[Elecraft] Balun loss

[John Magliacane]

--- Rick Dettinger <k7mw@cmc.net> wrote:
> The thread about feedline loss reminded me of a question that I have been
> meaning to ask:  If my K2 is sitting on my wooden bench on its rubber (or
> plastic) feet with no ground wire and powered by battery, is the output
> ballanced or unballanced?


Rick,

The output of your transmitter is unbalanced since the RF is being
generated and amplified with respect to the transmitter's "ground"
(local point of reference).  The transmitter defines this ground
as being zero volts at every point in the RF cycle.

Now consider the following: Take your unbalanced transmitter, feed it
into a length of 50 ohm coax, and terminate the coax with a 50 ohm
resistor (of suitable power rating).  The resistor, having no relation
to any "ground", represents a balanced load to an unbalanced transmission
line.  Yet, the coax will not radiate!

However, if the same transmitter and coax is directly connected to
a balanced half-wave dipole having a radiation resistance equal to
the resistor in the above example, the coax will radiate.

The difference between the two scenarios is that when the coax feeds
the dummy load, the ground of the transmitter, coax, and the "ground"
end of the dummy load are all set to the same RF potential as defined
by the transmitter.  The transmitter's "ground" could be 500 volts above
earth ground.  It doesn't matter.  The ground potential will be the
same throughout the entire circuit.

In the second case, the one side of the dipole that's connected to
the shield of the coax cannot assume the same "ground" potential
defined by the transmitter as in case #1. The reason is as follows.
Each side of the dipole act as quarter-wave impedance "inverters"
that transform the impedance of free space (120*pi ohms, or about
377 ohms) on each open end of the dipole down to about 35 ohms.
(Since each end of the dipole are essentially in series, the
radiation resistance of the antenna becomes twice this, or about
70 ohms.)  The loss incurred in this impedance transformation
is primarily the result of radiation resistance, which is what
converts our RF energy into electromagnetic radiation.

I suppose if you could elevate your transmitter's "ground" to about
35 ohms above "earth ground" at the transmitter with exactly the same
phase relationship as that of the shield connected end of the dipole,
then the coaxial line would not radiate when attached to a balanced
half-wave dipole.  But my guess is that it would be somewhat difficult
to do, especially over a broad range of frequencies.

In essence, this whole balanced/unbalanced situation comes down to
matching RF potentials on both ends of the coax shield.  The "ground"
at one end must be exactly the same as the "ground" at the other end.

In a similar situation, if a balanced transmission system is for some
reason thrown out of balance, the transmission line will radiate.


73, de John, KD2BD


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