Owen wrote:
Roy Lewallen wrote:
. . .
See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.


Roy, doesn't this suggest that there is benefit in twisting an open wire
feedline to attempt to expose each conductor to similar coupling to the
external fields. Clearly the benefit will be better for a higher twist
rate. Whilst achieving sufficient twist rate with a wide air-spaced line
may be impractical, it is probably quite realisable with ladder line
(notwithstanding the downsides of ladder line).

No. The problem isn't that the coupling is different to each conductor
of the transmission line, it's that the coupling is different from the
transmission line to each side of the antenna. Twisting the line won't
change the coupling of this common mode current by any appreciable
amount. Unfortunately.

I suspect there is not much one could do to minimise the effects of
current on a coax shield other than placement of ferrite suppression
sleeves or loops in the coax at several places on the feedline to spoil
resonances in much the same way as one would try to prevent parasitic
excitation of a guy wire by breaking into non-resonant lengths with
insulators.

The problem is exactly the same for coax and for two-wire line, and the
solution is the same for both -- common mode chokes (current baluns)
spaced about a quarter wavelength apart to break up resonances. In
practice, implementation is usually easier with coax.

Roy Lewallen, W7EL

On Sat, 25 Jun 2005 15:47:30 -0700, Roy Lewallen
wrote:

Owen wrote:
Roy Lewallen wrote:
. . .
See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.


Roy, doesn't this suggest that there is benefit in twisting an open wire
feedline to attempt to expose each conductor to similar coupling to the
external fields. Clearly the benefit will be better for a higher twist
rate. Whilst achieving sufficient twist rate with a wide air-spaced line
may be impractical, it is probably quite realisable with ladder line
(notwithstanding the downsides of ladder line).

No. The problem isn't that the coupling is different to each conductor
of the transmission line, it's that the coupling is different from the
transmission line to each side of the antenna. Twisting the line won't
change the coupling of this common mode current by any appreciable
amount. Unfortunately.

Yes, of course. Twisting helps differential coupling (which in most
cases of sound installationwill not be an issue by virtue of the
relatively large distance from any other conductors to the feedline),
but not common mode.

Owen
--

On Sat, 25 Jun 2005 15:47:30 -0700, Roy Lewallen
wrote:

Owen wrote:
Roy Lewallen wrote:
. . .
See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.


Roy, doesn't this suggest that there is benefit in twisting an open wire
feedline to attempt to expose each conductor to similar coupling to the
external fields. Clearly the benefit will be better for a higher twist
rate. Whilst achieving sufficient twist rate with a wide air-spaced line
may be impractical, it is probably quite realisable with ladder line
(notwithstanding the downsides of ladder line).

No. The problem isn't that the coupling is different to each conductor
of the transmission line, it's that the coupling is different from the
transmission line to each side of the antenna. Twisting the line won't
change the coupling of this common mode current by any appreciable
amount. Unfortunately.

Yes, of course. Twisting helps reduce differential coupling (which in
most cases of sound installation will not be an issue by virtue of the
relatively large distance from any other conductors to the feedline),
but not common mode.

Owen
--