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

[Terminator3]

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?

 

[biff44]

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

 

[vfone]

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