[Elecraft] Another feedline question

[Vic Rosenthal]

drcuthbert@micron.com wrote:

> Below about 1 GHz coaxial cable losses are virtually all conductor
> loss. 

Thinking out loud:

I was going to respond in the same way, but then I asked myself "why 
does an air-insulated parallel line made of no. 14 wire have so much 
lower loss than RG-213/U which also has a no. 14 (or 13) center conductor?"

I think the answer is that such lines are usually higher impedance (450 
or 600 ohms, for example) and operate at a higher voltage and lower 
current, which reduces the I squared R loss in the conductors.  Of 
course if the SWR is greater than 1:1 there are current loops and nodes, 
but the latter would balance out the former.

This also explains why the dielectric loss becomes more important with 
parallel lines than coax at HF -- why air-insulated line is so much 
better than 300-ohm TV twinlead, for example.  The dielectric loss is 
greater when the voltage is higher (unlike the copper loss, which 
depends on the current).

Vic K2VCO