Antenna PCB track

[Scarr]

Hi,

I am designing a new PCB with a LR8905 GPS reciver on it, this GPS engine has a pin for the antenna and it has a note saying "The circuit line must be matched to 50ohm impedance or resistance"

Now I'm not a RF or analog guy, I can program a PIC and throw a digital circuit together, so my questions id how do I make sure my PCB track is OK. is it as simple as making it have 50ohm's resistance?

Thanks

 

[stevez]

The PCB track is essentially a transmission line. To get maximum power transfer and with many circuits, to get it to function properly (or at all) the output must look like 50 ohms resistance. In order to do that the antenna or load must look like 50 ohms resistive. The problem - you need to connect the output to the antenna via some means. That "means" is the transmission line. Typically transmission lines are coaxial cables. They are designed to have a characteristic impedance of 50 ohms - or some other standard value. Tracks on PCBs can be designed to have a characteristic impedance.

 

[Nigel Goodwin]

If it's any help?, the impedance of coaxial cable is governed by the ratio of the inner conductor to the outer screen - if the inner conductor is thicker (for less loss) the outer has to be larger to keep the impedance the same.

How long a PCB track are you planning having from the module to the aerial? (or the socket for the aerial).

 

[Scarr]

I think I understand

OK, from what you both have said what we are taling about is the antenna I connec to it, am I right?

Would I be right in saying keep track short (40mm) and wide and let the off the shelf GPS antenna worry about resitance (as long as I but a 50ohm one that is)


Thx

 

[Nigel Goodwin]

Re: I think I understand

OK, from what you both have said what we are taling about is the antenna I connec to it, am I right?

Would I be right in saying keep track short (40mm) and wide and let the off the shelf GPS antenna worry about resitance (as long as I but a 50ohm one that is)

I don't see any reason to keep the track wide?, I would tend to keep it a few mm wide, with a wide space around it. Or you could use a piece of 50 ohm coax directly from the pin to the aerial?.

 

[Scarr]

Confused again

Thanks for the reply Nigel,

I thought the reason for keeping the track wide was to reduce the resistance so I don't have to worry about this 50ohm's, then just connect a normal 50ohm GPS antenna

So it appears I still fon't understanf this, if I have a track 40mm long to a MMCX connector do I need to worry about it's resistance size / shape clearance or not? from what your saying Nigel, just make a thin track (2mm) with space around it then just again connect a normal 50ohm GPS antenna to it.

Thx

 

[Nigel Goodwin]

Re: Confused again

Thanks for the reply Nigel,

I thought the reason for keeping the track wide was to reduce the resistance so I don't have to worry about this 50ohm's, then just connect a normal 50ohm GPS antenna

So it appears I still fon't understanf this, if I have a track 40mm long to a MMCX connector do I need to worry about it's resistance size / shape clearance or not? from what your saying Nigel, just make a thin track (2mm) with space around it then just again connect a normal 50ohm GPS antenna to it.

It's not the resistance, it's the impedance - as I mentioned before, with coaxial cable it's set by the ratio of the inner and outer conductors - I don't know how you would calculate the impedance of a PCB track?. But, presumably?, it's the relationship between the conducting track and any nearby ones?.

Doesn't the module datasheet give any suggested layouts?.

Bear in mind, it's not as critical for a receiver as a transmitter, mismatch on a receiver will decrease sensitivity (which is best avoided), but mismatch on a transmitter may well damage the transmitter.

 

[stevez]

The track that you describe seems to be analogous to a transmission line. In a receive application the track or transmission line may or may not be critical.

Let's look at another application. If you purchased a transceiver for 146 mHz to install in your auto you would also need several more things. An antenna, to radiate/receive the signal and a means to connect the antenna to the transceiver. The transceiver is likely to have been designed around the 50 ohm standard which means whatever it is that you connect to the coaxial connector has to look like a 50 ohm resistor - ideally no inductive or capacitive reactance. One way of accomplishing that is to install an antenna that can be adjusted to look like 50 ohms resistive at 146 mHz AND to connect the antenna and tranceiver with coaxial cable/connectors that have a characteristic impedance of 50 ohms. If you connect the antenna with something other than a 50 ohm coaxial cable it will most likely not appear, at the transceiver, to look like a 50 ohm resisitive load.

Your PCB track serves as the coaxial cable described above. Ideally it would need to be designed to look like a 50 ohm transmission line - and it also should be low in loss in terms of resistive losses, dielectric losses, etc.
As with many other things we strive for the ideal but it might not be achieveable in precise terms. I can tell you that one of my amateur transceivers is pretty tolerant of what it connected to the output. It will handle a significant departure from 50 ohm resistive. I have another transceiver that is less tolerant - it automatically powers down or shuts off on severe mismatches. I would like to think that in a receive application there would be less harm to be done and possibly less reduction in performance.

I have seen explanations of PCB track designs that would enable you to etch the right tracks but I can't put my finger on the websites. The PCB thickness, copper thickness/width and PCB material all play into the calculation. I don't recall it being all that difficult to calculate. The challenge would be integrating that into the rest of your design. I did a quick google on "PCB transmission line" and some interesting sites popped up.

If I were in your shoes I'd be inclined to do as Nigel suggested and use a short piece of 50 ohm coax - or just etch something that you think will work. Try it an see - keeping this as a future improvement area.

 

[Russlk]

The term for a transmission line on PC board is: microstrip. You can try Googling for that. A two sided board is needed for the "ground plane".

This chart is from the ITT handbook:

 

[Nigel Goodwin]

The term for a transmission line on PC board is: microstrip. You can try Googling for that. A two sided board is needed for the "ground plane".

Cool! - I knew someone would know 8)

 

[Scarr]

Stop, STOP )

OK, you lost me! I think it's best if I point you at the PDF and let you take a look at what I'm talking about.

Here is the GPS engine PDF link

https://www.tdc.co.uk/technical/downloads/leadtek/9805Manual_20040130.zip

See "note 1" on page 11. now please just take it easy on me , I really only want a guru to say "make it short and fat" or "long and thin" or "just use a meter and make sure it measures 50ohm" or even better "it really does not matter for a PCB track"

Please keep in mind it's going to a MMCX socket so I can plug in a off the shelf GPS antenna.

Thx everyone


P.S. There is a photo of a PCB using a track on page 19 if it helps.

 

[Nigel Goodwin]

Re: Stop, STOP )

OK, you lost me! I think it's best if I point you at the PDF and let you take a look at what I'm talking about.

Here is the GPS engine PDF link

https://www.tdc.co.uk/technical/downloads/leadtek/9805Manual_20040130.zip

See "note 1" on page 11. now please just take it easy on me , I really only want a guru to say "make it short and fat" or "long and thin" or "just use a meter and make sure it measures 50ohm" or even better "it really does not matter for a PCB track"

It clearly states, in the datasheet, that "The circuit line must be matched to 50Ω impedance or resistance". The diagrams on pages 12 onwards also show it!.

The graph posted above shows you the doublesided PCB details required, it looks about 3.8mm wide for 50 ohms.

Please keep in mind it's going to a MMCX socket so I can plug in a off the shelf GPS antenna.

Thx everyone


P.S. There is a photo of a PCB using a track on page 19 if it helps.

You could always always try copying that?.