Two GPDSOs 1 Antenna - Page 1
Forum section: electronics / rf-microwave

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 29, 2022, 09:23:59 pm
content: I have a GPSDO that I have hooked up to a outdoor powered antenna and this is working well. My Leo GPSDO has stopped locking with the antenna indoors. I would like to hook the Leo to the outside antenna as well as the Samsung GPSDO. I need the Samsung hooked up for the 1pps output in to the rubidium. But I like the adjust ability of the Leo and could use it's functionality in the lab. Both GPSDO's are powered so I need a least a DC block on one leg of the system. Seeing if this is common and if anyone has any ideas.Thanks.

author: TheSteve
author_rank: Unknown
timestamp: March 30, 2022, 02:04:57 am
content: What you want is a GPS splitter such as a Symmetricom 58535A.

author: cdev
author_rank: Super Contributor
timestamp: March 31, 2022, 02:25:16 pm
content: You can use a generic LNA and a multiport resistive splitter which will attenuate the signal by 3-6 db but allow you to use one good GPS antenna for multiple GPS receivers without deterioration of signal. You also might be able to use a splitter without an LNA.The Samsung is much more sensitive with its Ublox receiver than, say a Furuno TRAIM unit.Not long ago I bought a fairly flexiblle logarithmic spiral antenna on tindie which has coverage from 300 MHz up to above 2 GHz and can be used for both LHCP and RHCP. (use RHCP for GPS) Its a better GPS antenna than the usual patch antennas. Its fairly large, around the size of an LP recor made of vinyl and comes with two suction cups so it can be mounted on a window. The feedline exits from one side and its clearly marked LHCP and RHCP, so it can be flipped over to use the reverse polarization. That might be useful for some experimental applications, as two identical antennas might be useful in determining what polarization a signal was, that would tell you if it was a reflection from a reflective surface like a building or the Earth. This acts like a passive radar. One can use the reflected GPS signals to directly recived GPS signals ratio to determine how wet the Earth is in a given spot and to what depth. This would be very useful info to farmers, enabling their saving water. Lots of water.I also have a PCTEL GPS-TMG (26) family GPS antenna which internally has a quadrifilar helix in it. It is specifically intended for timing, has an upward pattern and good rejection of LHCP so low multipath, and even works (only partially) for Glonass, etc. (The Spiral antenna does receive multi GNSS signals well. I have found the larger tindie spiral antenna makes an excellent and sensitive GPS antenna. As it should if it does its job well) The quality of the antenna signal rather than the gain ofthe LNA is what matters the most. An LNA is unlikely to make the crappy signal from a crappy antenna better as far as the parameters that make it good for GNSS. It might even make it worse..There is another log spiral antenna they sell for less, thats smaller. Its advertised as covering 800 MHz and up. That also is likely to make a decent GPS antenna for multi-GNSS but I have not tried it. I only have one GPS that receives multi GNSS systems, a fake Ubloz M8N (its also likely a fake) (I think) It receives GPS and GloNass.. and does well at that. Despite not having a decent TCXo, The antenna I have is colored white. Its flat. The spiral is fairly incongrouous.. It does not look like a wide broadband space-oriented antenna. Years ago I made my own log spirals, several of them and they all worked as semi decent GPS antennas, compared to other GPS antennas that I tried to make at the time. The spirals worked better than the patch antennas.The cheap patch antennas all had a quite noticable pattern lumpiness issue/problem which was reduced somewhat by using ferrites on their feed line as an RF choke..Buying a GPS antenna thats claimed to work on glonass is a gamble because many dont work well at all. Especially the patch ceramic antennas.. The smaller the patch, the less likely it is to work well on both. I could explain why this is so but as I already have in the past on this forum, I'll spare you the reason unl;ess somebody specifically want it. Suffice it to say the cheap coax cable and LNAs these antenna use is a leaky combination.You can test the passive ones with a SAA-2 / nanovna2, but I dont know how one would test an active one and most of the ones people here use are active.. and contain an LNA, often with an SAW filter built in. This will make it better for GPS but worse for glonass.. which uses a higher frequency.One could also make their own multisystem GNSS antenna and I could offer some input as Ive tried to do that with a fair amount of success.One could do much worse than a log spiral antenna.. One big plus of the log spiral is the clarity as far as the GPS phase center and that is the most important thing if you intend to use it for GPS positioning.A spiral starts out almost ideal for multi GNSS system use.. The one improvement I would suggest is make a spiral with four arms rather than two. To do this one will need a 90 degree hybrid coupler. This is to delay by 90 degrees the phase of two of the arms. (depening on whether you want RHCP or LHCP, you could make that switchable,. which would also be useful.For behind the antenna, RF absorbent material or a ground plane would be helpful.The spiral pulls in more signal (even without any LNA) than any other GPS antenna that I have, especially when its above a conductive ground plane.. Using it I could locate a GPS in the basement of my house without needing any window placement at all. I could see that as being useful. And the GPS signal is low multipath and high quality. despite it being indoors, inside a wooden house containing lots of metal and wires..+ eletronics.. If I placed it outdoors, on the foof, with a clear sky view, it might be an exceptionally good GPS antenna for GNSS use..And I know a lot about the specific subject and have tested a great many GPS antennas for this using RTKlib (using GPS only, not GPS+Glonass) But I have no reason to suspect that the GLONASS performance of the tindie spiral antennas is inconsistant with its GPS performance which is good. Log spirals are a unique form of antenna thats broadband, maintains its polarization selectivity (rejection of LHCP signals which are usually undesired reflections from buildings and the ground) and has a well defined phase center, so is accurate as far as the vertical axis, more so than patch antennas which only achieve circular polarization by means of a sort of hack.There are other antennas you can build which are better GPS antennas than the ones you usually see.. many of which are not very good..This will make a difference for timing too. Maybe a lot.Quote from: Zenwizard on March 29, 2022, 09:23:59 pm

