[time-nuts] Choke Rings and Points North

Jim Lux 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 
so forth.

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