On Nov 20, 2007, at 12:44 AM, ka9qjg wrote: > Well I set it up for 2 Watts and to My surprise the Heat sink got > Hot , So I took it back and got the 2 Watt unit and it ran cold at > two watts . So I learned My lesson more is not always better , Now > if someone wants to put the Reason in Layman terms that would be > nice too
I'll take a shot at the layman's terms for why that happens in some designs... but I'll start with some explanation.... and of course, without a schematic and a detailed analysis of the specific rig, your mileage may vary... First off, a trip through the ARRL handbook or other sources on the differences between different class Amplifiers is a necessary starting point I won't type up here... but suffice it to say, we're not going for ultra-linearity in amplifiers for FM service, so generally we're using Class-C amplifier designs. http://en.wikipedia.org/wiki/Electronic_amplifier#Amplifier_classes These CAN be up to 90% efficient -- power is converted easily in Class C designs to RF... but... they can only do it within a designed window of power output levels. Class C amps in mobile rigs aren't very efficient at low power amplification because they're designed to put their most efficient part of the power draw vs. power output curve at the maximum power output. Then they get less efficient below that (and above it, in theory, if you follow the curve on past the most efficient point, but you'd blow up the device before you noticed, in most designs). You can see this inefficiency "curve" of the whole radio as a system, if you plot out the DC current draw of the example rig you gave that could go from 2W to 25W here at 2w, 3w... on up to 25w. It won't be a straight line going up as you turn the RF output power up. To oversimplify it -- imagine the radio is an electronic component in a schematic that has a connection to a power source, and then looks like a giant resistor that has both an RF tap coming off of it and a return to the power source. In the simplest terms, that's all it is to the power source... a big lossy resistor that draws a lot of current and converts some of it to RF. -------- So -- for the layman's explanation after that prelude: Your 2w radio was designed for maximum efficiency of it's PA at 2W. Your 25W rig was designed for maximum efficiency at 25W. Both still had to produce 2W of RF when set at 2W, but the 25W radio was FAR more inefficient at it, than the 2W radio was. Thus, by design, it had to run hotter to produce the same output RF power. -------- -- Nate Duehr [EMAIL PROTECTED]