author: cdev
author_rank: Super Contributor
timestamp: March 31, 2022, 06:02:24 pm
content: Any similar CATV-ready splitter that works okay at 1575 MHz will likely work. hats a pretty high frequency so much of the typical Radio Shack level CATV stuff is not up to the task. The better product might be however. Not too hard to find. if you restrict yourself to the better brands.The older CATV cable often is RG-59 and not so good for UHF. RG-6 is double shielded and much better. Its actually better usually than RG-8U and much cheaper. Just 75 ohms. Not 52. That said, many GPS companies swear by it for use with their GPS gear because its better shielded.It will work much better at 1575 MHz.GPS antennas often need a fairly low positive voltage supplied via the coax center conductor to function. They contain a low noise amplifier which is situated near the antenna input so it lowers the front end noise figure significantly more than would otherwise be possible. This is standard with receivers intended to receive satellite communications from space vehicles. It wouldnt be possible to receive those weak signals without an LNA right near the antenna. By being the first thing the signal path encounters, the signal doesn't get eaten up by the significant loss in the coaxial cable which can be substantial at high frequencies.Making the use of a good LNA essential when trying to receive signals from space of all kinds.. The web site UHF-satcom is about receivng signals from space and goes into detail on how normal people can capture them.. (its elusive).People have received signals like lunar rovers and satellites of all kinds, using what my wife would call "junk" bought on auction sites.. One person's junk is another person's gold..Precise timing is a huge help to space monitoring.. So is low noise figure front end hardware.. Soetimes cooling the front end devices to low temperatures is desirable.. The Earth itself is warm and it emits radio noise.. Cooling your front end improves the signal to noise ratio..The best space reception is often on cold nights.. Some people cool their equipment with thermoelectric coolers.. (peltier devices)Astrophotography is similar.. cool CMOS sensors are less noisy.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: March 31, 2022, 08:02:41 pm
content: Splitter/combiners that work at 1.575 GHz are cheap on eBay. I use a Mini Circuits ZA3PD-2 3-way splitter. If you get a DC-pass splitter like mine, as you said, you'll need to put an inline DC block on the receiver that isn't responsible for antenna power. But if you get a DC-blocking splitter like the Mini Circuits ZAPD-2DC series, you'll need to put a bias-t inline with the antenna cable to inject power -- usually 3.3V but sometimes up to 5V depending on the antenna.

author: cdev
author_rank: Super Contributor
timestamp: March 31, 2022, 09:05:01 pm
content: A DC block is implemented by inserting a decent microwave capacitor of usually under 1 nf to block DC but allow RF to pass through.A bias tee will pass DC up to the antenna LNA or down converter but block the DC out of your receiver letting it use the power but blocking the DC out.

author: cdev
author_rank: Super Contributor
timestamp: March 31, 2022, 09:15:13 pm
content: I use MiniCiruits components such as their SMT bias tee.. which is a decent deal and specs out pretty well.But their premanufactured bias tee and bespoke LNA is way overpriced.. (>$45) Good for corporate customers who have money to burn. But not decently priced for me.I buy the parts and make them using the LNA MMICs.. Easy and fun to make LNAs optimized for a specific task.. That's MiniCircuits core competency. Making those components and mass producing them in quantity.Not making $45.00 LNAs (for me) although they do that very well when they do. And the price isnt that bad because you know youre getting a decent product. But you could buy 20 MMIcs for that and make a bunch of them and learn a lot more.And your customers would be getting areallygood low noise amplifier, world class, quite possibly. By picking the optimal components. AND building it. Its up to you.Thats what they would rather do.. If you talk with them you'll realize that. They are makers like us.

author: cdev
author_rank: Super Contributor
timestamp: March 31, 2022, 09:36:03 pm
content: Too many fakes unfortunately.. But there are lots of good deals there to be had.. But be careful!Quote from: Melt-O-Tronic on March 31, 2022, 08:02:41 pmI've often upgraded cheap CATV hardware with better components. If you have the hardware to make it easy to participate in the upgrading of splitters. Its a 5 minute job.

author: cdev
author_rank: Super Contributor
timestamp: April 02, 2022, 03:23:54 pm
content: You should try using a splitter. If the antenna works okay without one, chances are good that both GPS's will do fine with it.Better than not that they both will work