"Simon" wrote in message
om...
Thanks alot for your quick replies guys,

As far as I know 433.92MHz is license exempt throughout Europe, if you
know otherwise let me know, but I've seen it written on the suppliers
website, so I'm inclined to believe them, plus I'm sure I used
transmitters at that frequency for a university project, but that was
a few years back, so I could be mistaken.

I'll look up the Yagi design, and see what I can build, would placing
a yagi inside a horn shaped tube be worthwhile, to get the best of
both worlds? or would a setup like that not benefit directional
transmission, my thoughts here are if a horn shaped tube almost works
and a Yagi antenna almost work, then together, they might give the
results I'm looking for. In an ideal situation I'd like to get the
transmission beam down to 1 degree, 10 degrees might be acceptable I'm
not sure.

My final point is that I've also seen 868Mhz transmitters, assuming no
license is required for these, could this frequency be used to create
a directional transmission more easily than the 433MHz?


the higher the frequency the smaller the antenna is for the same effect.
yagi designs are nice but can be complex and interact with their
surroundings, so putting a yagi in a can is not a good idea without some
very careful and complex modelling. the simplest way to get a reasonable
beam pattern at uhf and above is probably a horn, though they must be built
reasonably well and fed with a proper feed system. if fed at the wrong
location or with the wrong type of element they can be as bad as just using
a random hunk of wire. another simple one is a corner reflector. the exact
design you need will depend on many factors including:
1. path length, obstructions, frequency(determine losses between tx and rx)
2. transmitter power and receiver sensitivity(how much signal can you get to
rx)
3. required s/n ratio(how much signal do you need to detect the data you
want to get)
4. rejecting interference sources(preventing received interference)
5. allowable sidelobes on transmit (preventing interference to others when
you transmit)

all these factor interact and can offset each other. i.e. if you can double
your transmitter power you can probably go with 3db lower gain in the tx
antenna, but not always, one tree in your path can almost totally block
signals at certain frequencies and be negligable on others, ect... go
through your college library for books on rf communications system design,
but don't expect it to be a simple formula to plug in, there are lots of
variables and choices to make. it may also be that you are overdesigning,
if the path is short with no obstructions and you have reasonable tx power
many of the other factors can be ignored... but if you are pushing for
distance or have a tough path then you may have to include more.