using an Omindirectional antenna(Gt=1dB) as the Tx antenna and using a signal generator and feed the power (Pt) to the Tx antenna. with a distance of L, place the one your made as the Rx antenna and feed the power received to a powermeter.
now rotate the Rx antenna horizontally, and note the powermeter read when you get phi=40degree as (Pr).
now, considering FSL model, and the following formula Pt+Gt-FSL=Pr+Gr (all in dB)
FSL=20log(4piR/λ) dB where R is the distance between Tx and Rx, λ is the wavelength of the signal from signal generator and in meter.
hope this helps..