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Magnetic Deviation (was: declination??)

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Subject: Magnetic Deviation (was: declination??)
From: (Bill Coleman)
Date: Wed, 23 Oct 96 12:01:01 -0400
>From:        Jamie Tolbert,
>Today I heard a carribean dxpedition for the cqww concerned because he 
>wasnt sure how to adjust for the difference between true and magnetic 
>north...Is there really a big difference??

Depends on what part of the world you are in. Much of the US is 
reasonably ( +- 5 degrees ) close to agonic line (where true north and 
magnetic north coincide). So, if you are in the Midwest, Plains, South or 
Mid-Atlantic states, you probably don't need to worry about it.

>I think for my latitude its around 4 degrees..

Uh, the magnetic deviation is NOT constant over true latitude. (Heck, 
magnetic latitude isn't even a circle! Too much ferrous material in one 
place can bend the magnetic field considerably)

>Should ones antenna be set for true or magnetic north?? 

Depends on where you are trying to point your antenna -- to a true or 
magnetic heading? Most likely, you are trying to point to a true heading. 
So, if possible, you should calibrate your rotator to true north. (or, 
you can compensate on the rotator controls by adding your magnetic 
deviation each time you set the heading)

A good source for magnetic deviation information can be had at your local 
pilot shop for about $6. Get a sectional chart for your area. (or find a 
pilot throwing an obsolete sectional out -- they are revised every 6 
months or so) The isogonic lines should be easy to find. Topographical 
charts of your area should also list the magnetic deviation (just in case 
you are hiking cross-country by compass)

Here this part of Georgia, we're east of the agonic line, so we correct 
about 2 degrees west. This means you subtract 2 degrees from the true 
heading to get the magnetic heading (ie true north is 002 degrees 

Frankly, unless you are running long-boom (multiple-wavelength)  beam 
antennas on HF, and you can predict the refraction angles off the 
ionosphere, I can't see that you should be terribly worried about this. 
Beamwidths for a typical monobander are on the order of 20 degrees or 
more. Ionosphere propagation can sometimes refract a signal to a 
different heading than the true one. (east-west propagation on 10m during 
sunspot minima is sometimes possible only with beams pointed at the 

So, it all adds up that a few degrees probably aren't going to make the 
difference. Unless you pin everything precisely, it is doubtful your 
antenna will stay calibrated, anyway. Just point the antennas until 
people sound loud and don't worry about it.

Bill Coleman, AA4LR           Mail:
Quote: "Not in a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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