Dear Friends, Thank you all for all your replies and points of view regarding this thread. I was unable to reply to you individually as I got too many answers in a very short time. StilI had the chance during the day to summarize all your information and presented what is more than representative of the fine work of this enthusiastic and knowledgeable discussion group. Time Nuts is now well known at CENAM (National Metrology Center). All of today's round table participants were pleasantly surprised by all the replies to this topic. In the end the round table discussion was way better than yesterday and for your information it was set democratically as follows:
a. The use of GPS splitters along the transmission lines is a practice avoided in high end metrology labs. They rather keep things simple and predictive. Including elements in the GPS signal chain is undesirable as they represent complex variables and as the traditional definition of it, they are outside the control of the experimenter. b. If the characteristics of the splitters are known in deep, there is no reason why they should be avoided in general. The use of splitters is generally a matter of convenience, and as such they should be taken in account for their impact when doing precision work. c. The decision of using splitters should be considered in a case by case basis. It is as important as deciding the quality and type of transmission line and the associated RF connectors. d. As a side benefit, it gives food for thought for the amateur time and frequency community and their experimentation (this part was included by me as a side comment). I toured the time and frequency labs at CENAM today. If it is appropriate, I could upload the pictures I took to an image hosting server, showing a little bit of Mexico's time scale UTC (CNM) and the current work with Cesium fountain and frequency comb clocks. My apologies beforehand if my offer could be a little of topic and probably way off the general interest of the group. Please kindly advise. I wish you all a nice evening and a pleasant Wednesday. Thank you! 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. Thank you. On Oct 9, 2012, at 2:39 PM, Magnus Danielson <[email protected]> wrote: > Dear Edgardo, > > On 10/09/2012 04:31 AM, Edgardo Molina wrote: > >> 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. > > Cross-talk can occur, indeed. The main issue should be the > connected/unconnected reflections, only very secondary would be leakage of > signals from one GPS receiver to another. Both is being handled by measuring > and require sufficient port isolation. > >> 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 > > There can be. Therefore you should select to use antenna splitters that can > 1) Select which port to be feeded from. > 2) Back-signal the loss of antenna to the GPS receiver. > > There are double frequency GPS splitters able to do that. I will soon get > some for my private lab. I will measure one I borrowed tomorrow. > > A third issue would be that passive splitters will give you loss of signal > level. Therefore is active splitters recommended. > >> 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? > > Just using a transmission line style Wilkinson power splitter alone is not > recommended. The isolation effect between ports is terrible. Using three-port > couplers allows for the ports reflection to be loaded into a separate > resistor. I think I recall seeing that on the splitters that I have openend > up. Thus, it is fairly trivial to isolate the splitter. > > The Agilent L1 splitters uses resistive power-dividers, which has better > isolation at the cost of higher loss, but then they include an amplifier to > overcome that, but the amplification is at the input side. > > If you require isolation between ports, it can be designed in and if you get > the devices I have seen, it should be fair isolation. > > Similarly, the power require similar isolational effects, and again > commercial devices addresses these issues. > > When in doubt, measure the effect. Network analyzers for 1.2-1.7 GHz isn't > hard to come by these days. I'll toy around a little with the one at work. > >> Better ask as to start buying more antennas or feel comfortable with >> my original RF distribution design using splitters. > > The joy of using multiple antennas is one thing, but if you have a good > antenna up, then good cabling in combination with good splitters should work > well. > >> Your kind comments and expert advise is always welcome. Thank you! > > My 2 öre contribution to the discussion. > > Cheers, > Magnus > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
