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How the voltage ratio varies with frequency for an air core transformer?

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usernam

usernam

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Hi,
I need to know how the voltage ratio between primary and secondary varies with frequency for an air core transformer. What I have noticed with a transformer I'm using is that at 1 Mhz the voltage ratio is 2:1 but at 2 MHz the ratio comes to around 3.3:1.
Can anybody explain this phenomenon please.
 

coupling factor air core transformer

if your examination wherenot in no load conditionincreasing frequency add impedance of transformer and decraeses voltage drop in leakage impedance of both side and resistor impedance but i am not sure
 

air core transformer design

What is happening is you are approaching the natural resonant frequency of the transformer. All coils have some small amount of parasitic capacitance to ground, which forms a resonant tank. If you continue to increase the input frequency, you'll find that the output peaks at some point and then rolls off above that frequency. The peak point is the resonant frequency of the system.
If your function generator only goes up to 2MHz, you can verify this by simply connecting more capacitance across the secondary of the transformer, say 500 or 1000pF, then run another frequency sweep. You'll find the resonant freq has decreased proportional to the increase in capacitance.
 

    usernam

    usernam

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air core toroid transformer

Hi,
Thanks for the reply. I believe thats what is happening. I tested it as you said and it resonated at a lower frequency.
BTW, what is approximately the best coupling factor I can achieve with a solenoid air core & with a toroid air core?
And is there a book or something I can refer to for air core transformer design basics?

Thnx
 

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