[TowerTalk] Coax, conduit and toroids

Steve Hunt steve at karinya.net
Thu Jun 28 14:38:28 PDT 2012

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 ;)

Steve G3TXQ

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