Electronic – How to connect GPS module to usb cable

The ATMega has a wide VCC range, some run at 3.3v, some at 5v. It's pins are not tolerant of an input at a higher than VCC voltage.

The Raspberry PI is 3.3v signal only. No 5v Tolerant pins.

The USB to Uart with 3.3v signal but 5v Tolerant pins means you can connect a 5v ATMega to the 3.3v USB-to-Uart, and the Uart will be fine. The opposite is not true. Connecting a 5v Uart to a Raspberry PI or 3.3v ATMega will not end well.

If you need to connect a two way signal between a 5v ATMega and 3.3v Raspberry Pi, you would want a level translator on the ATmega TX -> RPI RX line. The other way round, from RPI TX -> ATMega RX, should be good, as the ATMega has a (VCC * 0.6) Input Voltage High threshold. In this case, 5 * 0.6 = 3v. A 3v or higher signal will be read as a logic high, which works fine.

Update: I take it you mean connect a RPI to an ATMega through a usb-to-serial adaptor connect to the RPI? Then you don't need to worry about level translation. A 5v or 3.3v USB-To-Serial adaptor will work, since the adaptor is a buffer between the two.

A full size patch antenna (normally about 25 × 25 × 4 mm) will typically have a gain of around 5dB and for an SMA connector pair insertion loss for even fairly cheap connectors is likely to be under 0.5dB at 1.5GHz. For coax RG-174 is a pretty popular (and cheap) cable often used on consumer grade GPS antennae. Running the numbers through this Attenuation and power handling calculator over that short distance the loss at GPS frequencies is only 0.3db. So you should still have a net gain of over 4dB.

Most modern receivers will work well with that and for example the Fastrax IT520 documentation recommends a loss of less than 1dB when using a passive antenna which the above combination achieves. In a vehicle application sometimes the effect of too much gain can be mixed, you can for example start picking up weak reflected signals that can cause multipathing effects. Other than extended testing with a particular module there's not really any way to predict that reliably.

For ESD protection I don't really see an issue if you're using an encapsulated antenna and the SMA connectors are normally always mated. But a little extra ESD protection isn't a bad thing and while the low capacitance protection diodes made for that purpose are small they do normally meet all the usual ESD standards.

So in general I'd say give the external passive antenna a shot and compare against an active in some real-world tests. Some of the cheaper antennae are actually quite good, so grab a few of each from a variety of manufacturers and unlike reworking a PCB design they can easily be compared. It sounds like you've talked yourself out of an on-board antenna for other valid reasons, but I don't see signal loss to be significant enough over that short distance to worry about that side of things.