Jim Lux wrote:
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.
Thanks for the explanations. I'm not terribly concerned about time, other
than knowing when it's time to eat and sleep... I'm more of a frequency nut
than a time nut. I have a USGS map and recent survey of my property, so I
know where North is, to a pretty good certainty.
Thanks!!
Dave M
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