Analysis of tower with Guyed ropes

I am trying to design a 40m tall tower with tubes, triangular in plan, stabilised with guy ropes. I am providing an initial tension of 5Kn. How can I check the deflections in the cable under the provided tension. I would prefer the cable not to sag. Staad section displacement is not providing any result.

Also, I am providing a member spec "PRIS YD 20" (that is 20mm). Can I provide specification for actual wire ropes provided by Say Usha Martin etc?

Thanks.