Cecil Moore wrote:
Ian White GM3SEK wrote:
However, any valid explanation of practical loading coils must
predict zero phase shift for the boundary condition where the coil
displays no other properties except pure inductance.

Translation: A model must accommodate conditions that
are impossible to achieve in reality. I'm sorry, Ian,
but that is pathological thinking not uncommon on this
newsgroup.

No, Cecil, that is a complete MIStranslation of what a boundary
condition is.

A software model that blows up when R=0 is perfectly
acceptable in the real world. It is a software bug,
not a statement on reality.

It's only your model that blows up. If your model sees lumped-component
behavior as an impossible singularity, that cannot be correct.

Other people's models of antenna loading do not have this problem. They
recognise that lumped inductance is often a good approximation to
reality, so they very sensibly use that as their starting-point. Then
they can progressively apply corrections for the distributed properties
of a real-life inductors. The smaller those corrections are, the simpler
the model becomes.

In practical terms, a lumped-inductance model will take you straight to
a buildable prototype. The necessary corrections can then be applied by
mechanical adjustment, without needing to model the distributed
properties of the loading coil in detail. Such models are to be found in
G4FGQ's MIDLOAD program, ON4UN's 'Antennas for Low Band DXing' and other
handbooks.

There was also an excellent theoretical treatment by Boyer in 'Ham
Radio', which shows in detail how the model of an antenna as an
unterminated transmission line is COMPLETELY capable of incorporating
lumped inductance:
The Antenna-Transmission Line Analog, 'Ham Radio', April and May 1977.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek