Although the idea is sound, easy to implement in one form or another
there are many places in the US (and the rest of the world) where
magnetic and true North differ substantially. The extremes in
continental US are in the NE and NW. IOW New England states (-20 deg)
and parts of Washington state(+ 20 degrees) with the East coast running
-10 and the N Central US down to Mexico and the tip of the Baja
peninsula running +10. Zero degrees runs from the Sestern edge of
Hudson Bay to near New Orleans.
http://en.wikipedia.org/wiki/File:IGRF_2000_magnetic_declination.gif
Western Australia is zero while Eastern is +10, but New Zealand runs
from + 20 in the North to about + 25 in the South. Tasmania is some
where between + 10 to 20. The N end of Madagascar is -10 while the
South end is -20.
NOTE this map does not take into account local variations which can be
as much as 30 or 40 degrees, nor does it show anything related to the
South Atlantic anomaly.
Check the magnetic deviation in your particular area if you decided to
use this simple and effective means of keeping track of your antenna
heading as it can easily be 10 to 20 degrees off particularly in the
heavily populated coastal areas. The map linked above makes a good
reference and starting point, but get an aviation chart for your area if
in the US and check the "Isogonic lines" (didn't think the spell checker
would know that one). for your area.
Mineral deposits as found in Upper Michigan, Wisconsin, and Minnesota
are particularly annoying. Man made structures including power lines
*May* affect readings.
However I should add that many wind indicators in wireless weather
stations "do not" depend on the earth's magnetic field, but rather
depend on them being oriented to North (true or magnetic is up to the
installer) They use a burst transmission on the 430 band, but I've
never heard the one I have up. Batteries are going on 3 years even in
our weather.
73
Roger (K8RI)
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