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Does watch size matter for the GPS accuracy?

silentvoyager
over 3 years ago

There is no good information that I could find about the Fenix 6 series GNSS antenna design. According to some indirect information, the metal bezel itself serves as the antenna, although if that was the case I'd expect the bezel to have a gap somewhere, which doesn't seem to the case.

Anyway, thinking about the possible antenna design makes me think that perhaps the watch size matters.

GPS L1 wavelength is 19.05 cm. The best signal amplification would be at antenna sizes that are fractions of the wavelength such as 1/2, 1/4, 1/8, etc. Perhaps the size of bezel of 6X is less ideal than 6 or 6s. I think I've seen way more accuracy complains from 6X owners than from 6s owners, but I don't know for sure.

Adding dual band GPS support in Fenix 7 will make this even more complicated. GPS L5 wavelength is 25.48 cm.

The other indirect indication that the size does matter is that the most accurate Garmin watches seem to be FR 745 and FR 945 LTE - both are smaller watches. Specifically, 945 LTE seems to be more accurate than 945 based on a few examples that I've seen. Anyone else thinks the size might matter for the accuracy?

  • 0 over 3 years ago

    Very possibly. See this thread for some technical discussion including references to patents from a few years back. The designs then were the bezel acting as a continuous annular antenna inductively coupled to an internal antenna  

    My view at the time was that the X was likely to be the best, and there would be some challenges for multiband.  I’m intrigued to see what the F7 performance might be.

    FWIW I reckon my F5 was similar to my F3, and my 6x is better than either. I would speculate that variations in the external-internal antenna coupling might mean there are some units with worse performance than others. 

  • 0 over 3 years ago

    All I know is that despite people's endless complaining about GPS accuracy on this forum over the years, I've always been quite happy with the GPS in my 6X. I haven't tried other models to compare it to though.

  • 0 over 3 years ago in reply to C.sco

    When in the same trail race FR 745 measures 15 miles and 6X measures fewer than 14, there is little reason to be happy with 6X accuracy. The official race distance is 15.6 miles. 

    When running on a road on an open terrain, 6X accuracy is adequate.

  • 0 over 3 years ago

    Yes, the size matters somewhat and the 6X should be slightly better, theoretically. But all is much more complicated then saying that it must relate to some specific fraction of wavelength. There are various designs, shapes, sizes with different radiation patterns, required matching networks and so on. Because antenna is so close to other parts of the watch and to the human body changes the behaviour by a lot. All that makes effect, not only the antenna element. It is difficult to achieve good performance with such highly integrated devices like a smartwatch. Requires some serious EE black magic. Sometimes the engineers manage to get it right, sometimes not really. Multiband antennas are of course even bigger headache. I guess this is not the place to talk if you are highly interested (consider EE forums).

  • 0 over 3 years ago in reply to mcbadger

    This is very interesting. So the way I understood the linked discussion is that the bezel is an integral part of the antenna - so called EXO antenna. It is inductively connected to an inner antenna, which in turn connected to the board via the spring rods. 

  • 0 over 3 years ago in reply to mcbadger
    mcbadger said:
    FWIW I reckon my F5 was similar to my F3, and my 6x is better than either. I would speculate that variations in the external-internal antenna coupling might mean there are some units with worse performance than others. 

    Confirmed with a slight difference, my F5+ was similar to my F3HR (note1: F3HR with earlier firmwares were much worse than F5+, while the latest fw I tried with F3HR made F3HR even slightly better than F5+, note2: I could not see the same evolution regarding F5+ firmwares), and my F6X is much better than those two.

    One of the proofs is that it my first Fenix where I did not play the “GPS vs GPS+Glonass test” game.

  • 0 over 3 years ago in reply to silentvoyager
    silentvoyager said:
    So the way I understood the linked discussion is that the bezel is an integral part of the antenna - so called EXO antenna. It is inductively connected to an inner antenna, which in turn connected to the board via the spring rods. 

    Garmin Fenix 6X Pro Disassembly or Teardown whatever you say…


    http://www.f-blog.info/garmin-fenix-6x-pro-disassembly-or-teardown-whatever-you-say/?fbclid=IwAR3MeFcYqoR_ozgizZcYgjqHj6pD6BNTqhLrg4dEg2f6FRKSE8ioFb9qzl8