To accurately compute the resonant frequency of an equilateral triangular patch antenna (ETPA) on composite and suspended substrate simple CAD formulae based on cavity model has been proposed. The present model is very simple, fast, accurate and valid for wide range of permittivity and thickness of composite and suspended substrate as well as single substrate patch antenna. This is for first time effect of composite and suspended substrate on the resonant frequency of an ETPA has been investigated. So, we have employed commercial softwares (HFSS and CFDTD) to validate the present model for triangular patch antenna on composite and suspended substrate. Our computed values of a triangular patch on single substrate are compared with different theoretical and experimental values available in open literature and the present model shows very close agreement with the experimental results.