The impedance may not change, but I'm pretty sure the insulation in Romex is vinyl, and I'm pretty sure it has a serious loss factor above HF. There's a good reason you see polystyrene, mylar, mica, glass, and teflon capacitors, but not vinyl capacitors.
Isaac
Romex is vinyl jacketed PVC. Not the best of insulation for RF. I couldn't find a value for loss tangent, probably because it varies by composition, manufacturer, color doping, position of the moon, etc.
You're probably right that it's not useful at UHF, but I'm too lazy/busy to test it.
The ham doing the Romex testing was Dave Stoddart W8FLL (SK) who died in 2008.
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