At 10:19 PM -0700 7/29/03, Chris BONDE wrote:
> David Jordan <wa3gin@erols.com> wrote:
> > ...shields tied together at the antenna but not
> > connected. At the tuner side the shields were tied
> > together and connected to ground.
>
>
>What is really meant by, '.. tied together ... but not connected... '?
He _should_ have meant that the shields of the two coaxial cables
were connected together, both at the antenna end and at the tuner
end; at the tuner end, the shields of the two coaxial cables were
also connected to the "ground" (chassis/enclosure) of the tuner;
however, at the antenna end, the shields of the two coaxial cables
were connected to nothing else.
>Also, if you twin lead is, say 600 ohm 6in apart do the centres of
>the coax have to be 6 inches apart ?
The two coaxial cables should be as close together as possible, e.g.,
taped together, shrink-wrapped together, or cable-tied together,
throughout their lengths. It would help if their shields were
connected together electrically at intervals, to reduce the length of
the electrically open slot between them. You do not want ambient E-M
fields to induce differential current or differential longitudinal
EMF between the two shields. It would also help to _twist_ the
coaxial-cable pair. The two cables should also be identical, to
maintain symmetry/balance.
You _certainly_ should not deliberately do anything to increase the
space gap them.
The characteristic impedance of this "shielded twinlead" will be
twice the Zo of either one of the cables. Yes, this means that
you'll have a mismatch if you connect this line to a 600-ohm line.
If the mismatch bothers you, use a broadband matching transformer.
Making a good one is no more difficult than making a good balun (IMO
it's easier); consult Sevick's book, "Transmission Line
Transformers." You can get pretty much any impedance ratio that you
want, and it can be made practically lossless.
73 de Chuck, W1HIS
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