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Re: [TowerTalk] Calibrating antenna direction and using chokes with a be

Subject: Re: [TowerTalk] Calibrating antenna direction and using chokes with a beam
From: K8RI <>
Date: Fri, 26 Oct 2012 13:53:38 -0400
List-post: <">>
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.


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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