Hi If you look at the way NIST sets up one of their "time modem" installations, they do indeed worry a lot about this sort of stuff. There's a major choke / isolator between the antenna and the feed line. The claim is that they see in building grunge causing trouble without it. I'm sure that will be a variable depending on your building.
The next claim is that without the isolation and a choke antenna, there is a possibility of multi-path issues. Since most of us do not have a choke ring antenna the isolator may be overkill. The NIST site in Bolder is definitely multipath challenged. They probably have some pretty good data on that. If you are after sub ns level timing, things are a bit different than if you are happy with tens of ns error. Few of us have an adequate survey of our location to *really* worry about sub ns numbers. If you are one of those lucky few that can worry about sub-ns, yes mismatch and voltage and a whole long list of things matter. The temperature coefficient of your antenna also gets onto that list at some point. Receivers are often designed with an "I'm by my self" approach. Some designs do indeed feed crud back up towards the antenna. Digitization clocks and all sorts of other things can be a source of these signals. Given the high gain of the antenna preamp, they can get away with a certain degree of sloppy design. I suspect that there are cases of GPS A not liking the spurs from GPS B. For most of us, none of this matters. If we're 10 ns off, we'll never know it. If our whole setup varies 2 or 3 ns over a day, we simply don't have the gear to spot the problem. It also does not matter to most of the people who use GPSDO's in systems. I have yet to see a surplus GPSDO arrive with a non-zero cable delay in it's eeprom. I doubt that people have zero delay cables. Without measuring the cable delay and compensating for it, you can easily be off 100's of ns. Yes, you can get out your TDR and come up with a cable number to 100ps or less. You can get a survey that's good to centimeters. You can get a good antenna (cheap if you are lucky or patient). Do all that and more, you can get into the sub ns range. Calculated cables based on length, estimated location based on self survey, easy to get antennas, not going to cut it. As always, the answer is "it depends on what you are doing". Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Edgardo Molina Sent: Monday, October 08, 2012 10:31 PM To: Discussion of precise time and frequency measurement Subject: [time-nuts] To use or not to use transmission line splitters forGPS receivers Dear Group, Good evening. I just arrived home after the first day of conferences at the Electrical Metrology Forum 2012 at Mexico's metrology center CENAM. I attended several presentations of time and frequency, very interesting indeed. At last I understood some concepts hard to land in theory. I will be attending the whole week long and half of next week. I will have the pleasure and honour to meet Dr. Judah Levine from NIST, who has shown a very nice attitude towards me and accepted to talk a little bit around my thesis in network synchronization. Now to the point if you kindly allow. I got involved in a round table discussion around the use of GPS antennas for time and frequency GPS receivers. I tried to make some points from my personal perspective. I got resistance from the audience and the topic went hot very quickly and didn't set at the end. Honestly I would like to share my doubts and opinions with you in order to enhance my experience about the topic. In the end it could also be beneficial to close this debate tomorrow while attending to CENAM's time and frequency forum. Facts and thoughts: 1. The time and frequency attendees at CENAM`s time and frequency forum is reluctant to use GPS antenna splitters for a number of reasons I couldn't conceive. 1.1 They argued that cross talk could happen among ports. I doubt it with the newer models. I have experience with HP and Symmetricom units and they state their products cancel cross talk. 1.2 They argued that there could be problems from the power feeding of the antenna and mismatches at the receiver antenna port voltages. Again I doubt it if one uses receivers in the same voltage range 2. I have been experimenting with GPS constellation coverage with different brands of antennas. I have found different gains, different radiation patterns and as a result different satellites in view for identical GPS receivers. 2.1 I have found that using a single antenna and a two port HP splitter I get the same radiation pattern, gain and identical satellites in view for a set of identical receivers. For comparison purposes I feel this is an adequate scenario. It is easier for me to take care of the transmission line length and errors caused by phase differences, attenuation and delays. 2.2 Two identical GPS receivers each one with it's own antenna, could eventually cause spatial diversity reception for a system of two receivers conceptually set as one for comparison purposes. Different satellites being tracked on each receiver if not connected to a common antenna. Even if antennas and transmission lines are identical. Question is: Am I wrong doing the above mentioned assumptions while considering the use of GPS transmission line splitters? I which scenarios are the splitters recommended? In which cases they are rather not to be considered? Better ask as to start buying more antennas or feel comfortable with my original RF distribution design using splitters. Your kind comments and expert advise is always welcome. Thank you! Best regards, Edgardo Molina Dirección IPTEL www.iptel.net.mx T : 55 55 55202444 M : 04455 20501854 Piensa en Bits SA de CV Información anexa: CONFIDENCIALIDAD DE INFORMACION Este mensaje tiene carácter confidencial. Si usted no es el destinarario de este mensaje, le suplicamos se lo notifique al remitente mediante un correo electrónico y que borre el presente mensaje y sus anexos de su computadora sin retener una copia de los mismos. Queda estrictamente prohibido copiar este mensaje o hacer usode el para cualquier propósito o divulgar su en forma parcial o total su contenido. Gracias. NON-DISCLOSURE OF INFORMATION This email is strictly confidential and may also be privileged. If you are not the intended recipient please immediately advise the sender by replying to this e-mail and then deleting the message and its attachments from your computer without keeping a copy. It is strictly forbidden to copy it or use it for any purpose or disclose its contents to any third party. 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