Jeff wrote:

On 04/08/2015 08:30, Jeff wrote:

But there is a difference. With the resistor load the current flowing
from the inner lead of the coax *must* (by Kirchoff's 1st law) balance
the current flowing on the shield inner surface. So there is no current
remaining to flow on the shield outer surface. No diagram needed.

For the case of the dipole, the load is no longer balanced with the
shield outer surface connected to one side and there is no requirement
for the current in the two arms to be equal. So in this case current
flows on the shield outer surface in an amount inversely proportional to
the impedance seen by the current in the antenna element and the shield.


No, the dipole is still balanced, just as it would be if it were fed by
twin wire balanced feeder. The power will ALL be applied to the dipole
and, if perfectly matched, will ALL be radiated (or lost as heat).

The only current that will flow on the coax outer is due to the induced
currents from the radiated field, or that caused if the dipole is not
truly balanced.

Try a NEC simulation of a dipole in free space with say 5m of feeder
attached (wire that simulates the outer for NEC purposes) and look at
the differences in current flow in these 3 scenarios:

1. Coax outer connected to one side of the dipole.

2. Coax outer NOT connected to the dipole outer by a very small
distance. (NEC cfeeder still connected)

3. Coax outer connected to one side of the dipole and an inductance
simulating a choke (balun if you wish to call it such) close to the dipole.

You will find very little difference in the currents flowing in the coax
outer in all 3 cases.

The pattern will be distorted by the currents induced in the coax outer,
and there will be other small differences due to the small lack of
symmetry in the model. What you wont see is a huge difference in the
current in the outer when it is directly connected to one element of the
dipole.

Jeff

To save you the effort here is a nec model for you to try

CM dipole with coax
CE
GW 1 10 -0.489 0 0 -0.05 0 0 1.e-3
GW 2 10 0.05 0 0 0.489 0 0 1.e-3
GW 3 3 -0.05 0 0 0.05 0 0 1.e-3
GW 4 50 0.05 0 0 0.05 0 -5 1.e-3
GW 5 3 0.05 0 -5 -0.05 0 -5 1.e-3
GE 0
EK
TL 5 2 3 2 -50 0
EX 0 5 2 0 1 0
GN -1
FR 0 1 0 0 145 0

Jeff

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?



--
Roger Hayter