worst common-mode problems.

Chokes and baluns: Three recommended designs for HF What’s bad: 1. You have to use those cores – no substitutes. 2. You have to make your own. 3. 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. )

Chokes and baluns: Three recommended designs for HF Remembering what’s good about these chokes… 1. All use the same type of core, by Fair-Rite 2. Cores are in stock in the UK (about £ 2. 70 each) 3. 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.

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

Thanks for reading! 73 from Ian GM 3 SEK