[amsat-bb] Re: How to calibrate the azimuth angle?
rlong3 at columbus.rr.com
Thu Dec 27 08:04:35 PST 2007
Here is an amsat-bb post from Tom Clark, K3IO, formerly W3IWI, on
this subject. Wonderfully detailed. Thank you Tom. I have removed the
From line since the original address from 12 years ago may not be
accurate. ron w8gus
Date: Tue, 17 Jan 95 20:12:13 UTC
From: Tom Clark
To: amsat-bb at AMSAT.Org
Subject: North/South vs East/West
The recent thread of Magnetic vs. Geodetic north-south for the
calibration of antenna azimuths has overlooked an even easier way to
do the deed. The idea is not new -- I believe it is the way the
ancient Egyptians had their Nubian slaves establish the orientation
of the great pyramids to accuracies of about a half-degree. Instead
of trying to establish a North/South line,
determine East/West instead!
Here is the recipe (with some footnotes):
1. Wait for the morning of a clear, sunny day.
2. Find a level, flat area.
3. Poke a 3-6 foot (1-2 meter) stick in the ground vertically (south
of the flat area in the northern hemisphere or north in the southern
4. Several times during the day, mark the location of the tip of the
shadow of the stick.
5. The points you just marked will be on a straight line which is
nominally East/West. Drive stakes at the ends of the line (or if your
shadow is on concrete, paint the line).
a. The accuracy of the east/west line is best at the solstices
(Dec.21/Jun.21) and worst at the equinoxes (Mar.21/Sep.21). The sun's
declination is a cosine curve with extrema of +/- 22.5 degrees
reached at he solstices. At the equinoxes, the declination changes
about 1/2 degree/day so if you are observing at the equinoxes over an
8-hour period, the line will be in error by about 10 arc minutes.
b. The diameter of the sun's disk is about 30 arc minutes, so the
shadow is "fuzzy" at this level. You need to be consistent in picking
the point corresponding to the tip's shadow to minimize the penumbral
error. The diameter of the stick should be big enough to give a solid
shadow -- i.e. the stick should subtend an angle > 30 arc minutes
when viewed from the level, flat area. I recommend that the stick
have a length/diameter ratio of 20-30. A typical tall antenna tower
will not cast a sharp enough shadow to use.
c. The level area need only be level in the east/west direction. A
north/south slope will not affect the accuracy. If you don't have a
level area, get a 4'x8' sheet of plywood and prop it with short
stakes to make it level and cast the shadow onto the plywood. You
should strive for east/west "levelness" of one degree or better if
you want results good to about one degree.
Hope this idea helps -- 73 de Tom.
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