[time-nuts] Choke Rings and Points North

[Bob Camp]

Hi

Here is some actual RF data on a number of antennas:

http://www.ngs.noaa.gov/ANTCAL/

There also is a pretty good document on how the calibration is performed. 

It appears that if you are after ps or mm then a calibration file on the antenna type being used is needed. Pointing north to be able to use that file would be a requirement. If we ever get into the sub ns world for Time Nuts time transfer, calibrated antennas may be needed.

————

*IF* an uncalibrated antenna is much worse than the survey antennas, then you might get into the couple of ns level as the sat’s moved around. Still not a real big deal for a GPSDO. Un-corrrected ionosphere issues versus angle will be a larger issue. 

Bob



hOn 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.
> 
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