[time-nuts] Choke Rings and Points North

Bob Camp kb8tq at n1k.org
Tue Dec 16 07:09:53 EST 2014


Hi
> On Dec 15, 2014, at 11:20 PM, Dave M <dgminala at mediacombb.net> wrote:
> 
> 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.

Since the only way you get frequency is by processing time, you do indeed care about time if you are looking for frequency :)

That said, the error is indeed only a fixed offset and it would not matter in any time solution. There are much larger issues in a time solution. For a precision time application, you would locate the antenna first. Next you run a survey receiver through the same antenna. Once you process the results of the survey, it would take out any phase center error from the antenna. 

All that said, yes, it’s not worth worrying about in your case.

Bob

> 
> Thanks!!
> Dave M
> 
> 
> 
> 
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