"Michael" wrote
"Marc Battyani" wrote
Hello,

I've made a Class-E amplifier which works rather well as I get 25VRMS
into
a
50 Ohm resistor.
But heating a resistor is not really want I do ;-)

I want to send this power into a loop antenna to transmit power by
induction
to another antenna 10 to 20 mm away.
The transmit loop antenna is in series with a capacitor and the receive
antenna is in parallel with a capacitor. Both antennas are tuned to the
correct frequency.

The problem is that though I have 125Vpkpk at the transistor drain, I
only
have 15Vpkpk at my loop antenna and also 15Vpkpk in the receive antenna
(not
loaded)
I guess I need to to some impedance matching but how do I compute the
impedance of those loops antennas (is it just the L//C ?) and how do I
compute the output impedance of the class-E amplifier ?

Any suggestion, URL etc ?

It sounds to me like you are talking about audio (or low) frequencies?

Sorry I forgot to mention the frequency
It's at 8MHz.
The antennas are one turn loops.

I think you are correct in that you need some impedance matching. The
impedance of your antenna is lower than 50 ohms so does not develop as
much
voltage.

The best way to work out the impedance of the loop is to measure it. If
you
are absolutely sure it is at resonance then it will be purely resistive
and
you can measure the impedance of the loop with the equipment you have - if
it is not at resonance there will be phase errors in the following
measurement.

Connect your 50 ohm resister in series with your loop and then connect
this
to the output of the amplifier. Measure the voltage across the output of
the amplifier. If it is reading over 100v pk2pk turn the drive down so it
does not saturate the output transistors.

Next disconnect the voltmeter and connect it across the 50 ohm resistor
and
note the voltage (Vr). Disconnect the voltmeter and connect it across the
loop and note the voltage (Vl). You know one resistance Rr = 50 ohms.
Since the current is the same in both loads and they are in phase you can
use ohms law.

Vr/Rr = Vl/Rl

rearranging you get

R_loop = (E_loop * 50ohms) / E_resistor

it doesn't really matter if you measure RMS or pk2pk

good luck and let us know how you go.

I will try something like this to measure the impedance of the loop+cap.
But my problem for the impedance matching is that I don't know the amplifier
output impedance.
It's a basic class-E amplifier (A self + an IRL510 MOSFET)
Is there a simple way to measure it or to compute it ?

Thanks,

Marc