Roy Lewallen wrote:
Chuck Olson wrote:


Hi, Larry,

An off-center fed antenna doesn't necessarily produce unbalanced feeder
currents - -


I'm sorry to say, it does. There's coupling from the feedline to each
side of the antenna, and unlike a symmetrical dipole with feedline at
right angles, the coupling from each side is unequal. This results in
unbalanced feeder currents. If the feedline is twinlead or open wire
line, this shows up as unequal currents on the two conductors. On coax
it manifests itself as current on the outside of the shield. In both
cases, the result is the same -- feedline radiation.

This can be shown quite dramatically by simple modeling. I've also seen
it myself by direct measurement(*). A current balun (NOT voltage balun)
at the feedpoint will reduce the conducted common mode current just as
it will with a dipole, but you still have to deal with common mode
current due to coupling. Depending on the frequency and the feedline
length and orientation, you might get lucky and not have too much
feedline current. On the other hand, it can be bad enough to aggravate
RFI problems if the feedline is routed near power or telephone lines, or
cause trouble with your rig or shack accessories.

just a different feeding impedance level, which he will no

doubt take care of with a tuner or a 4:1 ratio in the balun if single
band
use is all he wants and he can find the right spot for the feeder. Of
course
the balun takes care of any unbalance that might be due to the feeder
not making a 90 degree angle with the antenna. There's nothing wrong
with his idea.


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).

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.

Owen