In message , K8JRM
writes
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!

Presumably, your ATU has a balanced output for the 450 ohm feeder.

If you already have the two (presumably fairly short) pieces of coax
lines already in place, I reckon that the first thing you should try is
to ground the two shields to the case of the ATU (which I presume is
your shack safety ground, and maybe also your RF ground). This should
screen the inner conductors from other nearby conductors, and keep the
feed balanced. If you don't ground the shields, you might as well not
use coax.

Obviously, using coax will change the impedance of the line for a short
distance, and may affect the settings of the ATU (more on the higher HF
bands, of course). But with luck, the ATU will still tune OK. The
increase in feeder losses should be negligible.

An alternative is to go from the ATU and through the wall using low
impedance twin feeder. The spacing between the conductors will be much
smaller than the for the 450 ohm feeder, and it should not have much
opportunity to become very unbalanced due to the proximity of nearby
conductors. It might be advantageous to twist it. I understand that, if
the insulating material is suitable, twin mains power cable can be used
as twin feeder, especially on the lower HF bands. The current rating of
the cable will, of course, depend on what power you intend to run.
Ordinary lighting twin flex should handle a couple of hundred watts.
But, if you've got a bit of 50 or 75 ohm twin, all the better.
--
Ian