[time-nuts] Choke Rings and Points North
dgminala at mediacombb.net
Mon Dec 15 23:20:01 EST 2014
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.
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