[time-nuts] Choke Rings and Points North

[Bob Camp]

Hi
> On Dec 17, 2014, at 8:49 AM, Jim Lux <jimlux at earthlink.net> wrote:
> 
> On 12/17/14, 4:36 AM, Bob Camp wrote:
>> HI
>> 
>>> On Dec 17, 2014, at 1:07 AM, Chuck Harris <cfharris at erols.com> wrote:
>>> 
>>> I would venture that the extent of the magic was to note the physical
>>> center of the array, and call that the phase center.
>>> 
>>> As long as you always orient the antenna in the same direction, any
>>> errors that might exist in the real phase center will be consistent,
>>> and could be corrected for by noting the offset from a benchmark.
>> 
>> I’m afraid that’s what they do as well. Just spin it and see what a dial indicator reads sort of thing. I think that I’d want something that actually did some microwave tests ….
>> 
>> B
> 
> The UNAVCO data is an actual RF test.
> 
> And from a manufacturing standpoint, I would imagine that typical tolerances are better than 0.001" (25.4 microns).  Changes much bigger than that would show up as VSWR changes, which *is* something that they check in manufacturing.
> 
> The Leica "artichoke" multiband choke ziggurats (they're not flat, so I have a hard time calling it a "ring") are cast and then machined. Casting isn't what I would think is a precision operation, but it probably is real consistent from unit to unit.
> 

There are some *very* accurate casting techniques these days. They do an amazing job on multidimensional gizmos like antennas or golf club heads. It’s not a cheap thing to set up or keep under control. It’s probably cheaper than a full blown machining process. It would not surprise me to find that most of the errors are scale errors rather than errors in any one dimension. If you “inflate” the whole structure by 0.1%, I doubt that impacts a receiving antenna a whole lot. Getting back to the phase center question, it should have very little impact on the phase center. Stability wise, a casting is often a good idea. Having the center (where ever it is) stay put is more important than it being “perfect”. 

There is a sub issue to all of this. You can have an antenna with a good “averaged” phase center. You also can have one that truly has the same center no matter which way the signal comes from. With a timing setup trying to do per satellite data,  *that* parameter would indeed matter quite a bit. I doubt that it’s a big design issue on your run of the mill $3 antenna. I believe it would be quite a bit better on one of the fancy antennas. Pathogenic issues in phase center could indeed be part of some of the 12/24/48 hour stuff one sees in GPS plots. Separating them from multipath in a field setting could be a bit difficult. 


Bob

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