Is possible to use wave guide to decrease beam width?
Rule of thumb: physically larger antennas have narrower beam width for the same frequency.  

If you want to make a 30 MHz monopole more directional, you can't just cover it with a piece of metal.  One solution would be to get a bunch of antennas and build a linear array, where the array factor would give you the gain.  This array will need to be at least several meters long.  Adding a reflector behind the antenna would increase the gain a little bit, about a factor of two.  Bottom line, if you want a high gain at 30 MHz, you're going to need a REALLY big antenna.