"Toni" wrote
En Richard Clark va escriure en Wed, 12 May 2004 00:32:35 GMT:

These designs also have the
benefit of low pass filters aiding a smooth transition of Z from the
device Z on the order of 0.5 to 1 Ohm to the System Z of 50 Ohms.

There is 0.5 Ohm when the transistor is on but not when is it off. So the
resulting impedance should probably be more than that. ??

This is something I have always thought: When driving loops, very
short monopoles (VLF) or other low-impedance antennas, why raise
transmitter Z to 50 ohms, send it through the line and lower it
again to whatever required? Couldn't the amplifier be built
directly at the antenna base so _no_ transmission line is
required and couple directly the low Z at the final transistor(s)
to the low Z at the antenna?

This is what I have. The amplifier is directly connected to the loop.

Here is a scope screen copy of the output of the amplifier:
http://www.fractalconcept.com/scope-screen.jpg

The C2 trace is the drain voltage and the C3 one is the 50 Ohm resistor
voltage.

The schematics is very simple:
http://www.fractalconcept.com/schema.pdf

The first one is the one that works on the scope screen.
The second one is a one that does not work at all: the torus of the 680nH
melted

I have a little more success by just connecting the drain to the loop
through a capacitor but it's still not good (25% efficiency)

Any idea ?

Marc