At 07:26 AM 7/31/2003 -0700, Richard Karlquist wrote:
> > My favorite idea was to use circular or square duct; loosely assemble
> > and install in-place the entire 70-foot length or at least long
> > sections of the duct/shield (which in my situation could be mostly
> > straight with only one 90-degree bend) with just a pulling rope
> > inside; make spacers from hollow, thin-shell, stiff plastic spheres
> > having outside diameters just slightly smaller than the inside
> > diameter of the duct, drilling holes through each sphere
> > symmetrically about its center for the wires; thread the wires
> > through the spheres with a large "needle"; fasten the spheres to the
> > wires somehow (hot glue?); pull the resulting "string of beads"
> > through the duct; peek and as necessary reach through the (so far
> > loose) joints of the duct to check and if necessary reposition the
> > hollow spherical spacers; and seal the seams of the duct with wide,
> > adhesive, metal-foil tape (such the strong aluminum-foil tape that I
> > found in the HVAC section of a Home Depot).
> >
> > There's no reason why you'd have to assemble the entire duct and then
> > pull the entire line through it. You could make short sections of
> > line, each with spacers and wire conductors inside; then you could
> > join the sections, soldering the wires together and pulling them taut
> > as you went.
>
>I have been kicking around the idea of making my own air dielectric
>coax using 2 or 3 inch irrigation tubing for the outer conductor.
Presumably this is to replace the openwire line you're currently
using? Dielectric losses aren't all that significant at HF frequencies
(except, perhaps on 10,12,15 m, but even there...) unless you're running a
huge mismatch with large circulating currents in the feedline. If you need
lower impedances (which coax, in practical ratios of radii) can give you,
you can run multiple openwire lines in parallel.
For ohmic losses, the higher Z of open wire line reduces the current,
reducing the ohmic loss.
If you're looking to reduce the effect of surroundings on your transmission
line, then some form of quadroline or hexoline might also serve well, as
they tend to contain the field "within the line". A dielectric tube
around the line could serve to keep it clean and and keep the weeds,
children, cows, etc. from touching it. PVC, ABS, or Styrene pipe might
work well (watch the loss from carbon loaded plastics, though).
Air dielectric coax is a great idea when you are high enough in frequency
to worry about dielectric loss, but too low for waveguide (which coax sort
of is, anyway)
On the other hand, maybe you just want to make giant air dielectric coax
for the challenge and novelty. You might want to look into styrofoam disks
as the supports. or, even, styrofoam wedges. You can get low density foam
shapes diecut/stamped out pretty cheaply.
|