If you want to learn more there is a whole thread at DHC on the subject:
Stacking, Ganging, Combining TV Antennas
http://www.digitalhome.ca/forum/showthread.php?t=95148
You have to login and refresh to see all the images in posts on that forum.
Sometimes they use slightly different terms there. One above the other is stacking; one beside the other is ganging. That is the purist definition. I'm a ham. Most hams use the term stacking for either.
DHC has a thread you should read:
OTA FAQ & Knowledge Base
http://www.digitalhome.ca/forum/showthread.php?t=41102
In that thread there is a post (#15) about stacking:
Can I Use 2 or More Antennas For Better Reception?
(Just to make it even more confusing, in that post
intravino uses the term ganging instead of horizontal stacking for two Delhi 10Y13s.)
http://www.digitalhome.ca/forum/show...3&postcount=15
They have two other threads that might interest you:
Antennas Direct 91XG Hardware Hacks
http://www.digitalhome.ca/forum/showthread.php?t=111398
Antennas Direct OTA Antennas & Gear
http://www.digitalhome.ca/forum/showthread.php?t=41076
Final Comments:
I consider stacking to be a last resort. Horizontal stacking gives a narrower beamwidth which makes aim more critical. Before doing it, I try every possible location for just one antenna. Sometimes I measure an improvement of 10 dB or so when I find a "hot spot," and often the increase is more than stacking would give me.
Granted, it's easier for me because I watch the reading on my signal level meter when moving the antenna.
You stated that you couldn't move your antenna in a horizontal direction because of the trees. But, it is also possible to move it in a vertical direction. The general rule is Higher is Better, but the increase in signal strength is not uniform as you increase height because there are hot spots. Sometimes a foot higher or lower for UHF can make a difference.
Stacking requires extra effort and expense, but a dedicated DXer can make it work.
Best regards,
rabbit