[time-nuts] Choke Rings and Points North

Bob Camp kb8tq at n1k.org
Tue Dec 16 21:30:25 EST 2014


> On Dec 16, 2014, at 9:15 PM, Dave M <dgminala at mediacombb.net> wrote:
> Jim Lux wrote:
>> On 12/16/14, 5:59 AM, Tom Van Baak wrote:
>>>> Clever idea, but..
>>>> Most rotary joints have more phase and amplitude variability than
>>>> the antenna.
>>>> So you're stuck with rotating back and forth with a cable that's
>>>> flexing and now you get to measure the phase variability of the
>>>> coax.
>>> I was thinking of some sort of non-contact RF bridge that would allow
>>> either side to rotate independently. Converting to optical would make
>>> this easy, but there must be a way to do it at 1.5 GHz too.
>> two nested coils forming an air core transformer or slip rings are the
>> typical approach.
>> (Waveguide at 1.5 GHz is somewhat unwieldy in size).
>> The trick is in holding mechanical tolerances tight enough.I guess,
>> 1mm mechanical tolerance (easy, easy) would be comparable to the phase
>> center displacement.
>>>> You really need to have your entire GPS system antenna and receiver
>>>> on the rotating table (which will need to have temperature
>>>> controls, etc.)
>>>> Oh, what a pit one can descend into with the goal of reducing
>>>> everything to a minimum error.
>>> In this case the goal is actually not minimizing error. The goal is
>>> to vary each possible error source with its own prime modulation
>>> period. Collect lots of data and the FFT tells you how much each
>>> error contributes to the pie.
>> Yes, but how do you know whether it's coax flex or phase center
>> displacement that's causing your 17 hour periodicity.
>> I was thinking not so much reducing error in the overall measurement,
>> but in reducing the uncertainty in the estimate of the size of each
>> contributor to the overall system.
>>> For example, instead of temperature control you modulate temperature
>>> by 5C over a 13 hour period. Instead of voltage control you modulate
>>> the 5V antenna power by 10% every 1.7 hours, etc.
> Inquiring minds surely are in high gear!!  And to think, all I wanted to know was how close I needed to to point to north!!

The need to point north is a legitimate question. There is a chance that they designed some magic into it to deliberately shape the response. 

Based on the analysis done so far, you have to wonder just how they set up to check these things for phase center. Given the money you pay for them, I would hope they have a definitive test. Getting back an answer like “some guy named Bob did some math” in response to a request for traceability would be a major downer ….


> LMAO!!
> Dave M 
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