On 9/25/2011 1:24 AM, Tom Thompson wrote:
> On second thought, if all the impedances are equal, the currents are
> equal but I doubt that can really occur in the real world.
It's highly unlikely as open wire/ ladder line/ window line is highly
sensitive to nearby objects.
It could be a perfect balanced match at the antenna, and still end up
unbalanced by the time it reaches the rig.
Nearby metal objects, such as towers, roofing, antenna supports all
effect the currents in each leg differently. It may be small or large,
but it does affect them.
Then when you take into account the way balanced line or rather open
wire line is used, it typically has highly unbalanced currents. Even
with center fed, horizontal dipoles it's difficult to get the currents
matched perfectly.
"I think" one answer *might* be a remote, coax fed, balanced tuner with
link coupling as close to the antenna as possible. Even then the chokes
"might" be a good idea.
Every antenna installation is pretty much unique and particularly when
antennas are fed with parallel line. You put them up, see what you get
and then go from there. There are many out there that work great with no
special attention and there are those that never seem to work right.
Perhaps that is the point to look into remote tuners, and chokes or
even chokes at both ends of the open wire line. That I've never seen
done. What I don't like is the current and common practice of just
feeding the parallel line with a 4:1 balun in the tuner.
However when it comes to open wire line, after this many years I've made
it a practice to *never* end up in the same tent with some one using it
on Field Day! <:-))
Although I didn't operate last year, I did visit the site and shoot
photos. I did notice the station using open wire line was by himself
over in the far corner of the operations area.
73
Roger (K8RI)
>
> Tom W0IVJ
>
> On 9/24/2011 11:13 PM, Tom Thompson wrote:
>> On 9/24/2011 10:07 PM, Jim Brown wrote:
>>> On 9/24/2011 5:20 PM, Peter Voelpel wrote:
>>>> But the question concerned that perfectly! balanced open feeder which
>>>> certainly carries no common mode current.
>> From Wikipedia: In telecommunications
>> <http://en.wikipedia.org/wiki/Telecommunication> and professional audio
>> <http://en.wikipedia.org/wiki/Professional_audio>, a *balanced line* or
>> *balanced signal pair* is a transmission line
>> <http://en.wikipedia.org/wiki/Transmission_line> consisting of two
>> conductors of the same type, each of which have equal impedances
>> <http://en.wikipedia.org/wiki/Electrical_impedance> along their lengths
>> and equal impedances to ground
>> <http://en.wikipedia.org/wiki/Ground_%28electricity%29> and to other
>> circuits.
>>
>> This does not imply that the currents in the wires are equal and
>> opposite and if they are not, then a common mode current is present.
>>
>> Tom W0IVJ
>>
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