Your ‘x’ vector is a single point, ‘xd1’, because the step size is larger than the terminating value. Use the linspace function to create ‘x’. You also have to use the hold function and semilogy to plot and see both vectors:
a=0.0001;
l=0.0003;
eo=8.85e-14;
er=11.9;
es=eo*er;
f1 = 550e3;
f2 = 1650e3;
q=1.6e-19;
c1=1/(4*pi*pi*f1*l);
c2=1/(4*pi*pi*f2*l);
xd1=(a*es)/c1;
xd2=(a*es)/c2;
n1=sqrt(es)/(4*pi*q*sqrt(l)*sqrt(a)*(2.2e5)*xd1^1.5);
n2=sqrt(es)/(4*pi*q*sqrt(l)*sqrt(a)*(2.2e5)*xd2^1.5);
% x =xd1:0.01:xd2; % <— REPLACE WITH ‘linspace’
x = linspace(xd1, xd2, 10); % 10-Element Vector
k = sqrt(es)./(4*pi*q*sqrt(l)*sqrt(a)*(2.2e5)*x.^1.5); % <— DO ELEMENT-WISE DIVISION
figure(1)
semilogy(x,k)
xlabel('xd');
ylabel('nd');
hold on % <— ADD ‘hold’
plot(x,log10(k));
hold off % <— ADD ‘hold off’
