[TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much

[jimlux]

Rick Karlquist wrote:
> 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
> 
This is quite interesting...

It would be interesting to get a bunch of samples and test them. 
Tricky, because you need enough length so you're not trying to measure 
0.001 dB differences. I wonder if some cable vendor with a large 
selection would be interested in having someone come with a good 
calibrated network analyzer (not because you need fancy, but you want 
automation of the measurements and the ability to dump the data into a 
file) and doing a "measure-fest"?

As to reasons for things varying all over the place:
perhaps skin effect and cladding of various sorts, as the current 
density profile moves across the boundary.   Most of the loss is in the 
center conductor, but the effect of cladding is easily modeled, and 
probably doesn't account for everything. How uniform is the cladding? 
What about the losses in the shield, particularly for those cables with 
both braid and foil? How much effect on loss is there from braid moving 
around?