Here's an interesting paper by Wes Stuart N7WS on the topic:
http://www.scribd.com/doc/7132619/Ladder-Line
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 matched-loss 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 lower-loss line yourself simply by using
thicker wire. Also, if it's open-wire line, it wont suffer anything like
the increase in loss when wet that the commercial window line does.
Steve G3TXQ
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