I also transition from ladderline to coax in order to get through an
unfriendly environment to get to the shack equipment.
However, I avoid many if not most of the problems you raise by using a
simple W2DU ferrite bead 1:1 balun at the transition point. This
minimizes if not eliminates any possibility of common-mode, outer-braid
current.
I chose the 1:1 balun since impedance studies showed that on some bands
I would see a rather low impedance looking into the ladder line, and I
did not care to make the matching problem any worse by reducing it by a
factor of 4. That, of course, assumes that the balun would still be
functioning as such when faced with a low and reactive impedance. The
bead current balun does not suffer from noticeable frequency dependence
and does its job well in the face of reactive loads.
Analysis, borne out by subsequent analyzer readings and tuner
performance, shows that the transition from 40 feet of ladderline to 12
ft of 50-ohm coax introduces negligible loss and the resulting
"impedance bump" is readily compensated with the MFJ 989C tuner.
I run 5 watt CW QRP a great deal of the time here and have found that my
88 ft 20-meter Extended Double Zepp performs exceptionally well on all
bands from 80
through 10 meters. I seriously doubt that I am losing much power in
either the tuner, the coax or the balun.
An alternate solution to this problem is the use of two coax lines as
shielded extensions of the ladderline. This approach has been widely
used and is described in the ARRL Antenna Book, among others.
72/73, George W5YR - the Yellow Rose of Texas NETXQRP 6
Fairview, TX 30 mi NE Dallas in Collin county QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
Steve Ellington wrote:
> Yes it does seem to defeat the whole purpose of using open wire feeder.
> The assumption here is that since coax is "shielded" then we can run it
> through walls, over pipes etc. but think about it: If we connect a
> balanced line to coax then rest assured there is going to be plenty of RF
> on the outside of that so called shield. So what did we accomplish that we
> couldn't accomplish by just running some insulated, open wire through the
> wall? Well what we accompished was the introduction of a random, unknown
> and sometimes extream impedance to a transformer which will have an
> unknown effect on the random mess we fed it resulting in who knows what on
> the 50 ohm coax which could have considerable loss on 20m and lower.
>
> In your situation, I would do my best to use balanced line all the way to
> the tuner and make sure it's well insulated. 450 ohm window line is fine
> and it only needs an inch or so clearance. Running it through PVC wouldn't
> help a bit if the pvc is against an electrical conduit. If you ran a 4"
> PVC pipe and somehow suspended the line down the center by using tension
> somehow then that would be great.
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