Hi Eric,
In a message dated 98-07-07 14:26:28 EDT, you write:
<<
While its true that the instrument is probably not sensitive to
common mode current, a lot depends on lead dress in a situation
like this. Even if the line is quite long, if it is not
decoupled from the antenna at the feedpoint, and not "made cold"
somewhere ahead of the point where the line length is changing,
changes in the length of the line can and will be reflected as
actual changes in the _system_ feedpoint impedance that the line
is connected across.>>
Due to an unwavering opinions on a newsgroup, I measured some common lines
Eric.
These are general statements, that apply to what I think are typical
installations...
By the time the cable is dressed down a tower leg for any distance, the tower
pretty well contains all the common mode current from high mutual coupling
between the line and tower. If the line leaves the tower some point above
earth, there can be common mode currents on the line. However, the line is
shunted by so many impedances and has such a high surge Zo coupled with so
much loss (from radiation and dissipation) there is almost always no
detectable change in feedpoint impedance when common mode line impedance is
changed after the line leaves the tower.
This is also true if the line is installed near earth for any distance,
through other cables or wiring, through a house, etc. The common mode
impedance of the line becomes mostly a moderately high value because of all
the things "sapping off power" as the radiating line weaves its way through a
complex surrounding.
It's only in the areas where the line is spaced far from other conductors, or
where the coupling distance is small, that altering the line at the generator
has a noticable effect on feedpoint impedance.
>Burying the line for a significant distance probably does make
the line sufficiently "cold" for metering in the shack purposes.<<
It isn't so much how "cold" the line is, as how any common mode impedance
change is reflected at the feedpoint. It's pretty darned difficult to make a
large change in feedpoint impedance from the shack end, no matter what you do.
Let alone by adding only two feet of cable on a 100 foot line threaded through
all kinds of stuff on the way to the tower!
I've looked at dozens of meters and NEVER found one with response to common
mode current levels found in worse case conditions. I have seen common mode
impedances change feedpoint impedances in specific cases. But the biggest and
most common problem I've found when SWR changes with line length has always
been when line Zo and meter Zo are dissimilar.
Most 50 ohm cables I see range from 48 to 55 ohms. Toss that in with a meter
that might be set at 55 ohms, or 45 ohms, and it's no wonder SWR readings
change when the meter is moved.
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