Can I use conduit to ground an antenna?
If the conduit is metalic, and at least size 3/4", and at least 8 ft. long.  Then you should be able to use it as the grounding electrode for the antenna. 

If the conduit meets all these criteria, then you'll want to bond the antenna mast to it using at least 10 AWG solid copper wire (810.21(H)). You'll have to use a connector designed for the purpose, so you probably can't just tuck the wire into the bracket holding the conduit to the building.


  National Electrical Code 2014
  
  Chapter 2 Wiring and Protection
  
  Article 250 Grounding and Bonding
  
  250.52 Grounding Electrodes.
  
  (A) Electrodes Permitted for Grounding.
  
  (5) Rod and Pipe Electrodes. Rod and pipe electrodes
  shall not be less than 2.44 m (8 ft) in length and shall
  consist of the following materials.
  
  (a) Grounding electrodes of pipe or conduit shall not
  be smaller than metric designator 21 (trade size 3/4) and,
  where of steel, shall have the outer surface galvanized or
  otherwise metal-coated for corrosion protection.
  
  (b) Grounding electrodes of stainless steel and copper
  or zinc coated steel shall be at least 15.87 mm (5/8 in.) in
  diameter, unless listed and not less than 12.70 mm (1/2 in.)
  in diameter.


Since you're using the conduit as a grounding electrode, you'll have to bond the conduit to the main structures electrical system grounding electrode. This must be done using at least a 6 AWG copper wire (810.21(J)). 

You'll also have to ground the coaxial cable, and install an antenna discharge unit at the house.  See this answer for more detail.