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
On 26/01/2012 13:21, Carl Moreschi wrote:
> Loss due to dielectric loss tangent (tan) can be very important.
>
> This term is proportional to frequency, so the higher you go, the
>
> more likely it will dominate overall loss (metal loss is only
>
> proportional to SQRT of frequency).
>
>
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