Jim Brown wrote:
> On Thu, 04 Feb 2010 20:45:09 -0800, jimlux wrote:
>
>
>> Tricky, because you need enough length so you're not trying to measure
>> 0.001 dB differences.
>
> Yes. 100 ft is sort of a minimum length, and more is better. My
> measurement setup is quite simple -- an HP generator and an HP spectrum
> analyzer. Nothing automated. To eliminate calibration errors in the
> equipment which can easily be of the same magnitude as what we're trying
> to measure,I must write down numbers and subtract the difference between
> a very short cable and the cable under test.
>
>> 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,
>
> I wouldn't be so sure of that. DCR is split between center and shield,
> in many cables the DCR of the shield is significantly greater than the
> center, and the composition of both center and shield can be complex.
>
Is it? the surface area of the shield is enormous compared to the
center conductor. If both were solid copper, the relative contributions
would be in the ratio of diameters.
Z is about 138/sqrt(epsilonr)log10(D/d)
D/d is 7.2/2.17 for RG-8, 7.2/2.74 for LMR400 (which has copper clad
aluminum)
or about 3:1 (I guess enormous isn't a good word.. just shows it pays to
actually look this stuff up..)
So an interesting observation is that the kind of dielectric will have
an effect on the loss at HF, not because of dielectric loss effects, but
because lower loss dielectrics (foam, air) have smaller epsilon, so the
D/d ratio is smaller for the same Z, meaning that for the same Outer
diameter, the inner conductor is bigger (and lower IR loss).
>> 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?
>
> Exactly the sort of thing that needs to be measured.
>
One might think that the Beldens and Alphas of the world will have done
these measurements, just the data is buried in some archive. or do they
just measure at a few spot frequencies for the databook numbers?
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