It's important to understand that any antenna is better than no
antenna and I'm speaking of incremental improvements in terms of an
antenna _system_, i.e. the matching network, feedline and antenna, and
the most transfer of power from the RF final to the load for a single
system. There is rarely a complete utter night and day difference,
between two somewhat similar systems, but an improvement will be
observable over time although it might be subtle. But a dramatic
improvement, such as going from a low dipole in town to one up 120
feet in the country will be instantly clear. In the case of open wire
line etc. it is more a matter of having a solid signal with one
antenna over several bands. With any antenna the best test is seeing
how well you can do with fairly low power, almost QRP on CW for
example. If you run enough power, a coat hanger will work, but if
you are frequently solid copy with the amplifier off, then you are
moving in the right direction.
>I'd love to hear more about the practicalities of routing open wire line
>from feedpoint through heavily wooded lots, through house walls, etc etc,
>to the shack.
There's a rich body of literature on spacing insulators that has been
published over the years but most hams now seem to settle on either
the ceramic spacers that can be found at hamfests, or the black
irrigation pvc pipe that's about 1 inch diameter and can be cut and
notched to make spacers. The pipe has the advantage of being
lightweight, but it can get spiders and bugs in it. It seems to be UV
resistant and although black, not a known carbon conductivity problem.
The ceramic spacers were made in a variety of lengths--you use the
holes on each end to hold the tie wrap wire; the feedline wire goes on
the ends of the spacers, not through the holes. They can add a lot
of weight to the line, but over land that is okay--I'd probably go
with a light weight spacer from ground up to a free hanging feedpoint.
Most people think they need to have a spacer every two feet or so
but if the line is 3 or 4 or 5 inches apart, then once it is strung
up, and has some tension on it, the spacers can be every 3 or 4 feet,
allowing more length for the number of spacers.
If you construct your own line you get to choose what wire you want to
use. I'd consider AWG 12 stranded as it is a good compromise between
power handling and size although I think you'd have to space it pretty
far apart to get 600 ohms impedance. The top spacer at the feedpoint
should be strapped to the center insulator of the dipole so that it is
perpendicular to the center insulator and directly below it, so as to
make a X when viewed from above. then the feedline wires can be
curved around and attached to the dipole ends so there is strain
relief allowing the feedline to move around in the wind and not
fatigue the connection points.
Supports over ground can be poles --the only thing that is important
is that they hold the line with insulators in the weather and maintain
the line geometry with a bit of sag but not too much, and get the line
high enough so you can get under it and/or mow under it. I have used
PVC pipe topped with T sections with light weight line.
The feed throughs depend on the wall. If you have a window with a
small pane, you can knock that out and put in a plexiglass pane
drilled for a pair of banana jacks or porcelain feed throughs, or you
can drill holes for porcelain feed throughs, -- some guys use a pair
of RG213 stubs through the wall. If the wall is stone or brick, it's
harder. I'd probably route to a window I could modify in that case.
73
Rob
K5UJ
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|