The general rule for any superheterodyne receiver, either HF or radar, is that the local oscillator power must be sufficient to cause the mixer to run in its non-linear region of its transfer characteristics. That way you get the classic products of f0+LO and f0-LO at the I.F. output.
For microwave applications the modern method is to use a balanced mixer (one sideband output) or double-balanced mixer (upper and lower sideband on separate outputs). These designs are based on balanced half-bridge or full bridge arrangements of either diodes or FETs and these need a reasonable amount of LO power to get everthing biased correctly (at radar frequencies this can be anything up to +10dBm).
If you inject too much LO then weak signals get lost in the excess noise produced by the mixer. Conversely, too little signal gives problems with dynamic range and image rejection problems.
As you can probably guess, this means that for most mixers there is an optimum LO power range, so look at the manufacturer's data and choose a mixer design that has the best characteristics for your application.
