Many thanks to all for the replies received. I finally
did what I should have done in the first place and
went to the fount of antenna knowledge - W4RNL. On his
web page there is an article which calculates expected
shortening for various conductor/sheath diaameters and
various relative permittivities:
http://www.cebik.com/amod/amod50.html
Plugging in the numbers for the particular wire I am
using I get a shortening of about 2.5%. He shows that
the shortening can vary very considerably and with
very thick sheaths can exceed 5%.
Thanks again to all.
73 Roger
VE3ZI
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