GPS chip antenna

Wireless & IoT
1

I want to use the GPS chip antenna with the MN5010HS GPS receiver. I have never used a GPS where I had to add an antenna, I only used the GS405 and the EM-406 so I am a bit lost on where to start.

Here is a section from the design guide for the GPS

MN5010HS Design Guidelines:
A passive antenna can be directly connected to the ANT input of the MN5010HS. Assuming the

antenna is well tuned for the ground plane it would sit on and any plastics that would enclose it,

matching of the antenna to the MN5010HS would not be required. However, good practice suggests

that footprints for a PI matching network would be prudent if space permits.

The ANT input to the MN5010HS directly feeds an LNA. If high levels of RF interference are

expected in the end product, the addition of a pre-select SAW or ceramic filter may be required to

provide good performance.

The rejection characteristics of the pre-select filter can be determined by measuring the out of band

signal levels and using the curved from the jamming curve of the MN5010HS as shown in the data

sheet.

Please help… I’m kinda lost. Here is what I think I know.

The antenna can be directly connected to the GPS, but some circuit can make it more effective.

The ground plane of the PCB effects the antenna and it needs to be matched.

The ground plane should not go under or around the chip antenna.

What I need to know…

What circuit should I add to the antenna?

How do I match the ground plane and the antenna?

How this work in someones pocket?

Thanks a lot,

Justin

2

They are usually used with an LNA, Atmel makes a suitable device.

Don’t bother with this sort of thing unless you have a lot of RF design experience.

Leon

3

The GPS has an internal LNA.

That did not really answer my question. Do I need to add the PI circuit or does that not really effect it? What about the ground plane what should I do with it? Looking at Sparkfun’s PCB with chip antennas they just seem to stop the ground plane near the antenna.

Thanks

4

The ublox site has some fantastic application notes for RF design for GPS, including exactly how to lay out your board, calculate your 50 ohm microstrip length/width, etc. They require you to register for free and that only takes a few minutes. Also, they have a good overview and history of GPS technology.

Will.

5

Thanks for the tip, I’ll check it out.

6

I checked out the site http://www.u-blox.com but can’t find what you are talking about. I did find a 132 page PDF that is useful but nothing about the layout. Please post a link.

Thanks a lot,

Justin

7

For general GPS information, read the GPS Compendium at

http://www.u-blox.com/customersupport/tutorial.html

I highly recommend it - it gives a good detailed overview of the technology behind GPS. As for layout, read the Antaris 4 System Integration Manual at

http://www.u-blox.com/customersupport/antaris4_doc.html

Although they are specific to the Antaris 4 line, they are quite similar to most other GPS units out there.

Will.

8

Thanks, I think I’ll be able to come up with something.

You have been a great help!

9

One more question. Since the chip antenna needs 50ohm impedance could I simply use a 50ohm resistor or does it have to be a microstrip?

10

It has to be microstrip. A 0.1" wide strip on FR4, with a ground plane on the other side, is roughly 50R.

Leon

11

OK, a 0.1" wide strip with a ground plane on the back of the PCB and the same side or just the back? How long does it need to be? I am going to order the PCB from BatchPCB so it will have those specs.

Again thanks

12

The strip is on one side and the ground plane on the other. You could have copper pour around the microstrip, but it might affect the value. I think the formula only applies to a strip on it’s own over an infinite ground plane.

It’s a transmission line, so the length doesn’t matter.

If the length is short compared to the wavelength of the signal, microstrip won’t make much difference.

Leon

13

Pages 54 to 58 of the second document I linked to provide some very useful details on microstrip size and placement - make sure you read it carefully. I suggest you find a waveguide calculator like the one suggested, or at least follow Leon’s guidelines as well keeping the trace length below 1 inch. Take a close look at Sparkfun’s board too, to see how they position the antenna with reference to the microstrip (the document only covers antenna connectors).

Will.

14

Thanks, I take it that the 50ohms refers to AC resistance not DC. That is what got me, I am not familiar with AC characteristics and a 50ohm trace would be either very long or very thin.

Thanks again,

Justin

15

Impedance can be thought of as AC resistance.

Leon