--- Ron Wright <[EMAIL PROTECTED]> wrote:
> Jesse,
>
> Then why do twin feeders have much less loss than
> coax??? Skin affect is even more of a factor there
> due to the differences in the area of the outer
> shield in coax vs the twin feeders wire.
>
> Maybe it is because of the larger C coupling in the
> coax due to the larger surface area of the shield.
> Coax has a lower R even with skin effect than twin
> line feeders.
>
> Skin affect is a factor, but a small one compared to
> the LC factor.
>
> 73, ron, n9ee/r
>
It is not open wire or coax that determins the power
loss. It is the impedance of the line and the size of
the conductors for frequencies up to 1000 Mhz or so.
To transfer 1000 watts of power , the voltage will be
higher and the current lower in most prectical open
wire lines. That is because the impedance will be
around 300 to 600 ohms. Coax is usually 50 or 70
ohms. To get 1000 watts of power through that
impedance line it requires less voltage and more
current.
This is the principal of regular 60 hz power line
transmission. Up the voltage to a few hundred
thousand volts and the current will go down. This
lowers the losses.
I don't care to take time to do the calculations, but
if you take some small guage wire (say # 20) and
space it close to make about 200 ohms line and figuer
the loss, it may be higher than some 1 inch or even
1/2 inch hardline.
At frequencies below around 1000 Mhz the major loss is
the IsqR loss in all lines. Radiation is a very small
part. In coax there is a point in which the current
on the shield is not flowing but jumping from point to
point where the shield wires cross. This causes some
resistance. That is partly why the foil shielded coax
and hardline is lower in loss than regular coax.
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