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Author Topic: Aligning a beam..  (Read 1361 times)
KC0RDG
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Posts: 126


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« on: September 17, 2006, 07:15:00 PM »

All,

When setting up a beam for the first time, you set the rotator to 0 degrees, put the antenna in with 'forward' facing 0 degrees and tighten it down.

Question is - when doing this, you align it with magnetic north or true north, taking into account devation?  I can't imagine a antenna would have such a narrow beamwidth that it would really matter, but I'd just like to make sure my assumption is right.

Thanks,
Elijah
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K8AC
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Posts: 1478




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« Reply #1 on: September 18, 2006, 05:26:26 AM »

Align it with true north.  Do a search on "solar noon chart" and you should find multiple websites that will give you a chart of solar noon times for the month for your exact latitude/longitude.  At solar noon, the shadow of your tower/beam will be directly north of the tower base.  
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W5CPT
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« Reply #2 on: September 18, 2006, 06:49:20 AM »

The difference between True North and Magnetic North in MN where you live is so small as not to be a concern as far as antennas go.  Unless you are installing a 20 element (or so) beam with a beamwidth of less than 10 degrees, the antenna will be broad enough to cover anything you want to point it at.

Long boomed satellite antennas and similar antennas are the exception.  

Stick it up there pointing in the general direction of North and you should be fine.

Clint - W5CPT
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WW5AA
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« Reply #3 on: September 18, 2006, 08:40:08 AM »

I directed my beam between magnetic and true, This makes it close enough no matter if the map being used is magnetic or true. The following is a great site for beam headings.

http://www.njdxa.org/dx-tools/dxcclist.shtml

73, de Lindy
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KC0RDG
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« Reply #4 on: September 18, 2006, 09:50:08 AM »

Thanks all for the replies!
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WB2WIK
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« Reply #5 on: September 18, 2006, 10:02:58 AM »

I use true headings, not magnetic, and on VHF-UHF it can make quite a difference.  I doubt anybody has a 40 meter beam sharp enough where it matters...

Where I am in L.A., we have a magnetic declination of more than 13 degees (east) at this time.  On 70cm where a long boom Yagi can have a 9 degree beamwidth, that's quite a lot of error.  On 20m where I have about a 45 degree beamwidth, it's just about nothing.

But "magnetic north" is constantly changing, so setting up by that today is quite different from setting up for it a year from now.  True north stays in the same place.

If you don't know your specific declination, you can find it on line, continuously updated, here:

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

WB2WIK/6
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