An article in QST about 10-15 years ago discussed loss in coax. It was
written by an engineer for one of the major cable companies (possibly
Amphenol?). It said that,in the HF range, the dominant loss in coax
cable is I squared R in the center conductor. Skin effect worsens the
loss as frequency increases.
For the same OD, a 50 ohm cable with foam dielectric requires a larger
center conductor resulting in lower loss. There are possible problems
with foam dielectric including moisture penetration and movement of the
center conductor off center in bends. Tom is "right on".
73, Dan, N5AR
Tom Rauch wrote:
>
> > Brilliant!
> > Just trust Tom that dielectric in the coax doesn't contribute to
> > losses. Some "evaluation" of Force 12 products.
> >
> > Yuri
>
> Dielectric losses in typical coaxial cables do not contribute
> significantly to loss at HF.
>
> The primary loss mechanism is conductor loss.
>
> The reason various dielectrics might *seem* to produce different
> losses is because the manufacturer, for a given overall diameter
> cable, can use a larger conductor when the dielectric has a larger
> percentage of air. Higher velocity factor cables, for a given diameter
> and impedance, have less loss because the center conductor is
> larger.
>
> This really is well-known and widely published, but it is probably
> worth repeating because the myth that the dielectric is somehow
> responsible for dissipating significant (or even a majority of) power
> keeps popping up.
> 73, Tom W8JI
> W8JI@contesting.com
>
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