Electronic – How and where to add a dc blocking capacitor for a gps loop antenna

"Biasing" in this context basically means: to provide power to the active antenna

Active GPS antennas have a built-in amplifier (you'll see the term LNA floating around, Low Noise Amplifier), near the actual antenna element. The signals from GPS are so small that losses from a long (~10cm or more) wire are incredibly detrimental to the signal.

For any amplifier to work, it needs power (a positive and a return, aka ground). Most GPS connections use SMA connectors, and they only have 2 pins. One for ground, and one for "data". So how do you "send power" to the amplifier that is built into the active GPS receiver when we don't have an extra power pin? This is where the inductor comes in - it's a trick to share the positive power line with the GPS "data" signal line.

This works because the GPS data is completely AC data. The inductor decouples the output signal from the high impedance positive power rail, allowing the output signal's AC component (the actual GPS RF data) to come back along the power line. Or another way to say it, prevents the power rail from washing out the signal that we care about.

And yes, that extra 22pF capacitor is just to help provide clean (low noise) power to the inductor. If there is extra noise on the power line, that noise can get through the inductor and wash out the GPS signal that you care about.

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.