[TowerTalk] Open Wire Line Spacing From Surrounding Objects-Matching.

[Wes & Linda]

In a message dated 12/10/00 4:43:47 AM Pacific Standard Time, 
majrabsr@rconnect.com writes:
> 
> I need to run 450-ohm ladder line from my shack in the basement to outside.
> Basement wall is concrete blocks, with two blocks showing above ground
> level.
> 
> Also, how far should the ladder line be spaced from the house?  Thanks to 
>all. 
> 
>Ron, KA9ALC

You can build a balanced, shielded, two-wire line using two lengths of
coax.  Tie the shields together at both ends and ground the input side to
the tuner chassis.

'Reference Data for Radio Engineers' gives a formula for the differential
impedance of a balanced line near ground.  I won't repeat it here for fear
of screwing it up in ASCII.  Assuming that all of the dielectric is air and
the wire diameter is much smaller than the spacing between the two wires
and ground, the equation shows that the differential impedance reaches 96%
of its final value when the height above ground is the same as the wire
spacing and is asymptotic at five times the wire spacing.

The common-mode impedance is a different story though.  It keeps increasing
as the height increases, behaving much as a single wire over ground (no
surprise).

So in the ideal sense, five to ten times the wire spacing should be plenty.
 If effects are noted as this spacing changes, it seems to me that the line
is not operating in a totally balanced mode, which is probably the usual case.

Wes  N7WS



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