How Can I Electrically Vary the Inductance in an RF Transformer?

[dnyberg2]

I have an inductor in a design that is one half of an air coupled RF transformer. The operating frequency is 50 MHz. The purpose is to provide power to the load side. It is for all intents and purposes an RF to DC power supply design over some gap. The primary is the fed radiator inductor, a planar spiral PCB coil near 300nH. The secondary is the pickup and has a similar construction. The thing is, I need to be able to vary the inductance of the radiator inductor a little, plus and minus of the ~300nH in order to peak the match of the system. This post has a connection to my last post about detecting mismatch and changing the value of a varicap. Now I think I might need to vary the radiating inductor a bit but can't fathom a method to do so electrically. One theory is to insert ferrite core inductor in series with the radiating inductor and put a second winding on the tuning inductor and vary the u of that inductor by varying DC applied to the control winding like saturable reactor, thus varying the inductance of the tuning inductor that is in series with the radiator. Ideas?? Thanks!

 

[Averagesupernova]

At first glance without crunching the numbers my recommendation would be to incorporate a varactor. A circuit can still be inductive even though it has capacitance.

 

[berkeman]

I have an inductor in a design that is one half of an air coupled RF transformer. The operating frequency is 50 MHz. The purpose is to provide power to the load side. It is for all intents and purposes an RF to DC power supply design over some gap. The primary is the fed radiator inductor, a planar spiral PCB coil near 300nH. The secondary is the pickup and has a similar construction. The thing is, I need to be able to vary the inductance of the radiator inductor a little, plus and minus of the ~300nH in order to peak the match of the system. This post has a connection to my last post about detecting mismatch and changing the value of a varicap. Now I think I might need to vary the radiating inductor a bit but can't fathom a method to do so electrically. One theory is to insert ferrite core inductor in series with the radiating inductor and put a second winding on the tuning inductor and vary the u of that inductor by varying DC applied to the control winding like saturable reactor, thus varying the inductance of the tuning inductor that is in series with the radiator. Ideas?? Thanks!

I haven't read your other posts yet, so you may have addressed this already... How are you shielding this 50MHz power radiating apparatus to be sure not to cause interference with other RF devices?

 

[dnyberg2]

Right, the K or coupling coefficient between the radiator and the pick up is very close, in other words, the emissions is very low and there is a license in effect for the system.

 

[dnyberg2]

At first glance without crunching the numbers my recommendation would be to incorporate a varactor. A circuit can still be inductive even though it has capacitance.

In other words, you're suggesting to just shift the C around and that might be enough??