On Tue, 13 Jun 2006 07:05:51 GMT, Owen Duffy wrote:
Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we

Richard, does Z mean the characteristic impedance of the line in the
"differential" mode or "common" mode?

Hi Owen,

The Common mode. The remainder of this discussion will undoubted be
known to you, so it is largely meant for our otherwise silent original
poster, Kevin. The ferrites will be more or less transparent to the
differential mode when the ferrites wrap around both conductors for
twin lead, or around the coaxial cable.

As this discussion has been largely coaxial based, the outer
conductive surface of the coax is the primary imbalance to a dipole
through its "third wire" connection at the dipole feed point. It
appears as one of three wires to the abstract source established at
that feed point and it presents an ad-hoc Z load. The value of this
load is rarely determined, except when one deliberately attempts to
make their feed line 1/2 wave long (or some multiple). Of course,
that means 1/2 wave for the velocity factor of the outside conductive
path of the coax. This is often accomplished through cut and try
rather than modeling or measuring currents, but these too would be
good first pass approximations.

However, it is simpler to add ferrites as their Z is well known and
rarely subject to externalities. They also allow for multiband
operation. They are simple to apply. If you aren't using hi power,
they are "set and forget."

In closing, it bears mentioning that this treatment should be repeated
a quarterwave down the coax and away from the drive point. This, in
effect, enforces a one band solution by its description, so choose the
band that counts the most. I would expect it would improve other
bands to some degree. I've simply distributed the same number of
ferrites over a 20 foot length of coax as the second treatment instead
of repeating the same lumped isolation. I cannot vouch for this
method's effectivity in maintaining a null pattern in the lobes of the
dipole as I never confirmed that.

73's
Richard Clark, KB7QHC