There has been a bit of confusion between these two words.
Declination is the angle of arc, north or south, between a body on the
celestial sphere and the celestial equator.
Variation is the difference in bearing between the north pole and the
north magnetic pole. The direction to the north pole obviously is true
north. the direction to the north magnetic pole is magnetic north, and
variation can be east or west, depending on where you are.
To convert a true bearing to a magnetic one, and the variation is west,
variation must be added to the compass heading. If the variation is east,
it must be subtracted. To convert compass headings to true headings, do the
reverse.
Lines of equal variation are "isogonic lines", and probably can be found on
the web. In the United States, the line of zero variation runs
approximately from western Lake Superior southeasterly to just off the
Eastern Florida coast. Places in the US east of that line have west
variation and in Northeast Maine as much as 20 degrees. Places to the
west of the zero line have easterly variation and exceed 20 degrees inn the
Pacific Northwest.
Very few directional antennas can tell the difference, and to find out what
the variation is wherever you are, call the local airport and ask. It's
printed on aeronautical charts.
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