I thought a 15dBi yagi has a much narrower beam. more like 10 degrees. Depending how good your antenna tracker is, you might want to consider a smaller, wider beam antenna. I have been known to basically fix a 6dBi yagi antenna (3 element, ~60 degree beam) aimed at the distant part of my flight plan.
No idea how your 'light woods" will impact range, except to guess they won't make it better :) I suggest you stash one radio somewhere and take your ground station for a walk with a GPS, see how it works in reality through the vegetation you might be flying behind (i.e. test the worst case scenario). I'd start with omni antennas, then experiment with changes until I had a reliable link over the required distance. Over short ranges but difficult terrain, you might get your best results using a tall mast (e.g. collapsible "squid pole" from eBay) rather than pumping up the gain (or transmit power).
Also, remember that you can safely experiment with long range comms without flying out of sight - keep your aircraft close to the safety pilot and set up your ground station remotely. Maybe you could test that as "phase 2", after you've learned what you can about the worse-case scenario from the walkabout testing.
P.S. note it's possible to trick the SiK radio firmware into injecting MavLink "RADIO" packets. The RADIO packet includes local and remote signal strength, local and remote background noise levels, transmit buffer saturation, etc. This is useful to analyse why you are not getting the range you expected, in case of RF noise from within your the plane, RF sources close your your flight path (or near your ground station), etc.
The trick is to configure it into MavLink mode and transmit a MavLink Heartbeat every now and then. Not what you want with paparazzi, unless you want to debug a link rather than send telemetry/telecommand.
I'm trying to build a yagi to mount on an antenna tracker, I think it should be 15dB(i?) to get me a 30degree beamwidth, with 20dBm of imput power. would this be enough to get me a range of 1.5 - 2 miles with light woods? I can't guarantee that I'll be able to maintain LOS...
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