I agree with K1TTT's comments, below.. comments interspersed..
Jim, W6RMK
----- Original Message -----
From: <k1ttt@arrl.net>
To: <towertalk@contesting.com>
Sent: Tuesday, July 29, 2003 6:30 AM
Subject: Re: [TowerTalk] Balanced Line using Coax ?
>
> > The dialectric has very little effect if you do not have the shield
hooked
> > up at both ends. Coax is coax only if you use the shield.
> >
>
> i don't think so. there is still the conductor there and the dielectric
is
> between them.
More to the point, there's an electric and magnetic field. You still get
ohmic losses and dielectric losses. For HF, the dielectric loss is quite
small, but the ohmic loss is signficant. If the source and load are
perfectly balanced, you'll have some current, I, flowing in one center
conductor and a current -I flowing in the other. Just like in any other
transmission line, the current on the outside surface of the center
conductor is neatly balanced by the current on the inside surface of the
shield. There shouldn't be any current on the outside of the shield.
However, because the surface area of the shield is huge compared to that of
the center conductor, the losses come mostly from the center conductor.
Skin effect means that copperweld is fine as a center conductor (unless
you're sending DC through the coax), because the current flows only in the
surface.
Running a pair of coax as a balanced pair is just hooking two transmission
lines in series to make a line of impedance 2*Z0... It's no different than
hooking two lines in parallel to get Z0/2. If you don't connect the shields,
of course, then what you've got is two funky wires with a capacitor to
ground with some horribly indeterminate impedance. If you don't ground the
shields, but they are connected, then the shield will tend to float to the
common mode voltage. Impedance wise it will look ok, but you'll lose the
"shielded twinlead" effect.
you can't get rid of the capacitance between them, nor can you
> get rid of the losses in that capacitor. i would expect very different
> effects based on the lengths of the coax if you don't connect the shield
since
> it would effectively be a floating conductor. this could lead to strange
> resonances and possibly very high voltages near the ends if it happens to
be a
> resonant length.
>
> i would also expect that you may lose the advantages of the shielding for
> running them as a balanced line if you don't connect the shields. leaving
the
> shields floating in this case would cause coupling between the floating
> shields and would probably open you up to problems with nearby conducting
> objects which is one of the few advantages to using the parallel coax this
> way.
>
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