On an unmatched line, losses vary with length because there are current
peaks and troughs along the line. And because at HF losses are almost
entirely copper losses, where the current peaks the losses will be high
and where it dips the losses will be low.
Here's an example:
* Terminate 35ft of Wireman 551 in a 50 Ohm load. The total loss at
3.8MHz will be 0.5dB
* Now double the line length to 70ft; the loss will be 0.58dB, not the
1.0dB you might expect.
The reason is that the initial 35ft contains a current maximum (at the
load) and consequently has relatively high losses, whereas the next 35ft
contains a current minimum with relatively low losses. In fact that
second 35ft of line with its 8:1 SWR contributes less loss than if the
line was perfectly matched!
73,
Steve G3TXQ
On 09/08/2013 22:26, Stuart Rohre wrote:
The comment someone posted about the losses varying depending on SWR
might be why results are not linear with length.
Stuart
K5KVH
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