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

[Azelio Boriani]

Crosstalk? With the same signal?

On Tue, Oct 9, 2012 at 4:31 AM, Edgardo Molina <xe1xus at amsat.org> wrote:

> 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
>
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>
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