Yep, almost. The word "balun" is a portmanteau of "balanced" and
"unbalanced." They are transformers or auto-transformers and their job
is to keep a balanced feed to the antenna balanced when transitioning to
unbalanced coax. Since they're transformers, they can also transform
the impedance.
Common mode chokes act as a series high impedance to current on the
outside of the coax shield. For RF, coaxial cable is really a
3-conductor circuit because of the so-called "skin effect" ... inner
conductor and inside of shield form one path are is unaffected by a CM
choke. Current can also be induced on the outside of the shield and is
the common-mode current. When transmitting, they can radiate close to
the shack and get into places that don't like RF. They can also distort
the radiation pattern of the antenna. When receiving, the outside of
the shield acts as a vertical antenna and can pick up noise.
CM chokes are often a string of ferrite toroids taped together with the
coax going through them. Sometimes, the coax is wound several times
through a larger toroid. First place to put them is at the antenna-end
of the coax. In some cases, a CM choke at the station entrance will
help as well, but it's not the first choice.
Jim, K9YC, has prepared "Common Mode Chokes [and other things] For
Dummies" at http://audiosystemsgroup.com/RFI-Ham.pdf It's full of
information, including some charts demonstrating that all ferrites are
not created equal.
73,
Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org
On 10/25/2015 12:32 PM, Bob McGraw - K4TAX wrote:
My take on this is baluns are baluns and common mode chokes are common
mode chokes. In many cases baluns do not provide adequate common mode
choking requirements. Thus both may be needed. Usually, the balun is
located physically at the load and the common mode choke is located at
the source. In certain antenna configurations, a balun is used at the
feed point along with a common mode choke and then a 2nd common mode
choke at the source is required as well.
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