Okay well the answer is relatively simple.

"Height is might". Antenna system should be as high as possible, clear
of path obstructions and provide gain in the direction of interest. For
equal coverage in all directions this generally means a vertical
collinear design. The idea is to have a very small angle vertical
pattern as close to horizontal as possible. (ie radiates most of its
power to the horizon)

In my experience, end fed (ie at the bottom) collinear systems
(especially home made ones) tend to have current imbalances in the
elements such that the lower ones get slightly more than the upper ones.
the result is that the pattern skews undesirably upwards at a slight
angle. You also get less gain per unit length using systems that feed
from one element to the next rather than (say) a number of dipoles fed
directly in parallel. There is an optimum spacing between elements for
best gain that is difficult to get in element to element systems.

Of course 2.4GHz antennas are harder to size accurately and test
equipment can really drain the hip pocket!

If I was doing it for a community system I wouldnt bother doing antenna
construction as such. I'd probably go and buy maybe 4 of a well known
base design of moderate gain and stack them for gain. These may end up
being in panels rather than simple verticals. Keep in mind though what
you are trying to achieve and best bang for your buck. Doubling your
antenna size gives best case an extra 3dB (often less) of gain. In a
free space model the extra distance covered is double the distance for
every 6 dB. This initially sounds good but once you are no longer line
of sight, that gain is nothing compared to the losses. I dont know the
2.4GHz numbers off hand but the range numbers might be something like
5km, then 5.1km with an extra 6dB. Of course those users that have a
signal already will get a slightly stronger one and thus may get a
higher data rate but there will be a point where more gain and a
different system design is better. Using multiple channels and WAP
repeaters or bridges/links although likely bringing the bandwidth down
will give you greater coverage albeit sometimes at the tradeoff of speed.

I dont have any modeling stuff for 2.4GHz networks as such. You tend to
need a lot of obstructive rather than terrain data information. You can
however take some photos and hand enter some near field terrain info to
look like buildings. Have a look at "Radiomobile" as a prediction tool.
As far as I remember it is GPL software that you will find other users
have used for WiFi networks. You can see the effects of gain and height
very easily and it will give you a lot of go/nogo help for particular
areas you want to supply access from.

Apologies for the length!

Bob

Penang wrote:


It's for a community-type wifi network, and I'm cracking my head
trying to figure out a way to make the basestations' broadcast cover a
wider area.

I am reading all your suggestions right now and am trying as hard as
possible to digest them all.