[TowerTalk] RG400 Measured Data

[jimlux]

On 3/16/18 12:33 PM, Jim Brown wrote:
> Here's measured data for Harbor Industries RG400.
> 
> k9yc.com/RG400DataMF-HF.png
> 
> Data was obtained by S11 sweeps (1-30 MHz) using VNWA, exporting as S1P 
> files to AC6LA's Zplots spreadsheet for Excel. ZPlots does the math to 
> compute and plot Zo, VF, and attenuation vs frequency, and finds the 
> constants K0, K1, and K2 in the Johnson transmission line model. The 
> lower plot is computed from the raw data, the top two plots come from 
> the model. Zo (nom) and VF (nom) are the values to which the curves are 
> asymptotic at VHF.
> 
> These curve shapes are predicted by the wave equations and are typical 
> of virtually all transmission lines.
> 

Interesting data..
So I wonder what truly horrible coax would look like - that is Z vs freq 
or loss vs freq are generally smooth curves (and, so, well modeled by 
k0,1,2, etc.)

I wonder if a "figure of merit" for coax could be developed by 
"deviation from model"  - that is, we all know RG-174 has more loss than 
RG-213 or 1" hardline, but that's mostly a matter of geometry and materials.

But maybe "good" coax has a better fit to the model than "poor" coax - 
or the plots move around as you move the coax or there's perodic dips 
and peaks (at an enlarged scale) from a manufacturing roller that has a 
flat spot, or the cable being stored with a big weight on the spool on 
one side, or....

The CATV business has measurements and criteria that attempt to 
characterize shielding effectiveness (not in an idealized modeled sense, 
but as in "what you measure in a standard test fixture"), and I'm sure 
there's some way to measure IMD from cable/connectors.

Is there some "doable by a ham with simple test equipment (which today 
means an inexpensive 1/2 port VNA)" test that can distinguish between 
"good" and "less good" coax?

Preferably with a one port measurement (so you don't have to have access 
to the inside of the spool or other end of the cable).