At 7:18 PM -0400 7/30/03, Roger D. Johnson wrote:
>If signal pickup is the criterion, why not use shielded
>twisted pair such as the RG-22 series[?]...
I don't know the loss and power-handling capability of that stuff
(for transmitting); but, in small-signal applications, shielded
twisted pair is great.
A few years ago I considered making 70 feet of low-loss, high-power,
shielded-twisted-pair or -quad transmission-line for HF with a shield
diameter of, say, 8 or 12 inches and with air dielectric. I was
thinking of cutting/drilling/punching a bunch of spacers out of thin
but rigid plastic that would fit inside a light sheet-metal duct such
as HVAC systems use, and which would support the parallel wire
conductors of an "open-wire" transmission line inside the duct.
Obvious questions were how to keep the spacers in position and
properly oriented within the duct, how to assemble and join sections
of duct, and how to "seal" it electrically.
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.
It wasn't clear to me that the ducted/shielded "open-wire" line had
to be twisted, as long as it and the duct were symmetrical, but I had
a few schemes for twisting it within the duct, too.
In the end instead I used 7/8ths-inch semi-rigid coax (Heliax
LDF5-50) and the mother of all baluns (required by the combination of
high power and high SWR), but I still wonder whether my
HVAC-duct-shield idea has merit. BTW, this 70-ft. line was/is
entirely indoors, from my upstairs hamshack to the far end of the
house, through the attic. At the far end, large Steatite feedthrough
insulators take it outside; and, from there, ordinary unshielded
open-wire line, suspended in air, goes to my antenna.
Have any TowerTalkers built anything like this? Do high-power
broadcasters use something like it (professionally built of course)?
At broadcast stations I've seen some pretty big rigid coax, but never
anything that I suspected was a shielded _balanced_ line.
73 de Chuck, W1HIS
|