cost-effective ferrite chokes and baluns

Cost-effective Cost-effective Ferrite Chokes and BalunsFerrite Chokes and Baluns

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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?

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.


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?

Wrong!


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.

Same as saying…No “common-mode”

RF current on feedline


Fields around a balanced dipole: the textbook picture

Feedline doesn’t radiate

Feedline could be either coax or

parallel line

Only the antenna radiatesWRONG AGAIN

Real life isn’t like this!


Reality looks like this…

Common-modeRF current on feedlineHigh

risk of T V I

… or many other kinds of R F I (RF Interference)

Feedline radiates

Induces RF current on TV

downlead

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.

3. The outside surface of the coax shield is a completely separate conductor from the inside surface.

4. The outside surface of the shield is part of the outside world. This is where the common-mode currents flow.

Coax basics:

Antenna feedpoint

Common-mode current flows on outside surface


Common-mode RF current

RF earth

Mains earth

“RF in the Shack”

INJECTING RF into the mains

Feedline is radiating

Another example…

Coax-fed dipole


Answer: To block unwanted common-mode currents

“RF in the Shack”

INJECTING RF into the mains



It all starts here: the antenna feedpoint


Common mode choke here

Common-mode RF current“RF in the Shack”

problems

RF into the mains



Common mode choke here

Another choke here

Mains choke

More realistically, you may need two or three chokes:


Common mode choke here = “balun”

“Feedline choke”

“Mains choke”

The same chokes, doing the same job

Different names for (almost)the same chokes


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 current on that particular cable.

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).



Why might I need one?

Next:

What makes a good choke?

Chokes and baluns: What makes a good choke?

Remember, this is an EMC problem.

• 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.

The Curse of EMCis that every situation is different.


What does “high performance” mean?

For hard EMC problems, that means…

• Impedance of several thousand ohms

• Maintained across a wide bandwidth

• Impedance must be mainly resistive

In any RF choke,high performance = high impedance

Why?

See the Radcom article.


Chokes that don’t work well…or may only work for ‘soft’ EMC problems.


Chokes that don’t work well

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.

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.


Chokes that don’t work well

Strings of ferrite beads




Next:

• Three recommended designs

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.


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…


Three recommended designs for HF

What’s good:

1. Cost-effective: all use the same type of core, available in the UK (about £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.

covering the low, middle and high HF bands

(Developed from an idea by W2VJN, in the 2010 ARRL Handbook)



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.)

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.






Three recommended designs

More informationwww.tinyurl.com/inpractice

Thanks for reading!Thanks for reading!

73 from Ian GM3SEK73 from Ian GM3SEK

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