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.
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