[time-nuts] To use or not to use transmission line splitters for GPS receivers

Edgardo Molina xe1xus at amsat.org
Mon Oct 8 22:31:24 EDT 2012

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


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