how to simulate a monopole using CST?

hello,everyone,
I simulated a quarter wavelength monopole vertically placed on a infinit ground plane using CST MWS,opting the discrete port,template was antenna(with ground plane) ,other parameters such as mesh properties as same as the dipole that is given in CST examples,but I were surprised to find the directivity of monopole was 5.3dB on the phi plane,I don't kown where the fault is ,As an CST expert,Can you tell me how to simulate the monopole ?hope your reply!

you can find an example of a monopole antenna on a ground plane in this web site.
http://www.cst-korea.co.kr/