with the use of 450 ohm feedline and a manual tuner.

Where I can purchase this kind of line ?
(some online mall that delivery outside USA/Canada),
because I can't find it here in Italy.

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Frank, the exact impedance of the high impedance line doesn't matter at all.
You don't even need to know what its impedance is. Line loss is small just
because it has a high impedance.

Any line where the pair of wires are spaced apart more than 25mm will be
satisfactory. Wire spacing can change along its length without any bad
effects. The only effect will be a change in tuner component settings.

If you are unable to purchase a line then it is not difficult to make. You
can make the stuff in less time than it takes to order and deliver from a
manufacturer.

Use 1.6 or 1.3 mm diameter wire. Space the wires 50 mm to 100 mm apart.
There are many ways of fixing the wire to the ends of rigid plastic spacers.
The distance between spacers along the line can be anything from 100 to 200
mm. Construction depends on your judgement of what weather conditions, wind
velocity, etc., the line will have to withstand and the time in months.
Failure due to flexing in the wind can be a problem.
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Reg, G4FGQ