Readers with access to the EZNEC User Manual will be able to check the
full version of Roy Lewallen's comments on modelling radiating
feedlines, rather than just Jim's selective quote! Go to "Building the
Model" > "Using Transmission Lines" > "Modeling Coax Cable". You will
see that Roy also says:
"A radiating coaxial cable can be modeled quite well with a combination
of transmission line model and a wire. The transmission line model
represents the inside of the coax, and the wire represents the outside
of the shield".
Look at W8JI's web pages, or those of VK1OD, and you will see that's
what they have done in their analysis of coax common-mode current.
All of the modelling examples I have quoted have been of dipoles _fed
with coaxial cable_ and followed Roy's guidelines. When I quoted my
first EZNEC results on a coax-fed dipole, Jim replied that NEC has
serious limitations in the analysis of the common mode circuit; as you
can see, that's in direct disagreement with Roy Lewallen's position. Jim
then gives us a selective quote from the EZNEC manual that appears to
support his position, but it actually has nothing to do with coaxial cable!
I shall post no further on this topic. If others want to ask me
questions on the topic I shall be happy to answer by private eMail. I
should have learned my lesson - I discovered several times in the past
that trying to have a technical discussion with Jim feels like trying to
herd cats ;)
73,
Steve G3TXQ
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