Simplest DIY GPS patch antenna - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: August 02, 2017, 01:29:49 am
content: This is for people who want an easy to make right hand circular polarized antenna for GPS use. This should work pretty well and its likely to be considerably more broadband (for GLONASS, etc) than the ceramic version of it.https://web.archive.org/web/20120322102621/http://home.iae.nl/users/plundahl/antenne/patchant.htmI would pass on the LNA design and use something a bit more modern if you want to add gain.Note, you can also use 5 mm thick styrofoam board (foam board) (only) for the spacer which makes this antenna *really* easy to make with just a square of copper flashing (or even copper tape). Styrofoam has a dielectric constant of "1" just like air.The ground plane's (back) size is completely irrelevant to the non-critical andtuning once it gets a bit larger than the top part and actually my recommendation is to make it circular - not square and substantially larger. The bigger the better. Ideal would be 200 mm diameter or more, depending on the stiffness of the material, you really don't want it moving in the wind.Also take care that the shield is soldered to it well and that there isnt any play or any sharp bends in the center conductor. It should be as short as possible - 5 mm long and the ground should be connected to the pcb on both sides if bent or all sides of the hole if the coax is going straight back (recommended) For stationary use this antenna can be placed on top of a small tripod or run down a length of PVC pipe with the feedline going straight down.Cover the patch part with an RF transparent material to keep rain and debris out.I used a very thin hard plastic dome taken from a closet light I have as a cover.If everything is smooth most birds won't land or sit there because there is nothing to hold on to..

author: Lord of nothing
author_rank: Super Contributor
timestamp: August 02, 2017, 02:51:06 pm
content: The sold also as: Outernet Antenna very cheap.

author: cdev
author_rank: Super Contributor
timestamp: August 02, 2017, 07:47:58 pm
content: If you use anything other than air (or styrofoam) as the dielectric (even just PCB) the measurements suddenly become ultra-critical.So, I would stick to air unless you have a network analyzer, etc. Also, make sure the gap is uniformly 5 mm. There are different thicknesses of foam core and it can get squashed at the factory so you should check and if your foam core is too thin use additional shims- something of uniform thickness thats nonconductive and nonlossy to move it so its 5 mm exactly.Foam core makes a great substrate for antennas.

author: metrologist
author_rank: Super Contributor
timestamp: August 02, 2017, 08:18:00 pm
content: Thanks for that. I might try it, although all my GPS units that are worth running now require an active antenna. Is there a more modern design plan? I tried a simple MAR3 and MAR6 and it was not as good as no LNA, maybe because I don't know what I'm doing.

author: cdev
author_rank: Super Contributor
timestamp: August 03, 2017, 01:05:03 am
content: They have all in one combination GPS LNA and SAW filter SMD devices that cost around $2. Look around.

author: ConKbot
author_rank: Super Contributor
timestamp: August 04, 2017, 12:38:31 am
content: Quote from: metrologist on August 02, 2017, 08:18:00 pmAll GPS receivers require an active antenna or an LNA so close its basically the same thing. The signal is well below the thermal noise floor (~-130 dBm GPS signal at the earths surface, -113.9 dBm noise floor for 1 MHz bandwidth) a bit of antenna gain on a patch (3dB or so) and a whole lot of CDMA processing gain can make it barely usable.Where was the MAR3/MAR6 LNA? Because of the way noise figures work (https://www.microwaves101.com/encyclopedias/cascade-analysis) your first amplifier and any passive losses beforehand plays a huge factor in the final noise floor and SNR, everything else, not so much. As long as your cable loss doesn't eat up most the LNA gain, you wont significantly degrade the signal. Hence why little magnetic patch antennas have leads made from RG-174, despite its loss being horrific (almost 15 db in 10 meters) without a problem.TLDR: get that antenna LNA as close to the feed point as you can.