[time-nuts] Choke Rings and Points North

Dave M dgminala at mediacombb.net
Tue Dec 16 21:15:40 EST 2014

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!!


Dave M 

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