Good advice from ADTech. If trees are in the signal path, they will always be a problem. There is no magic antenna that will see through trees as if they weren't there.
If PBS is really important to you, make a temporary test with a UHF antenna set up in the clear to see what might be possible. If it does well, it can be combined with your 7694 for the other channels using a custom ch 36 combiner.
http://www.tinlee.com/PDF/AC7-custom...kup%20Info.pdf
http://www.tinlee.com/MATV_headend.php?active=4
A less expensive version from Jan Jenca who sells on ebay:
http://www.ebay.com/itm/Selective-co...item1c6d4d6faa
http://www.antenne-komponenty.eu/eng...zlucovace.html
Another approach is a selective amplifier for ch 36:
http://www.ebay.com/itm/UHF-selectiv...item1c6483bd0a