Is this a pukka installation or a DIY job? (Genuine question)
Perfectly pukka. You can buy the same antenna from Garmin, just costs more. Plugs into the antenna socket on the GPS unit. The actual antenna, a matchbox-seized thingy, has suction cups (also comes with a detachable magnetic plate, which you perhaps don't want to use near the compass
).
You stick it low on the windshield, so it looks as much skywards as possible, thereby eliminating the shielding of the airframe. Comes with a 5-foot or so cable. Long enough to position the GPS unit anywhere in the cockpit (mine lives on a yoke mount).
I've had perfect reception with this setup even in parts of the world where only a handful of satellites are visible and the unit could only barely 'lock on' without the antenna.
One caveat though - this antenna is 'active', meaning it draws power. If you run your handheld off batteries, have spares to hand!