Your intention is to use WiFi or WLAN (just to doublecheck since the
question was posted on a GSM forum)?

I'd suggest to use as short antenna cables as possible, and good stuff. I
understood you already had the AP in the tower instead of having it on the
ground and feeding the antenna through a long cable. Equally the other end
should preferably have short cable. If you accept Ethernet cables, could use
a separate access point and Ethernet from there. Another option would be to
use a WLAN repeater (see DLINK or similar).

Directional high gain antenna are one option to improve the coverage. The
base should probably have an omnidirectional antenna but could still have
better gain than a standard access point antenna (I wonder if you have
sufficient lightning protection at the tower?) or are the receivers at a
limited angle from the tower, then you could use a directional antenna there
too. Unfortunately good antenna area easily more costly than WLAN access
points. You could build your own antenna but would need some experience to
do that.

How far are actually the network clients?

I assume ''only a few bars'' on the Sierra 860? If so, I'd try using a
small yaggi, and most of them are dual band (850/1900). Do you
know which band you're trying to get cellular internet over?

A sixty foot tower oughta be sufficient for your wifi links, but if
possible you'll want to put the AP on the tower, not just an antenna
for one. That much cable loss would be ridiculous. You might also
check ebay for some panels, they can do 90 degrees if you sort of
defocus them right. Or do you need omnidirectional up there?

Just FYI using a Sierra 775 a couple months back I was able to get
sustained downloading from my news server of 245kbs, a bit more than I
thought Edge was capable of, but I did have max bars in the Sierra
watcher. Using T-Mobile along I-25 in New Mexico. If that matters.

Bill