larry d clark wrote:

given that the length of a half wave dipole is
calculated by 468 / freq in mhz when velocity factor
is 1, ie 468 / 1.9 is about 246 ft.

i'm sitting here wondering why folks with small
city lots don't use (468 / freq in mhz) * velocity factor,
to construct a much shorter antnenna, particularly on
80m & 160m?

as an example, rg59 coax typically has a velocity
factor of .66, so plugging to the formula,
468 / 1.9 is about 246 ft, * .66 is about 162 ft.

why couldn't 162 ft of rg59 be cut in two, attached
to a 50 ohm, have the remaining ends of the rg59 shorted
together, and hoisted into the air?

so what am i missing? there are no free lunches:-)

larry
kd5foy

One wavelength in free space in feet is 984/F in feet (300/F in meters); a
half-wavelength is 492/F. 468/F assumes a vf of 0.95 of the antenna
radiating surface.
The vf of coaxial cable is dependent upon the dielectric used. 0.66 is
common for solid polyethylene. Foamed PE is typically near 0.80. As with
many RG cables, RG59 is available both ways.

In your example, the vf of the dielectric would not matter, since the
majority of the current flows on the *outside* of the shield due to
skin-effect. Thus, the formula for the length would be very close to (only
slightly shorter than) 468/F. Since I^2*R losses would be lower, bandwidth
would be better than a smaller-diameter wire.

Bryan WA7PRC