Your best bet is to use phase information. It is very hard to
confine an EM emission to a pure hemisphere (unless you're up in
the UHF or microwave bands). The best you can hope for is a
cardioide pattern.

If you told us more about the application, rather than your preconceived
ideas of what solution to use (no offense), we could probably
make more meaningful suggestions.


Jinx wrote:
>
> Is it possible to have two low power transmitters (different
> freqs) next to each other and have one radiate 180 degrees
> in one direction and the other radiate 180 in the opposite
> direction ? I've been asked to look at making a unit that will
> detect when an object goes past a certain point. The object
> has a device that is off on this side of a line (influenced by freq1)
> and switched on when it crosses a line (influenced by freq2).
> It has to be brought back into the influence of freq1 to change
> state again. If it goes past the range of freq2 it will stay
> switched on. I thought perhaps the transmitters could be
> separated by metal plate or mesh shaped to funnel or block
> the radiation pattern. The power needed is just enough to
> extend perhaps 20ft this side of the line and 10ft the other
> side, frequencies unknown at this stage. Any other type of
> detection system (IR, US, laser etc) is probably not practical
>
> TIA
>
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