Calculating the distance from the localiser centreline

Plymouth (EGHD) in the UK has a runway length of 3809 ft (1161 m) with ILS. There may be shorter ones in other parts of the world.Reverting to earlier points in this thread, ILS localisers don't have narrow beams. The ICAO standards requires them to produce a useable signal between 0 and 10 deg from the centre-line to a range of 25 nm, and between 10 and 35 deg to a rangle of 17nm. The localiser has two overlapping beam of the same frequency. One is modulated at 90 Hz and the other at 150 Hz. The aircraft receiver measures the difference in depth of modulation (DDM) between the two beams to get the deviation from the centre-line. The DDM is zero on the centre-line. The actual output of the receiver is a current in microamps which varies according to DDM.The Course Sector is defined as being bounded by values of 0.155 DDM (150 microamps), although the values can be greater than this outside thr sector. By convention/specification, 0.155 DDM is taken as the full scale deflection of the localiser instrument needle. The localiser is required to have a sensitivity of 0.00044 DDM/ft. The figure of 700 ft is twice the half-width given by 0.155/0.00044, suitably rounded. The maximum Course Sector angle mustn't exceed 6 deg.At Plymouth, FSX gives a beam angle of 10.7 deg which is outside the 6 degree limit. According to the UK AIP the distance from localiser aerial to threshold is 3889 ft which gives a Course Sector angle of 10.3 deg. Moving the 700 ft point by 3500 ft (1050 m) would reduce this angle to 5.4 deg and bring it within the allowable limit.