On 8/5/2015 5:43 AM, Jeff wrote:
If the dipole is truly balanced (and the coax is perpendicular to the
antenna), how does current get induced into the coax? My understanding
is that perpendicular wires have little current induction due to this
orientation.

Current does not need to be "induced" in the shield outside. There is
a voltage on the shield at the antenna feed point and a path for the
current on the shield outside. Current flows. Why would the current
prefer the antenna element over the shield outside?

Yes, the antenna has not changed and it is still "ballanced", but the
antenna is not the only load seen by the feed line. The current in the
two antenna elements are not equal because some of the current flows on
the shield having nothing to do with induction from the antenna.


Well NEC and reality do not agree with you!!

If the feeder sees an impedance that is identical to its own
characteristic impedance that all of the power will go into that load,
that is the situation with a 50 ohm resistor and the radiation
resistance of a dipole is no different. The feeder is not clairvoyant so
that it knows that it is a dipole is attached, and a perfectly matched
dipole will not reflect any power back down onto the coax outer, the
only load seen will be the dipole's radiation resistance not some
mythical addition load in parallel. In a perfectly matched situation the
current in the dipole element will be equal, it is only imbalance that
will cause currents other than those caused by induction to flow on the
outer. This is clearly demonstrable in NEC. NEC also shows that the
currents induced on the outer by radiation are much larger than any
currents caused by small imbalances in the dipole elements when
resonant. Those induced currents are there even if a balun or choke is
used.

Jeff, I am tired of discussing this. Any time you need to invoke
"clairvoyance" I think you have missed the mark. The simple matter is
that a dipole is *not* like a resistor load. The resistor load is a
closed path and does not accumulate charge (except for non-idealities
which we are purposely ignoring), so the current in one side is the same
as the current out the other side. A dipole does *not* have this
limitation and the currents into the two sides does not have to be the
same.

There are many ways to use simulation and only one will be correct. If
you simulate an antenna with a balanced feed you will not get the
results for an unbalanced feed. I found such simulations on the
Internet and posted the links somewhere, not sure which thread or if it
was even this group. I was discussing this elsewhere with people who
deal with EMC and RF for a living and they agree with me. If you have
done such simulations, please show us results. Meanwhile, I'll try to
dig up the info I found on the Internet.

--

Rick