Trouble using symsum function to solve the Radiation Resistance eqn. for a finite dipole

%Radiation Resistance for finite dipole

C=0.5772;%This is Eulers const.

k=(2*pi)/300;%wave number

l=140e-2;% length of dipole

eta=120*pi;%impedance of free space

Rad_resist=(eta/(2*pi))*(C+log(k*l)-cos_integral(k*l)...

+.5*sin(k*l)*(sine_integral(2*k*l)-2*sine_integral(k*l))...

+.5*cos(k*l)*(C+log((k*l)/2)+cos_integral(2*k*l)-2*cos_integral(k*l)));

disp(Rad_resist)

%%%%%%%%%%%%%%%%%%%%%%%%%%%% Two functions that are called

function [c_i] = cos_integral(x)

%Cosine Integral

%Taken from Antenna Theory, Balanis

%9/5/2019

syms k;

C_euler_const=0.5772;%Eulers Constant

c_i=C_euler_const+log(x)+symsum(((((-1)^k)*((x)^(2*k)))/(2*k*factorial(2*k))),k,[1 100]);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [s_i] = sine_integral(x)

%Sine Integral

%Taken from Antenna Theory, Balanis

%9/5/2019

syms k;

s_i=symsum((((-1)^k)*(x^(2*k+1)))/((2*k+1)*factorial(2*k+1)),k,[1 100]);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%

output doesn't make sense; real summation limits should be 0 to Inf

val =

(189713413132327*pi^2)/337769972052787200000 - (2898399760547415523*pi^3)/12159718993900339200000000 + (129692640763094342591*pi^4)/4559894622712627200000000000 + (142021588266823360627*pi^5)/45598946227126272000000000000000 - (31776518994577836803303*pi^6)/115422332637413376000000000000000000 - (994151117867763524389*pi^7)/51298814505517056000000000000000000000 + (934239861683258850080753*pi^8)/692533..... keeps going...

These are the formulas I used from the book. Just wondering if I should go about a different method to solve this equation or if I'm missing something fundamentally important when using symsum function in MATLAB. Thank you!

Answer 1

[Nishant Gupta]

Hi Brandon,

You can use vpa function to get the result as follows:

vpa(Rad_resist,4);

To see how vpa function works you can refer to the followign document:

https://www.mathworks.com/help/symbolic/vpa.html?searchHighlight=vpa&s_tid=doc_srchtitle