Ian White GM3SEK wrote:

Personally I would always use a balanced feeder for any wire antenna.
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That reel of cable marked "balanced feeder" is a myth!

Twin feeder is not self-balancing. It never will be "balanced" unless
your installation has MADE it balanced.

As it comes off the reel, twin feeder is capable of carrying unwanted
common-mode currents just as easily as the intended differential mode.
All practical antenna installations are unsymmetrical (either the
antenna, its environment or both) so there will always be some unwanted
common-mode current. Common-mode current will cause the feedline to
radiate and will conduct unwanted RF back into the shack, where it often
flows through into your house wiring... all of which is regarded as a
Bad Thing.

Twin feeder will only become more "balanced" if you - the user - have
taken some positive action to suppress the common-mode current. When you
install a typical link-coupled ATU designed for "balanced" feedline, the
low capacitance across the link coupling creates a high impedance to
block the entry of common-mode current into the shack. A suitable balun
would also create a high impedance to block the common-mode current, so
in this case you would need a choke balun (aka feedline choke or line
isolator).

When you insert this high impedance in the path of the common-mode
current, the current distribution over the whole antenna-feedline system
readjusts itself to take account of this new factor. The new current
distribution will be forced towards better symmetry on the antenna
itself, with a smaller common-mode current on the feedline... so the
feedline has now become more "balanced".
etc
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Ian , You are right ,I should have used the term 'twin feeder' instead
of 'balanced feeder'. Tnx for the correction.

In my situation the twin feeder from the dipole comes through a double
concrete block wall ,runs over the loft ,through the plaster board
ceiling (meeting very few metal parts) into the shack . No RF
interference anywhere in the house ,operating with approx 100 W PEP on
any of the HF bands between 3.5 - 29.7 MHz
I intend to make an RF current probe ( 2 half ring ferrite parts -diode
-connected to a DVM) to see whether there is any unbalance current .
This is more sensitive than taking the difference between measuring
individual currents in the feeder lines.

My next activity will be to extend the feeder to a suitable length
(Cecil style) such that I 'll be able also to operate on Top Band (1.8
-2.0 MHz) ,the dipole being 2 times 21.35 metres (2 x 70 feet)
With a twin feeder the actual length of the dipole is not critical ,
however in order to get a reasonably high current (low impedance) at the
matching unit (of the E-Zee Match variety and the like) it is important
that the total length of half the dipole plus the feeder is about an odd
number of quarter wave lengths at the operating frequency .
The velocity factor of the 450 Ohms twin feeder I use is published to be
0.99 ,so for the above purpose it can be taken as 1.
This total length is more critical at the lower frequency HF bands
because of the higher wavelength.

Frank GM0CSZ / KN6WH