At 7:53 PM -0400 10/3/02, K5RC wrote:
>One problem.... 468/f does not work with insulated wire. You have to
>consider the velocity factor of the jacket.
Yes and no; but mostly no. Coaxial cable has a velocity factor
significantly less than one, and equal to one divided by the square
root of the dielectric constant of the insulating material that fills
the space between the center conductor and the shield, because 100%
of the electric stored energy of the wave(s) is stored in that space,
in that insulating material. In other words, the capacitance per
unit length of the transmission line, which together with the
inductance per unit length determines the velocity of a wave
traveling along the line, is proportional to that dielectric constant.
With an insulated antenna wire in air or free space, typically the
thickness of the dielectric/insulation is smaller than the diameter
of the wire conductor and much, much smaller than the distance from
the wire to the ground or other part of the antenna where electric
field lines leaving the wire terminate. Most of the electric stored
energy is *not* stored in the insulating material. The capacitance
per unit length of the wire, which together with its inductance per
unit length determines the velocity of a wave traveling along the
wire, is only in small part determined by the dielectric constant of
the insulation. Per unit length, the capacitance of the wire is that
of two capacitors connected in series: the first being the
coaxial-cylindrical capacitor formed by the insulation; and the
second being the capacitance between the outer surface of the
insulation and the ground or other part of the antenna where electric
field lines leaving the insulated wire terminate.
It should be obvious that the second capacitance is much smaller than
the first, so that the capacitance of the series combination is
nearly equal to the capacitance that the wire would have if it were
solid metal, equal in diameter to the outer surface of the
insulation. (The exact value of capacitance is between the values
for solid wires having diameters equal to the inner and the outer
diameters of the insulation.)
The capacitance per unit length of a thin circular cylindrical wire
depends only weakly, logarithmically to be exact, on the diameter of
the wire. Thus the resonant length of a typical wire antenna, with
thin insulation, is not much affected by the insulation. Nothing at
all like the length of a standing wave inside a coaxial line.
As mentioned, the statement of the next-to-last sentence is an
approximation. Yes, the resonant length _is_ affected, just not
enough to worry about in a typical HF antenna. Other factors, such
as proximity to ground and other conductors and dielectrics, and
details of the feedpoint and ends of the antenna, are typically more
important. As you probably know if you've used insulated wire in HF
antennas, as I have.
73 de Chuck, W1HIS
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