Re: OT; Feedline question

Posted by alorona on Apr 18, 2017; 3:31am
URL: http://elecraft.85.s1.nabble.com/OT-Feedline-question-tp7629317p7629453.html

If instead of using the relative permittivity of air, 1.00059, for calculations of open-wire line, you use the effective permittivity including any spacers used to homebrew the line, this value will increase slightly.



For instance, I am using spacers made of a material called "nylon 6,6 30% glass fiber-reinforced" which happens to have a relative permittivity of 3.9. This has the effect of increasing the effective permittivity between the wires from 1.00059 to 1.08656. Another way of saying this is that the velocity factor goes from 1 (for pure air) to 0.959, or about 96% with the spacers. The effect of this is to make the characteristic impedance of the line drop from around 480 to around 460 ohms.

( See http://www.emclab.cei.uec.ac.jp/xiao/Wire/index.html )

This obsessive exercise on my part illustrates that spacers don't make a whole lot of difference, but they do make a difference. You may want to include their effect or not.

Al  W6LX
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