[time-nuts] Choke Rings and Points North
jimlux at earthlink.net
Mon Dec 15 21:05:15 EST 2014
On 12/15/14, 5:46 PM, Dave M wrote:
> With all the discussion about surveys & position accuracy, I have a question
> about my choke ring antenna. There is an arrow marked "N" on the underside
> of the rings. How accurately does the alignment need to be to "N"orth?
> True north or magnetic north (my thinking says True North)?
> Does the directional accuracy affect the precision survey? I'm assuming
> that it would have no effect on the accuracy of the 10 MHz frequency output.
> Or am I completely off base?
If you're using a standard antenna, they've characterized them for the
change in phase center with respect to the direction the signals are
coming from. It's assumed you'll install it level, so elevation is taken
care of. The remaining uncertainty is the azimuth, hence the north arrow.
Now we can find out how much of a nut you really are. On choke ring
antennas, I think the maximum shift in phase center with look direction
is on the order of single digit millimeters, or a few ps.
And how accurately do you know what direction is "north". That could be
a whole project in itself, ranging from moss on trees, to shadows of
sticks and rocks, to observations of Polaris through a theodolite, and
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