[TowerTalk] A word of caution Was Wireless tower direction indicator?

[R. Kevin Stover]

On Tue, 01 Feb 2011 01:19:30 -0500
"Rroger (K8RI on TowerTalk)" <k8ri-on-towertalk at tm.net> wrote:

> 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)


Here is a link to a page which will figure your magnetic declination
for you.

Plug in your zip or lat/long plus the date.

http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp

My QTH is 1 degree 27 minutes east so when pointing a beam on a tower
with an magnetic compass I'd subtract 1 degree. Problem is this number
changes every year as the north pole is moving, 7 minutes per year to
the west at my QTH.


-- 
R. Kevin Stover
AC0H