Multi-GNSS system antennas? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: January 30, 2019, 02:40:16 am
content: What antennas are people using for current and future multiple GNSS-capable GPSs?Because they need different kinds of antennas than a single band GPS does.There is a little problem with most of the designs I've seen to date."How can you two (or more) antennas occupy the same exact place at once".

author: hagster
author_rank: Frequent Contributor
timestamp: January 30, 2019, 06:28:44 am
content: Im not currently aware of any low grade multi-frequency GNSS receivers on the market yet. Most multi constellation rx's just use L1. This will probably change as there are now open signals on different bands. E.g for Galileo.

author: cdev
author_rank: Super Contributor
timestamp: January 30, 2019, 02:03:08 pm
content: It depends what one considers to be low price.Antennas should keep the phase center variation to a minimum.Otherwise different GNSS systems may resolve the position differently!

author: awallin
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:18:48 pm
content: latest one at work is this one, IIRChttps://www.novatel.com/assets/Documents/Papers/GPS-703-GGG.pdfdon't recall the price, maybe 1keur-ish?

author: hagster
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:24:04 pm
content: By low grade I mean less than 1-2K.

author: cdev
author_rank: Super Contributor
timestamp: January 30, 2019, 04:35:12 pm
content: Yes, thats the pinwheel design which is basically a inherently broadband, four armed spiral design, (likely using a 90 degree hybrid to deliver a 90 degree phase shift to smooth out the pattern slightly.)PlanarArchimedianandlogarithmic spiraldesigns are naturals for multi-system GNSS, as they have all of the desirable characteristics.They aren't new, they were first explored in depth by Dyson in the 1950s. I've built a number of different variations on them. The planar designs in particular are well suited to GNSS because thephase center variationis virtually nil because the center remains the same despite the active region of the arms moving, because they are balanced. Going to a 4 armed (rather than 2 armed) design is particularly useful in the GNSS context because it smooths the pattern out still more.Actually, in this post I kind of wanted to get a discussion started on this very useful and interesting family of spiral antennas and speculate as to how it might be possible to lower the cost of the (broadband) phase shifting network needed to make 4 or more arms practical.These spirals can be printed on single sided PCB and they work best if combined with an RF absorbent cavity or similar to prevent reflected signals from below from causing multipath. (Soil, i.e. earth might work just as well, especially if the antenna was placed right on the - right kind of ground. Thats something that would be interesting to determine experimentally using a program like RTKlib.)One particularly useful aspect of the spiral designs is their really excellent rejection of the non-desired opposite polarization. Much better than any patch antenna design, including the annular patch designs (>90% of current GNSS antennas are patch antennas)The move to multi-system GNSS is a shift which I think will force the GNSS community to re-evaluate these very interesting broadband antennas and hopefully we'll see them become more available, especially ones that can be used for two way communications on other frequencies.They have the potential to be UWB antennas, I've built spiral designs that seem to work over very wide (i the literature, as much as a 10:1 ratio) ranges of frequencies.Thats really a phenomenal accomplishment.

author: hagster
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:48:47 pm
content: Im not aware of any easy ways to create very broadband 90deg phase shifters. My guess would be that all GNSS frequencies are close enough together for it not to be a big issue.I do make some log spiral antennas that work well. I am trying to design an UWB EBG reflector for this, but its proving difficult with the simulation tools availiable to me.I havent chatacterised them for the phase centre though.https://www.tindie.com/products/15452/

author: cdev
author_rank: Super Contributor
timestamp: January 30, 2019, 05:02:33 pm
content: Very nice!Although going to four arms likely would improve theuniformityof the pattern substantially, I wouldn't worry about the phase center as its planar - so phase center should remain the same.It should be a good GNSS antenna, when its arms are oriented to make it RHCP. I've tried a number of different approaches to placement, tripods, ground placement, half-assed 'RF absorbent materials'.I think simply placing it on the ground outdoors works the best.What do you mean about GNSS bands? Using coax? They definitely aren't close enough to use coax. You'd need a really broadband "90 degree hybrid" that may not even exist yet in practical terms if you want both 4 arms and to be able to take full advantage of the huge frequency range.Simply for GNSS, they are out there but they will add to your cost. Not a super lot, but enough to make it significantly more expensive.