there is nothing ideal in life, inductors have parasitic capacitances and parasitic resistance . this may be the problem as when frequency change the parasitc caps effect may appear , i am not sure but this may be the case.
regards,
a.safwat
It is because of nonlinearity of inductor core.
This ninlinearity, change inductor value, when it's current or volatge magnitude changed.
I think if you use inductor without core (winding on air) it's inductance will be fixed.
Well inductors are made of conductors that are solated. If you think two wounds like charge and betwen them an isolator you have a capacitor. So in your inductor you have a Inductor, for low frequencies, but if you increases the frequency of operation parasite capacitancies will appear and cancell part of the inductance called DIP. AT some point of frequency capcitances will be great than inductance and your inductor will operate like a capacitor.
If you have access to a Universal Bridge you can comprobe that situation and select an area where your inductor stay almost constant.
For your case you must verify that your inductor won't be at any DIP or nearby a DIP.
In this case when you plot the XL vs frequency
DIP is a depresion in the XL (inductive reactance) when a little parasitic capacitance anules part of the inductance.
every inductor have a self resonance frequency because of the parasetic capacitors with it this self resonance change the value of inductor and after this freq the inductor behave as capacitor if u working bellow this freq u will see the inductance almost constant but near this freq u will find the inductance change greatly