Re: [time-nuts] Choke Rings and Points North
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 jim...@earthlink.net wrote: On 12/17/14, 4:36 AM, Bob Camp wrote: HI On Dec 17, 2014, at 1:07 AM, Chuck Harris cfhar...@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 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
HI On Dec 17, 2014, at 1:07 AM, Chuck Harris cfhar...@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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
On 12/17/14, 4:36 AM, Bob Camp wrote: HI On Dec 17, 2014, at 1:07 AM, Chuck Harris cfhar...@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 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
Hi On Dec 17, 2014, at 8:49 AM, Jim Lux jim...@earthlink.net wrote: On 12/17/14, 4:36 AM, Bob Camp wrote: HI On Dec 17, 2014, at 1:07 AM, Chuck Harris cfhar...@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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
To answer which North, it is True North, not Magnetic. Orbits, including GPS, are specified relative to the geographic pole. Magnetic North moves noticeably over time and place. True North moves somewhat over time but only very slightly. David On 12/15/14 9:05 PM, Jim Lux wrote: On 12/15/14, 5:46 PM, Dave M wrote: With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? If you're using a standard antenna, they've characterized them for the change in phase center with respect to the direction the signals are coming from. It's assumed you'll install it level, so elevation is taken care of. The remaining uncertainty is the azimuth, hence the north arrow. Now we can find out how much of a nut you really are. On choke ring antennas, I think the maximum shift in phase center with look direction is on the order of single digit millimeters, or a few ps. And how accurately do you know what direction is north. That could be a whole project in itself, ranging from moss on trees, to shadows of sticks and rocks, to observations of Polaris through a theodolite, and so forth. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
Hi On Dec 15, 2014, at 11:20 PM, Dave M dgmin...@mediacombb.net wrote: Jim Lux wrote: On 12/15/14, 5:46 PM, Dave M wrote: With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? If you're using a standard antenna, they've characterized them for the change in phase center with respect to the direction the signals are coming from. It's assumed you'll install it level, so elevation is taken care of. The remaining uncertainty is the azimuth, hence the north arrow. Now we can find out how much of a nut you really are. On choke ring antennas, I think the maximum shift in phase center with look direction is on the order of single digit millimeters, or a few ps. And how accurately do you know what direction is north. That could be a whole project in itself, ranging from moss on trees, to shadows of sticks and rocks, to observations of Polaris through a theodolite, and so forth. Thanks for the explanations. I'm not terribly concerned about time, other than knowing when it's time to eat and sleep... I'm more of a frequency nut than a time nut. I have a USGS map and recent survey of my property, so I know where North is, to a pretty good certainty. Since the only way you get frequency is by processing time, you do indeed care about time if you are looking for frequency :) That said, the error is indeed only a fixed offset and it would not matter in any time solution. There are much larger issues in a time solution. For a precision time application, you would locate the antenna first. Next you run a survey receiver through the same antenna. Once you process the results of the survey, it would take out any phase center error from the antenna. All that said, yes, it’s not worth worrying about in your case. Bob Thanks!! Dave M ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
On 12/15/14, 8:10 PM, Tom Van Baak wrote: But to prove us wrong, put the antenna on a 17 hour turn-table, collect data for 6 months, and then see if you see any 17h peaks in the FFT! 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. 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. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
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. 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. 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. /tvb ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
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. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
Hi On Dec 16, 2014, at 9:15 PM, Dave M dgmin...@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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Choke Rings and Points North
With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? Dave M ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
Hi If: 1) You are doing a survey of a number of points in an area 2) You want to hit the “1/5 mm accuracy” that your system is rated to :) 3) You really do care Then: You point the arrow north to the best of your ability to make all the antenna errors show up in the same direction. It becomes an offset to the entire grid of points rather than an error randomly added to each point. You do a correlation process at your stationary survey (DGPS) locations to remove the error. If you are not doing that sort of survey work … don’t worry about it. (Apologies to those who do this for a living, you do indeed care, it’s just the time side that does not care). I’m sure that “best of your ability” is indeed quite good. Bob On Dec 15, 2014, at 8:46 PM, Dave M dgmin...@mediacombb.net wrote: With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? Dave M ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
On 12/15/14, 5:46 PM, Dave M wrote: With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? If you're using a standard antenna, they've characterized them for the change in phase center with respect to the direction the signals are coming from. It's assumed you'll install it level, so elevation is taken care of. The remaining uncertainty is the azimuth, hence the north arrow. Now we can find out how much of a nut you really are. On choke ring antennas, I think the maximum shift in phase center with look direction is on the order of single digit millimeters, or a few ps. And how accurately do you know what direction is north. That could be a whole project in itself, ranging from moss on trees, to shadows of sticks and rocks, to observations of Polaris through a theodolite, and so forth. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? Dave M Dave, I agree with the others. If you're using dual-frequency GPS with post-processing it might make a few mm or ps difference. This only matters for absolute measurements, or multiple measurements where you plan to compute differentials. It also depends on the antenna. You can find the phase vs. angle profiles at http://facility.unavco.org For one example of an antenna calibration see: www.unavco.org/projects/project-support/development-testing/publications/trimchoke/trimchoke.pdf The GPS pros take their mm very seriously. If all you're using is a fixed location and commodity GPSDO then it makes no difference at all, not even close. But to prove us wrong, put the antenna on a 17 hour turn-table, collect data for 6 months, and then see if you see any 17h peaks in the FFT! /tvb ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Choke Rings and Points North
Jim Lux wrote: On 12/15/14, 5:46 PM, Dave M wrote: With all the discussion about surveys position accuracy, I have a question about my choke ring antenna. There is an arrow marked N on the underside of the rings. How accurately does the alignment need to be to North? True north or magnetic north (my thinking says True North)? Does the directional accuracy affect the precision survey? I'm assuming that it would have no effect on the accuracy of the 10 MHz frequency output. Or am I completely off base? If you're using a standard antenna, they've characterized them for the change in phase center with respect to the direction the signals are coming from. It's assumed you'll install it level, so elevation is taken care of. The remaining uncertainty is the azimuth, hence the north arrow. Now we can find out how much of a nut you really are. On choke ring antennas, I think the maximum shift in phase center with look direction is on the order of single digit millimeters, or a few ps. And how accurately do you know what direction is north. That could be a whole project in itself, ranging from moss on trees, to shadows of sticks and rocks, to observations of Polaris through a theodolite, and so forth. Thanks for the explanations. I'm not terribly concerned about time, other than knowing when it's time to eat and sleep... I'm more of a frequency nut than a time nut. I have a USGS map and recent survey of my property, so I know where North is, to a pretty good certainty. Thanks!! Dave M ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
