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GPS PCB Module Antenna (MIcrostrip)

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dknguyen

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THere is a PCB-mounted GPS module but it's antenna isn't built in. On the datasheet it says to run the antenna pin to the antenna (I presume some 50ohm SMA jack that an antenna connects to) with a 50 ohm microstrip.

Is there any way to accomplish the same thing without actually needing ot make a special PCB which would cost a ton in itself.

THe module is an LEA-5 from uBlox but I don't think that information is too pertinent.
 

JimB

Super Moderator
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I guess the first question is:
Where do you want to put the antenna in reletion to the GPS module?
ie how far away?

Next question:
What type of antenna were you intending to use?

JimB
 

RadioRon

Well-Known Member
dknguyen said:
THere is a PCB-mounted GPS module but it's antenna isn't built in. On the datasheet it says to run the antenna pin to the antenna (I presume some 50ohm SMA jack that an antenna connects to) with a 50 ohm microstrip.

Is there any way to accomplish the same thing without actually needing ot make a special PCB which would cost a ton in itself.

THe module is an LEA-5 from uBlox but I don't think that information is too pertinent.

That information is indeed pertinent, I think. The reason is that any RF connection at 1.5GHz will be extremely sensitive to the connection geometry, that is, how the transmission line approachs and connects to the antenna and ground pins. Since the connection geometry is entirely dictated by the physical design of the module and the type of pinout used, I can only give an answer for the LEA-5.

Since this module is a surface mount type with perimeter pads along two sides of the bottom of the module, I don't see how you are going to use it without a custom pcb. But if you are going to try by soldering wires to the pads or somethiing like that, then the way to connect an antenna is to use small diameter coaxial cable, diameter of about 1mm would be OK, but smaller is better. Prepare the end of the coax so that a solderable bit of center conductor sticks out of the coax just far enough to touch pad 16 while the bared ground braid neatly sits on pad 15. Then solder the braid to 15 and the center to 16. It is important to keep the center conductor exposed length (any part without braid around it) short, like 2 mm.

This fine coax is quite lossy so you may want to transition to a larger type and will need to use an adapter of some sort to get that done.
 

dknguyen

Well-Known Member
Most Helpful Member
I was going to stick the SMA connector right next to the module (and after that I don't think it matters too much where the antenna is since it has a impedance matched cable).

https://www.u-blox.com/customersupport/gps.g5/LEA-5_HardwareIntegrationManual(GPS.G5-MS5-07005).pdf

I wasn't sure about the antenna, but probably passive.

THere are modules from GLobalsat where the antenna connections (or the antenna itself) is already taken care of. BUt it seems these uBlox ones are better at what they do.

THis may also be a really silly question, but what does it mean when a wire is 50 ohm matched impedance? Because there are 1m long and 5mm short wires like that and they would have different resistances (at least if they are the same width). Or does it simply mean the total resistance of that wire is 50ohms and the 1m wire would be much wider and thicker than the 5mm one? Because a lot of coax cables of different lengths (and what seems to be similar widths) are all 50 ohm matched...

I don't mind a custom PCB (I always use those). What do I mind is a "custom custom" PCB since microstrip costs an arm and a leg doesnt it?
 
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JimB

Super Moderator
Most Helpful Member
I agree with RadioRon on terminating a coax cable right at the module, and keep the connections as short as possible.

dknguyen said:
THis may also be a really silly question, but what does it mean when a wire is 50 ohm matched impedance?

To be more correct, the cable has a characteristic impedance of 50 ohm.
This 50 ohm is not something you are going to measure with a multimeter, and is not a function of the length of the cable.
Have a look here:
https://en.wikipedia.org/wiki/Characteristic_impedance

for descriptions of characteristic impedance.

In an RF circuit, as the frequency increases and the wavelength decreases, the physical size and orientation of circuit components becomes more critical.

JimB
 
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