Jeff wrote:

I confess I haven't looked up the meaning of your model parameters, but
what happens if the feeder comes off the aerial at forty-five degrees in
the plane of the dipole?

In that case the current induced into the outer will be much greater,
and to some degree the feeder will unbalance the dipole which will also
increase the current seen on the outer.

With the feeder at 90 degrees, as in my model, the current in the feeder
is about 1/10 of that in the dipole elements in all 3 of my cases due to
the coupling into the coax and dipole imbalance. The increase in current
in the outer between the cases of a direct connection to that of no
connection or a choke is about 10%. Some of that current is due to the
fact that I have not spent enough time trying to get the swr down to 1:1
ie a perfectly matched dipole.

It helps if you change the coax & system impedance to 70 ohms, but I
could, if I had time, trim the element lengths etc, but it is still
difficult in NEC to make an exactly symmetrical model.

Jeff

The conclusion I would draw is that generally there is no need to for a
balun with a well set up coax fed dipole unless you get problems with RF
in the shack, or local RFI, more likely at high power. But if the
geometry is not very symmetrical a balun may improve the aerial
performance. Which, by a coincidence, is what the books generally say.
But the discussion has been useful, and the modelling you have kindly
done tends to confirm the textbook version. (10% is quite a lot if
running kWs, even if it doesn't affect the aerial much.)


--
Roger Hayter