"K8JRM" wrote in message
...
Hi Guys,
This is an age old question - what is the best way to get 450 Ohm ladder
line
into my shack antenna tuning unit (ATU)? Here is my situation; I have two
4"
PVC tubes inserted into the exterior wall that allow the entrance of
coaxial
cable into my shack. When I run the ladder line into the shack through
one of
these ports, the multi-band wire antenna fed with the ladder line goes
wild! I
can not get the antenna to tune with any of my ATUs.
I have received several suggestions on how I can overcome this problem.
The
most promising one seems to be to use two short pieces of coax with center
conductor connected to either side of the ladder line. These pieces of
coax, in
turn, would be fed through the PVC hole into the shack and the center
conductors connected to the ATU. The shield on the coax would float with
no
connection to any thing. This solution sounds good; however, common sense
tells me that ladder feed line will still suffer because of its proximity
to
other objects. Any thoughts on this solution?
Another possibility is using a piece of Plexiglas place under a metal
framed
window pretty much as shown in the old hand books. My problem with this
solution is that the window is on the other side of the room from
operating
table, the two coax entry ports and AC power.
Short of placing the tuner outside in the elements, is there a solution to
my
problem? I would be interested in hear what has been tried!
Tom, K8JRM in OKC
Howdy Tom
The two short pieces of coax with center conductor connected to either side
of the ladder line is perhaps the best solution. It was covered in QST/The
Doctor is In, June 2008. In the article it suggests using the best quality
coax on hand, keeping the run to the absolute minimum and the coax shields
should be tired at both ends and connected to the station RF earth. This
configuration introduces a small section 100ohm nominal impedance
transmission line into the antenna system which should have minimal losses,
but most important is that it is still part of the balance transmission
line. The losses for say RG213 at 4mtr would be almost nothing on 80mtr and
perhaps up to 2db on 10mtr.
Obviously the lengths of coax need to be exactly the same length.
If you would like a copy of the article drop me email.
Peter VK6YSF
http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm