On Sat, 2007-01-27 at 20:34 +0000, Steve Hunt wrote:
> Jerry,
>
> How do I get that? ..... mental aberration and mis-interpretting the
> "brackets", that's how :)
>
> At the risk of extending this thread for far too long .... what would happen
> if you "parallel connected" two lengths of 300 Ohm twin sitting on top of one
> another. Would the composite Zo be half the individual value (as it would be
> with coax), or because the fields would interact, would it act more like a
> single entity with fatter conductors but separated by the same distance?
>
> 73,
>
>
I think how the two 300 in parallel would show would depend on details
of their construction and so the paralleled conductor spacing. A line
with a good oval of solid plastic would come closer to holding the
majority of the field in the dielectric and so there would be less
interaction of the fields, while a line with a dumbbell cross section (a
thin web between conductor insulators) would tend more to act like a
line with fat conductors.
The most effective 300 ohm lines to parallel would be the shielded twin
lead lines. Belden used to call it 8290 IIRC. It was an oval twinlead
with a foil shield. I tried a sample on my tuner. First I set up series
tuned and dumped 200 watts into the line and tuner. The tuner didn't
heat and the line didn't melt. Of course, that particular tuner coil was
made of 1/4" wide copper strap edge wound. Then I set it up parallel
tuned for high voltage and had to add corona balls to the open end of
the line sample and it took all the voltage I could create, RF again. No
load other than tuner losses and radiation. I wouldn't think it would
handle a KW, but a nearby ham ran a run of it on his 100 watt packet
station on 20 meter for a few years. I think maybe RS has a shielded
twin lead yet, I can't vouch for how well it would handle RF power.
The unshielded twinleads laying against each other with the figure 8
shaped conductors would be a pain to compute characteristic Z because
most of the formulae have been developed around circularly symmetric
conductors. Even going to a square outer for coax begins to add
significant complication and approximation because of the loss of
circular symmetry. In the old days the fields and impedance might have
been computed from a cross section drawn in conductive paint on a large
sheet of carbon paper and then with voltage applied, the voltages were
plotted to find the field lines. Then they went to fluid emulators and
the math came later. At least to get a precise result is complex. A few
engineers seems like made a career of such computations.
73, Jerry, K0CQ
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