Choke Rings and Points North
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!!
LMAO!!
Dave M
Hi
On Dec 16, 2014, at 9:15 PM, Dave M dgminala@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 ….
Bob
LMAO!!
Dave M
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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.
Having taken a few apart, I very much doubt that. I think they try
to get the phase center coincident with the mechanical center, but
knowing that it won't be exact, they want you to orient it so that
the residual error is always in the same direction. You can point
the arrow in any direction you want, as long as you do it consistently.
Best regards,
Charles
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.
-Chuck Harris
Bob Camp wrote:
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
….
Bob
LMAO!!
Dave M
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HI
On Dec 17, 2014, at 1:07 AM, Chuck Harris cfharris@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 ….
Bob
-Chuck Harris
Bob Camp wrote:
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
….
Bob
LMAO!!
Dave M
_______________________________________________ time-nuts mailing list --
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instructions there.
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On 12/17/14, 4:36 AM, Bob Camp wrote:
HI
On Dec 17, 2014, at 1:07 AM, Chuck Harris cfharris@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.
Hi
On Dec 17, 2014, at 8:49 AM, Jim Lux jimlux@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@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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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@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@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.
time-nuts mailing list -- time-nuts@febo.com
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