I was curious as to the characteristic impedance sensitivity to height
variations of parallel line over an infinite perfect conducting plane (not
the same problem but it may give some insights to the pipe question). The
characteristic impedance formula for this appears in many handbooks and
transmission line books. Each line is a height h above the conducting plane
and the distance between the two lines is W. I chose the wire spacing, wire
diameter and dielectric constant to give a nominal impedance of 400 Ohms
when h=infinity. Here are some intermediate results:

h=0.5W , Z=363 Ohms
h=W , Z=388 Ohms
h=2W , Z=397 Ohms
h=3W , Z=399 Ohms
h=4W , Z=399.6 Ohms

The 3W rule offered by Dave is apparently very conservative and leaves some
margin for cable movement due to wind as he suggests.

Getting back to the iron pipe case, for h values less than W, other issues
such as Ohmic loss and ferromagnetic loss may come into play.

73 Dan


--
Dan Roth, N9NO


"Henry Kolesnik" wrote in message
om...
Back in the 1950s we used to install TV antennas on 30 and 40 foot steel
masts to get a 100 uvolt signal on channel 2 with a 10 or 12 element yagi.
We used regular 300 ohm twim lead and twisted it before putting it into
the slot of the standoff. the standoff were about 4" long. The twisting
did two things: minimized leadin pickup and equalize any stresses from
wind. What happens when you transmit with high SWR is out of my league.

--

73
Hank WD5JFR
"KC1DI" wrote in message
...
ml wrote:
I wonder exactly what happens when you run ladder line say next to a 1''
metal pipe spaced about 1" apprt

is it just that the ohms that change would it be a bit or does it
swing waaaay off?? tnx
Hi ML

Keep the Ladder Line 3X it's width away from the metal for 300 ohm twin
lead that would be about 2 " For 450 ohm Ladder line it would be about 5
" a twist in the portion of the line near the metal object about every
18" or so will also help. Remeber though that the portion of the line
near the metal object must not be allowed to sway in the the wind if it
does it will cause the impedence to very sometimes widly as the line
moves closer then further away..

I've found the above to be very accurate and have not had problems in
many installations over the years.

73 Dave