Bill B.
P-19 #454, Dream Catcher
Nampa, ID
Jim Nolan wrote:
>
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> Bill:
> The cable dielectric (insulation between the inner and outer conductors)
> alters the wavelength in the cable by the reciprocal of the square root of
> its dielectric constant. The formula you gave is for half wavelength for a
> dipole antenna which is about 10% shorter than the free space wavelength.
> The formula I use for free space is 11.8/f where f is in GHz and the
> wavelength is in inches. For foam core coax the alteration is neglible.
> However foam can hold water (not good). For most plastic dielectrics (which
> don't absorb water)the dielectric constant is about 2.5 so the wavelength is
> shortened by a factor of 0.6. All keeping the coax a constant multiple of
> wavelengths does is make the radio see the exact input impedance of the
> antenna. If the antenna and radio are properly designed the coax length
> should be a very small factor in performance. Your best bet is to have the
> proper antenna and radio impedances and not worry about cable length.
>
> Jim Nolan P-19 #426 Panache
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