Here's an interesting paper by Wes Stuart N7WS on the topic:
http://www.scribd.com/doc/7132619/LadderLine
If you look at his Equation 2 for the total loss of twin line you can
identify the copper loss term and the dielectric loss term. For 300 Ohm
line constructed from #16 copper wire with a polyethylene dielectric,
the copper loss exceeds the dielectric loss until you reach 3.3GHz.
At 30MHz the two loss terms turn out to be:
Copper loss 0.264dB/100ft
Dielectric loss 0.025dB/100ft
Steve G3TXQ
On 26/01/2012 14:34, Steve Hunt wrote:
> In a typical coax line, the dielectric losses don't exceed the copper
> losses until you get above 1GHz. At HF line losses are overwhelmingly
> copper losses, and dielectric losses can safely be ignored. Do the sums
> or take a look at the chart here:
> http://rfcafe.com/references/electrical/coax.htm
>
> As Jim said, the lower matchedloss of twin lead is down to its higher
> characteristic impedance (hence lower current) and, possibly, larger
> conductor diameter. Commercial window line doesn't have a particularly
> large conductor  you can make lowerloss line yourself simply by using
> thicker wire. Also, if it's openwire line, it wont suffer anything like
> the increase in loss when wet that the commercial window line does.
>
> Steve G3TXQ
>
>
>
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