5
\$\begingroup\$

How big a ground plane does a gps antenna need?

I am using a GPS puck antenna that has a magnetic base to hold it on to, typically, the roof of a car. I would like to use it from my room at home, on a piece of ferrous metal plate mounted horizontally outside outside a window. So my question really is: how small a ground plane can it reasonably be?

\$\endgroup\$
  • \$\begingroup\$ I highly doubt the puck is relying on the roof of the car to act as a ground plane. \$\endgroup\$ – Matt Young Jan 25 '15 at 16:55
  • 1
    \$\begingroup\$ I've used a GPS puck on the end of a piece of PVC. It worked fine. I'm sure there are better antenna choices, but the puck by itself suffices. \$\endgroup\$ – caveman Jan 25 '15 at 17:00
4
\$\begingroup\$

Ublox have a detailed application note on GPS Antennas that states the following for ceramic patch antennas which is the style of element normally used in those:

Patch antennas are ideal for situations where the antenna is mounted on a flat surface, e.g. the roof or the dashboard of a car. Patch antennas can show a very high gain, especially if they are mounted on top of a large ground plane (70 x 70 mm). Ceramic patch antennas are very popular because of the low costs and the huge variation of available sizes (40 x 40 mm down to 10 x 10 mm; typical 25 x 25 mm).

That seems to line up with the specifications I had on hand for a few similar end-user antennas:

  • Peak gain: 4 dBic ( based on 70 mm ×70 mm ground plane)

Having said that I agree with the comments that they work fine without an additional ground plane at all (there's a small one internally) and keeping them clear of obstructions and sources of multipath is more important in my experience. That especially applies with newer receivers that are much more sensitive than their older counterparts and often more antenna gain doesn't give any real performance benefit.

However if you do find perform isn't optimal without a ground plane the above document contains a graph of ground size versus gain for a few common ceramic patch sizes. It appears you'd get about 6 dB of gain for a 25 x 25 mm patch by using a 70 x 70 mm ground plane over the small internal one:

GPS antenna gain versus ground plane size

\$\endgroup\$
3
\$\begingroup\$

Increasing the ground plane can increase the positional accuracy in static positioned situations according to the "u-blox M8N - ground planes, antennas and positional accuracy" post at DIYdrones measured during 10-15 minutes at a static position.

For improved comparison I looked up all dimensions of the used patch antennes and guessed ground plane sizes based on PCB dimensions, and sorted them

  1. by GPS IC [older to newer]
  2. by ground plane size [smaller to larger]:

Lea-6H 25×25×4mm patch antenna on 38×38mm ground plane positioning
6H 25×25×4mm patch on 38×38mm

25×25×4mm patch antenna on 36×36mm ground plane positioning
M8N 25×25×4mm patch on 36×36mm

25×25×4mm patch antenna on 38×38mm ground plane positioning
M8N 25×25×4mm patch on 38×38mm

35×35×3.5mm patch antenna on 50×50mm ground plane positioning
M8N 35×35×3.5mm patch on 50×50mm

35×35×6.5mm patch antenna on ⌀80mm ground plane positioning
M8N 35×35×6.5mm patch on ⌀80mm

25×25×4mm patch antenna on ⌀90mm ground plane positioning
M8N 25×25×4mm patch on ⌀90mm

Note #1: All compared modules do contain a temperature compensated oscillator (TCXO).

U-blox GPS Antennas - Application Note

That DIYdrones post references the same U-blox antenna application note as in the answer from PeterJ. On the other hand the "Design considerations" (section 5) for "Patch antennas" (5.1) and "Ground plane" (5.1.1) of the U-blox antenna application note GPS-X-08014-A1 is not written very consistent.

  1. measuring 25 x 25 mm2 down to 10 x 10 mm2

Remark: The patch antennas do not measure mm2 but mm1 (or shortened: mm).

  1. Figure 17: Typical Radiation Pattern of a Patch Antenna

Remark: One should read the text above to note that this text should be: "Figure 17: Typical Radiation Pattern of a 16 x 16 mm Patch Antenna"

  1. Figure 18: Typical gain and axial ratio of a patch antenna with respect to ground plane size

Remark #1: "Figure 18: Typical gain and axial ratio of a patch antenna with respect to square ground plane size"

Remark #2: For which patch antenna dimensions?

  1. On page 19 the text becomes really sloppy:

50 mm2 square ground plane

Remark: A 50 mm2 square ground plane size measures sqrt(50) ≅ 7,07 × 7,07 mm1. What is meant here is a 50 x 50 mm1 = 2500 mm2.

  1. However in Figure 19 the horizontal label axis is correct:

Ground plane [mm x mm]

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.