All,

 BALUN is a contraction for BALanced-UNbalanced.  An unbalanced line is
one in which one side has a=20
 different impedance with respect to ground. COAX is an UNbalanced line.
 =20

 Alternately, twin lead is BALanced. ie both sides have equal impedance
WRT ground.

 A typical RF transmitter has an unbalanced output generally.  And if
you are going to drive a ground plane
 antenna, this is fine.  COAX is a good fit for this job.  However, if
you were to drive a DIPOLE antenna
 for instance, this type of antenna is BALanced.  Using COAX directly
will work, but it won't be optimum.
 And you'll also run the risk of radiating RF from the line at various
places where you don't want it to=20
 radiate from.  So, in this case, you could use some COAX connected in a
way to perform the BALUN function.
 This is a transformer of sorts.  And it transforms a single unbalanced
signal to a differential balanced=20
 signal.

 This is a rather simplified explanation, but should suffice to get the
idea across to most members of the
 list.  I'm sure there are several HAM radio operators on this list (I
am KA9QHR) that can verify (or
 refute) my explanation.

 Regards,

 Jim

> -------- Original Message --------
> Subject: Re: [OT]:: and you think that YOU have got wiring problems!!!
> From: "Olin Lathrop" <olin_piclist@embedinc.com>
> Date: Fri, March 04, 2011 7:48 am
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>=20
>=20
> Manu Abraham wrote:
> >> That's a balun, otherwise known as common mode choke. The are
> >> explicitly exploiting the fact that coiling adds inductance to the
> >> common mode path but not to the differential path.
> >
> > After writing that only, I thought that attenuation would be in the
> > common mode.
>=20
> It is.  That's exactly the point of a balun.  It adds impedence to the
> unwanted common mode signal while adding as little as possible to the rea=
l
> differential mode signal.
>=20
> Baluns are often used in RF applications as single ended to differential
> converters.  For example, the final transmitter output could be single en=
ded
> with respect to ground.  That works fine for the transmitter and for a
> single ended antenna such as one that relies on a ground plane.  However,=
 if
> you have a symmetric antenna you want both sides with equal and opposite
> signals with respect to ground.  A balun between the transmitter and such=
 a
> antenna achieves that for various levels of good enough.
>=20
>=20
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> (978) 742-9014.  Gold level PIC consultants since 2000.
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