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Bob Miller · March 14th 09, 11:39 PM posted to rec.radio.amateur.antenna

On Fri, 13 Mar 2009 19:22:24 GMT, (K8JRM) wrote:

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 use the MFJ feedthrough board # 4602. It has two ceramic feedthrough
insulators for balanced line, with other feedthroughs spread across
the board for coax lines, a random wire and a ground wire. MFJ now
makes a whole line of these boards, and they come complete with
weather stripping and burgler bar. No problem tuning the balanced
450-ohm line, coming in to the balanced posts on an MFJ tuner.

The whole setup also makes it very easy to disconnect antennas from
the outside when it gets stormy.

bob
k5qwg

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