Electronic – What are the differences between regular and disc Yagi-Uda antennas

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.

From the datasheet it always transmits over one of the antenna connectors and a software algorithm is used to determine the best one at the time to use for receiving:

BCM4312 has 2 radio chains, one is a dedicated Rx chain, the other is shared by TX and RX chain using an RF switch. For RX, It supports antenna diversity and provides four options. A software diversity algorithm is used to determine which option is used.

It doesn't mention what the four options are but in general the idea of using diversity mode is when you think the reception from one antenna is likely to be better than the other, it won't increase range otherwise because only one is actually being used at a time. For further reading here's an article Multipath and Diversity from Cisco that goes into quite a bit of detail on the use cases.

So it's certainly possible to connect a Yagi to each antenna connector and it shouldn't cause any problems as long as you don't use it to try and expand coverage to a wider area. But if it's in a location where multipathing isn't likely to be issue like say outdoors and line-of-sight it won't gain you anything either. In that case you'd be better to spend the extra money on a better antenna and low-loss coax and connectors.

From a practical point of view determining the best setup is mainly a matter of testing various configurations and doing a site survey to make sure all the areas you're interested in have good coverage. The last time I did something similar it was an embedded system that had logging and GPS anyway, but there are a number of phone apps around that may prove useful if the clients will be moving around. For point-to-point you could just measure throughput and reliability over a period of time to make sure it meets your needs.