Jim Brown wrote:
> On Thu, 4 Feb 2010 13:03:06 -0800, Rick Karlquist wrote:
>
> But Rick, I'm talking about loss at MF and HF, especially low HF
> (that is, 160M-40M), and to a lesser extent, 20-10M. Again, once
> you hit 5 MHz, skin effect becomes increasingly dominant. These
> data are not always published, because most higher priced coax is
> optimized for VHF/UHF, where the differences are more dramatic. But
> if you're buying coax to use on these lower bands, resistance comes
> much closer to telling the story than loss numbers for 50-500 MHz.
> Jim K9YC
Consider these cables that are spec'ed at 1 MHz:
Type DC resistance RF loss Ratio DCR/RF loss
8214 2.3 0.1 23
8237 3.1 0.2 15.2
9913 2.7 0.3 9
9258 7.6 0.3 22.8
9913F7 2.9 0.4 11.6
Not a very good correlation between DC resistance and RF resistance
If we compare 50 MHz loss to 1 MHz loss:
Type 50 MHz loss 1 MHz loss Ratio 50 MHz/1 MHz loss
8214 1.2 0.1 12
8237 1.3 0.2 6.5
9913 1.0 0.3 3.3
9258 2.1 0.3 7
9913F7 1.1 0.4 2.56
Again, not a very good correlation.
I will note that the theoretical ratio is the 7.07, the square
root of 50. 8237 and 9258 come close to this. It is very
unclear why 8214 deviates. Could the Belden catalog be
wrong? :-) 9913 is of course a "funny" coax and maybe we
should expect it to do funny things.
So it looks like neither 50 MHz nor DCR is well correlated
to 1 MHz loss. And you probably should take any data sheet
numbers with a grain of salt.
Rick N6RK
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