[TowerTalk] Calibrating antenna direction and using chokes with a beam

[K8RI]

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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