Antenna selection on generic GNSS modules (Ublox etc.) - Page 1
Forum section: electronics / rf-microwave

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 13, 2021, 04:33:53 pm
content: I have a number of GPS etc. modules which I'm using for experimental purposes, with (genuine) Ublox neo-xx cans onboard. In general I select these to have provision for an external antenna, very often they support both an external and an onboard antenna.I've seen various mutterings that a capacitor should be removed so that there is only a single RF source. Can anybody provide definitive or at least informed information on this?In more detail, the attached photo shows part of a GPS module with SMA and IPEX connectors paralleled at the bottom and left, and the centre connection of the onboard (ceramic patch) antenna towards the top right.The external antenna is connected to C8 and L1 in parallel, with C8 connected to U3 which is an unmarked six-pin device.The internal antenna is connected to C4 and L2 in series, hence to U3.The fragmentary instructions I've seen suggest that C8 should be removed if the external antenna is being used... which is of course rubbish since that's the in-path device. However the seller who advised that must have got the notion from somewhere.If U3 is a passive combiner, e.g. from Mini-Circuits, then is is reasonable that an unused RF source should be isolated since an out-of-phase signal would mess things up.If U3 is an active switch, e.g. from Peregrine as mentioned inhttps://www.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_%28UBX-15030289%29.pdf, then it's not reasonable to expect to have to remove components.I lean towards U3 being active, since the component layout on the module is approximately as described in the Ublox appnote- even if the open-circuit detection has been omitted or simplified. However I'd be interested in the community's opinion or experience.MarkMLl

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 13, 2021, 05:06:23 pm
content: The five-pin U4 is a voltage regulator, marked 4A2D.I suppose that one thing I could usefully try is looking at the six-pin chip (U3) and finding whether it has CMOS-style power on it, rather than being a passive splitter/combiner.MarkMLl

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 13, 2021, 05:56:30 pm
content: U3 is marked "LY". I noticed this when it started catching oblique sunlight, but now find that there's one very specific direction that LED illumination has to be applied which also shows it clearly... same applies to three modules branded differently so this looks like somebody's reference design.That means it might be a MAX2659ELT+ with gnd on 1 and 2, RF in on 3, Vcc on 4, shutdown/ on 5 and out on 6. Input is shown via series L and C.If that's correct it suggests that it's only in the "ceramic patch" antenna circuit, that the SMA/IPEX antenna is in parallel with its pin 6,In which case if the antenna on the SMA connector is being used the internal antenna should be disabled, and that if this isn't done by the Ublox module telling the amp to shutdown removing C8 is the obvious way.HOWEVER: component numbering isn't consistent, and on other modules that cap is identified as C2 etc... the important thing is to identify the SMA -> can line and look for a C and L in parallel.MarkMLl

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 13, 2021, 07:49:21 pm
content: Quote from: evb149 on June 13, 2021, 06:33:14 pmI suspect that by now "conventional wisdom" is going to be this thread since the situation has not been adequately explained elsewhere :-)I'd add that my workshop is thoroughly screened and even without sorting out that cap, I've been getting fairly good results with a cheap external antenna sold ashttps://www.banggood.com/GPS-Active-Marine-Navigation-Antenna-10-Meters-With-BNC-Male-Plug-Connector-New-p-1019248.htmlHowever the thing I want to "get into the record" is that the thread described there as "G3/4" actually matches standard conduit fittings... at least in the UK.MarkMLl

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 13, 2021, 08:46:47 pm
content: Quote from: evb149 on June 13, 2021, 08:14:58 pmI suspect that its quoted bandwidth- 1575MHz+/-5 is fairly accurate: it's good for GPS but I can't see any of the other constellations at all... and would prefer to.https://gssc.esa.int/navipedia/index.php/File:GNSS_navigational_frequency_bands.pngI'd normally rip it apart to see how well it's made, but my mast is in an extremely exposed position and right now I need it to stay waterproof. It might possibly resemble the one discussed athttps://www.reddit.com/r/RTLSDR/comments/fvycl5/how_can_i_modify_this_type_of_gps_antenna_for/I'm losing quite a bit since I'm putting it through one of these splittershttps://www.aliexpress.com/item/1005001614629367.html... they appear to "do what it says on the can" quite nicely.MarkMLl

author: Miti
author_rank: Super Contributor
timestamp: June 14, 2021, 01:06:11 am
content: I have two of those modules, U3 is LNA for the passive ceramic patch antenna installed on the other side. Remove C8 to use it with external antenna.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 14, 2021, 06:48:09 am
content: Quote from: Miti on June 14, 2021, 01:06:11 amExcept where it's marked U2 (as on two out of three of the modules I've just done), and we've thrashed out why it's only that cap that's significant.MarkMLl