On Thu, 13 May 2004 10:58:27 +0200, "Marc Battyani"
wrote:
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 ?

Hi Marc,

There are a number of questions.

First, how do you compute efficiency?

Where on the schematic are these scope connections?

What is your design source for this schematic, or if original, what
drove you to select the reactive components you did? The series load
on the drain does not resonate at 8MHz and is not particularly matched
to either the FET or the 50 Ohm load.

What is the inductance of the loop? Alternatively, what size is it?
This last is more important because it subsumes the radiation
resistance that must be known to perform any efficiency computation.

The two circuits are very, very different from each other to be
achieving the same purpose. The 50 Ohm resistor, as an equivalent
load appears to be grossly in error.

By all appearances of the magenta trace, you are running Class B or a
very long Class C. This is not a hallmark of high efficiency.
However, not knowing where this trace resides in the circuit, this is
simply a guess.

73's
Richard Clark, KB7QHC