Question: could some of this loss be caused by the capacitance in
the line ?

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Yes. It's another way of looking at it.

In addition to current in the load, there is a current which flows
between the pair of wires through the capacitance.

Increase the capacitance and this current increases.

There is negligible loss in the capacitance itself.

But the capacitor current has to flow along the wires to get there.

And so the additional capacitor-current loss actually occurs in the
wire resistance.

But this is just the same as saying that loss is greater because the
impedance Zo is lower (due to the increase in capacitance).

The opposite effect occurs by increasing inductance. An increase in
inductance increases Zo and so much reduces attenuation. That's why
88 mH inductive loading coils were used at intervals of 2000 yards at
audio frequencies in very long telephone cables. An invention of the
great but modest Oliver Heaviside which I think somebody else patented
and manufactured by many millions.

88 mH loading coils, spaced at 2000 yards, increases Zo from about 300
ohms to 1100 ohms, thus reducing loss in dB per mile to about one
third.
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Reg, G4FGQ.