Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Ferrite, Balun, Choke and Transformer?  (Read 10723 times)
KC2NLT
Member

Posts: 146




Ignore
« on: December 25, 2017, 05:35:40 PM »

Ferrite, Balun, Choke and Transformer?

Is there a difference between these four? Does each serve a different function?
Logged
WB6BYU
Member

Posts: 18462




Ignore
« Reply #1 on: December 25, 2017, 10:33:13 PM »

Ferrite is a material that increases inductance.

Balun is a function:  there are are different ways to accomplish a
balanced-to-unbalanced transition at different frequencies.

A choke, if properly constructed, can function as a current balun.  Chokes
serve other purposes in other circumstances.

A transformer is a set of coupled windings.  It is possible to build one that
serves as a balun, or an un-un for impedance matching on an unbalanced
antenna.  They can serve other functions as well.
Logged
NK7Z
Member

Posts: 2509


WWW

Ignore
« Reply #2 on: December 26, 2017, 05:58:27 AM »

Nicely explained!
Logged

Thanks,
Dave
Amateur Radio: RFI help, Reviews, Setup information, and more...
https://www.nk7z.net
AE5GT
Member

Posts: 396




Ignore
« Reply #3 on: December 26, 2017, 07:34:35 AM »

One minor clarification ,transformers are devices that transform  impedances or voltages . They don.t necessarily have to be made with coupled windings .
« Last Edit: December 26, 2017, 07:41:58 AM by AE5GT » Logged
K7NI
Member

Posts: 55




Ignore
« Reply #4 on: December 26, 2017, 10:00:33 AM »

Ferrite materials not only increase inductance but also introduce frequency dependent resistive loss.
Logged
WB6BYU
Member

Posts: 18462




Ignore
« Reply #5 on: December 26, 2017, 11:00:35 AM »

Let's look at each of these in more detail, to see if we can better answer the original question.


Quote from: WB6BYU

Ferrite is a material that increases inductance.



A coil wound on a ferrite core has higher inductance than the same number of turns without
the core.  For many types of baluns / transformers / chokes, when we wind them on an air core
(such as a piece of PVC pipe) we run into problems where, to get enough inductance to be
effective, we end up with long lengths of conductors and higher self-capacitance, both of
which can reduce the effective frequency range.  

For example, if I wind a balun using a pair of wires that don't have a 50 ohm impedance,
this acts like a length of transmission line and will transform the impedance, causing a higher
than desired SWR at the top end of the band (especially at the point where the wire length
is around 1/4 wavelength.)  Using ferrite I can get the same inductance with a much shorter
wire length, maintaining good performance at higher frequencies.


This includes a coil with a single turn, such as slipping ferrite beads over the outside of a
coax cable:  a straight wire has inductance, running that wire through ferrite beads increases
the inductance.

A common shape for the ferrite is a torus, or doughnut, which is then called a toroid core.

Note that toroid cores are also available in powdered iron, instead of ferrite.  Generally
speaking, ferrite has a higher permeability (increases the inductance more) and is better for
broadband use, while powdered iron is more repeatable / stable, and is preferred for tuned
circuits.  Of course, there are always exceptions.

Ferrite and powdered iron come in different mixes, or types.  These are often referred
to by a material number (occasionally a letter), as used by Amidon in their catalog (though
not all manufacturers / vendors use the same system.)  For example, #43 is a medium grade
of ferrite, while #2 and #6 are types of powdered iron material commonly used for HF use.
(Powdered iron cores are often painted to indicate the type, such as red for #2 and yellow for
#6.  Ferrite cores are usually unpainted, and are just grey and difficult to tell apart by eye.)



Quote

Balun is a function:  there are are different ways to accomplish a
balanced-to-unbalanced transition at different frequencies.



The basic application of a balun (of any type) is to couple a balanced antenna or load (such as a dipole)
to an unbalanced feedline, such as coax.  This is important because coax in effect contains three
conductors at RF:  the center conductor, the inside of the shield, and the outside of the shield.
We want all the power from the center conductor and the inside of the shield to flow into the balanced
feedline / load, without any current on the outside of the coax.

I recommend W7EL's article on baluns to explain this better.

