Cecil Moore wrote:
The latest QEX has an article by k9la: "The Impact of Load SWR
on the Efficiency of Power Amplifiers". The amplifier is designed
to drive a 50 ohm load and simulated measurements were made for
8 reflection coefficients of 0.333 with phase angles in 45 degree
increments.

One thing I don't understand. When driving a 100 ohm load, the
drain current is 33 amps and the efficiency is 62%. When driving
a 25 ohm load, the drain current is 12 amps and the efficiency
is 83%.

The amp is a Class E/F design from Jan/Feb 2004 QEX. Why would
the drain current fall when the load is decreased from 100 ohms
to 25 ohms?

The output filter of an amplifier can act like a 1/4 wave (or 3/4 wave)
transmission line, so a high impedance at the antenna terminal would
look like a low impedance at the final, and visa-versa. This extends to
the time-delay properties: older oscilloscopes with time delay used long
L/C filters operated way below cutoff to simulate even longer stretches
of transmission lines.

In fact, a filter that looks like this

___
--------o---UUU---o--------
| |
--- ---
--- ---
| |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

with all reactances equal to the design impedance (say 50 ohms) at the
design frequency is _called_ a 1/4-wave filter. Put two of them
together and you get a 1/2 wave filter:

___ ___
--------o---UUU---o---UUU---o------
| | |
--- --- ---
C --- 2C --- C ---
| | |
| | |
=== === ===
GND GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Stick on another section and it'll act like a 3/4 wave line, etc.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com