[Elecraft] K2 - Unstable power output on 40m (digital modes)

[Fred Jensen]

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.