There are many types of baluns, particularly at VHF/UHF where quarter wave structures are more practical.
Typically there are two approaches:  to make sure that the voltages on each side of the load are equal
or to force the currents in each side of the load to be equal.  If the load is perfectly symmetric, these
to should give similar results, but usually it isn't, and the current balun is a much better choice.

A balun can also provide an impedance transformation in addition to the primary function of balanced / unbalanced
connection.  The most common one probably is a 4 : 1 balun, often used for matching 200 / 300 ohm antennas
or loads to 50 / 75 ohm coax.



Quote

A choke, if properly constructed, can function as a current balun.  Chokes
serve other purposes in other circumstances.



Generally, a choke is a coil with enough inductance to limit the flow of RF (or some other range
of AC).  In the context of RF transmission lines, it usually means winding the coax into a coil
to limit the current on the outside of the coax.  (The latter is called "common mode current",
so this is called a "common mode choke".)   It turns out that eliminating common mode current
is exactly the role of a "current balun", and most effective current baluns are just a well-designed
choke.   You can make them by winding coax on a piece of plastic pipe or a shampoo bottle, but
it requires more coax and results in higher self-capacitance, which limits the useful frequency
range.  By winding it on a piece of ferrite it can use less coax and work over a wider frequency range.

Chokes can be used for other uses as well.  For example, at the base of a quarter wave vertical so
the feedline doesn't act like a radial wire.  That's the same function as a current balun, even though
the load isn't technically balanced - the function isn't dependent on what sort of connectors a
manufacturer puts on the ends of the device.


Quote

A transformer is a set of coupled windings.  It is possible to build one that
serves as a balun, or an un-un for impedance matching on an unbalanced
antenna.  They can serve other functions as well.



Quote from: AE5GT

One minor clarification ,transformers are devices that transform  impedances or voltages . They don.t necessarily have to be made with coupled windings.



Although you can have 1 : 1 transformers that don't provide any impedance transformation,
but serve other useful purposes instead, and there are a number of impedance transformation
circuits that I wouldn't call a "transformer".


A perfect RF 1 : 1 transformer wound on a ferrite core, for example, could serve as a balun (if the
capacitive coupling between the windings is minimized.)  Transformers are more often used for
wideband step-up or step-down impedance matching (for example, to match a 12 ohm mobile
antenna or a 300 ohm OCFD.)  Many common types to not provide good common mode rejection,
so must be used in conjunction with an effective choke / current balun when the balun function
is desired.

A current fad is to use transformers for end-fed wire antennas, because they can (in some
cases) reduce the SWR at the feedpoint enough to lower the losses when used with a tuner in the
shack.  These are often wound as an "autotransformer", with a single tapped winding, but the
principle is still the same as for separate coupled windings on a common core.  An end-fed half-wave
wire requires a very high turns ratio to match the high impedance, and such devices rarely give the
impedance transformation one might expect from the turns ratio due to stray capacitance across
the coil and other factors.  For wires that are not intentionally a multiple of 1/2 wavelength, lower
impedance ratios are commonly used, such as 4 : 1 or 9 : 1.  In practice the lower ratio transformers
are often wound on powdered iron cores, and act more as a shunt coil than a transformer on the
lower bands.



So they are all somewhat related, but refer to different aspects of the matching problem.  You can
have a choke that is a current balun, or a choke that doesn't serve as a balun.  A transformer may
be wound to act as a voltage balun, or as an unbalanced impedance matching device.  It doesn't help
that many hams - and some manufacturers who should  know better - misuse or confuse terms, or
even invent new ones to make their products seem better.
Logged
AE5GT
Member

Posts: 396




Ignore
« Reply #6 on: December 26, 2017, 03:28:23 PM »

For an accurate description of how Transmission Line Transformers work  I would suggest you read Transmission Line Transformers by W2FMI .
Logged
WB4SPT
Member

Posts: 777




Ignore
« Reply #7 on: December 27, 2017, 06:12:31 PM »

Semantics.    I can transform a 50 Ohm source to a 200 Ohm source with a resistor. 
Logged
KC2NLT
Member

Posts: 146




Ignore
« Reply #8 on: December 27, 2017, 07:06:22 PM »

Great overview. Thank you for the very informative posts.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!