How to plot frequency response after down sampling

Hi There,

I have an FIR filter expressed in this form:

Ha = 0 -11*exp(-j*w) + 0*exp(-j*2*w) + 74*exp(-j*3*w)+ 168*exp(-j*4*w)+ 74*exp(-j*5*w)+ 0*exp(-j*6*w)-11*exp(-j*7*w)+ 0*exp(-j*8*w);

And I know how to plot the frequency reponse (x axis is frequency, and y axis is magnitude) of this filter. For example, like this:

w=-2*pi:2*pi/400:0;

amp_max = max(abs(Ha));
amp_normalize = abs(Ha)/amp_max;
dB=mag2db(amp_normalize);%normliazed to zero db

fs=15*2*2; %sampling freq.

%plot magnitude response
plot(fs/2*w/pi,dB);

Now after this filter, one additional step is to down sample by factor of 2. That is, to discard alternate samples.

My question is: how to plot again the frequency reponse after down-sampling? Example code or pointer to example code is highly appreciated.

Thanks.