I'm currently redesigning a board that implements GPS positioning. Until now I've been using a Fastrax IT520 module, which is very nice but has been discontinued. This forces me to rethink my board and I am wondering how would these options compare for a GPS antenna connection:
- An on-board patch GPS antenna like this question. Perhaps a Yageo ANT2525B00FT1575A.
- A similar passive GPS antenna, off-board using SMA connectors and a 1 foot (30 cm.) cable, from a Chinese supplier.
- An amplified (LNA) GPS antenna, off-board using SMA connectors and a 1 foot (30 cm.) cable, from a Chinese supplier.
In my application I need to track moving vehicles and the GPS antenna is never attached in the best position. It's usually half-shadowed by the vehicle itself and I have little control over this.
I was using an amplified GPS antenna connected via SMA connectors and a 30 cm. RG174 coax. However this antenna is drawing ~8 mA from my battery powered device, and that's a lot. I wish I could get rid of this load without giving up too much GPS accuracy.
The antenna LNA gain is about 28 dB, which AFAIK might saturate the GPS RF input with a cable that short, although I don't think it's been a problem with the current boards so far.
Of course, removing the LNA would reduce the antenna power draw to zero, but I fear the cable and connectors could produce some noticeable attenuation, especially for cheap chinese antennas whose cables are manually soldered and crimped.
Placing the antenna on the board itself is not ideal because of the real state its ground plane needs. Also, it would strongly condition my board layout as where to place the antenna in the device (the external antenna gives me some freedom about it).
So, all three options seem to have advantages and disadvantages. My design goals include:
- The cost should not grow too much (a $5~10 increase could be acceptable).
- The dB budget should not be severely affected; a 3~6 dB attenuation at most.
- I'm hoping that an on-board antenna would make my GPS RF input less exposed to ESD, as the only ESD diodes I could find are extremely small.
The devices are low volume (50~100 units).
So, what do you think are the pros and cons of each solution? Thank you!