Thanks a lot MrAI , your comments very insightful and really summed up my problem , i will work on these lines and get the max range , first i have to build a good 8-order Active filter with a gain of 6 on the receive side that will work on +/-5.0V and i will test it and see i have good results . To make things even harder the Tx coil circuit will be running off 03 D-cells i.e 6.0V , so my concentration would be make receive side circuit filter and pick faintest 25Hz , figers crossed
Hi again,
The current through the coil driven by a voltage source will follow this form:
I(t)=(1/R)*E*(1-e^(-(t*R)/L))
where E is the voltage, R is the resistance of the coil, L is the inductance of the coil, and t is time, and from this we see that the peak current is:
Ipk=E/R
So with 240 ohms for R and 6v for E we get only 25ma, which isnt much.
If we assume we are using a higher voltage and simplify we get this:
di/dt=E/L
which means the change in the current is equal to E/L, and that means the higher E is the faster the current will shoot up with a given fixed value for L (which your coil has).
So you see either way E affects the peak current quite a bit. Resistance also plays a big role when the voltage is low, but to get less resistance you need thicker wire.
An idea to get higher voltage from the batteries is to use a boost circuit. Boosting the voltage two times gives you twice the current.
What else we dont know though is the inductance of the coil. It's got a magnetically active core, and without knowing more details we cant calculate the inductance. This inductance will affect the design a lot if it is high because the higher the inductance the slower the change in current is that can occur. A couple tests would tell us what the inductance is if you have a scope handy.
As a rough guide, the higher the voltage is the more the voltage source starts to look like a current source and that means it pumps current faster into the coil. Of course the current peak and time duration has to be limited in this case, so it would be very similar to driving a stepper motor coil.
Just for reference, the true change in current with time is:
di/dt=(E/L)*e^(-(t*R)/L)
so it also does depend on R when the voltage is relatively low.
The difference between using a resonant circuit and a boost circuit to raise the voltage is the boost circuit would provide better control over the current though the coil.