On 10/26/2012 12:12 AM, Rick Kiessig wrote:
Thanks for the replies so far -- as I said in my OP, I know how to find true
north. I already went through the compass/declination/rough-alignment when I
first put the antenna up. I know about the shadow technique as well [FWIW,
in the southern hemisphere the sun is in the north and shadows point due
south at solar noon]. What I'd like to do now is calibrate pointing
direction with some degree of accuracy, based on something more concrete
than just eyeballing it.
The shadow technique when combined with the boom of the antenna or used
with a Theodolite or transit is the most precise method of finding true
North available to a ham and is going to get you into seconds of arc
precision and transferring that to the antenna/rotator can still be kept
in the realm of arc seconds.
Actually with your watch synchronized to WWV, having some one count down
for you, and marking the end of the shadow right on the count for solar
noon you should be within seconds for actual accuracy. Even doing it
alone with a good inexpensive watch you can be well under one degree, or
even under a minute of angle.
Extend the line formed by the tower shadow at solar noon far enough to
be able to get a good view of the antenna end on. Just use a string or
wire from the center of the tower to the stake marking the shadow tip
and carry it on out in a straight line as far as is practical. The
shadow is as precise as your watch and doing it this way does not peg
you to working only at solar solar noon.
Assuming the shadow mark is done properly, its position will be at a
fraction of a degree accuracy (within seconds of arc) so the taller the
tower the better off you are. If the antenna/rotator is properly aligned
then when it is pointed due South (or North) at solar noon, its shadow
will form a straight line and the transit will see only the end and
underside of the boom which should line up with the vertical cross hair.
The point is the shadow method is not "just eyeballing". It is elegant
in its simplicity and outstanding in its precision. Using the transit
or scope, to transfer the alignment to the antenna/rotator eliminates
the old fashioned "eyeballing" it in. You can also use the shadow of the
boom but that is limited to very little time.
73
Roger (K8RI)
Also, I realize a 3-el Yagi has a reasonably wide 3 dB forward lobe (appx 66
deg). I'm not concerned with the accuracy of where I'm pointing the center
of the lobe; I'm ultimately interested in managing placement of its edges,
where gain drops off quickly.
73, Rick ZL2HAM
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