Why frequency meter gives different readings for different horn angles? +-100MHz

Hello! Here is some question:
Using frequency meter with horn attached, going to measure frequency of radiated LO wave, but then realize that moving board a little near horn gives different readings, more than 100MHz. LO is warmed up and stabilized around 5GHz, so why does this happens?
How to measure true frequency? The only way i see now is to put directional coupler / capacitor and attenuator and connect LO output directly to frequency meter. Is it possible to measure true freq using horn approach?

you do not give sufficient info to give a specific answer. But it sounds like you are perturbing the load impedance the oscillator is seeing, and that pulls the frequency the 100 MHz

If the LO is not changing frequency when you move around the antenna, this can happen only if there is a Doppler effect...but I assume you are not sitting in a F-22 Raptor when measure the frequency of the signal coming from the horn antenna