Ken Smith wrote:

In article ,
Nug wrote:
Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

[.. 1/4 wave and 1/2 wave ...]

An antenna looks like an LC tuned circuit loaded by the radiation
resistance. Your output stage has some impedance that correctly matches
to it (there are exceptions we will ignore) and it is this impedance you
want the antenna system to have. When the correct matching is done, the
antenna works as an impedance mathcing network that matches the output
stages impedance to the radiation resistance.

RF transmitters are not impedance matched to antennae in the sense of maximum
transfer of power. "Maximum transfer of power" is a small signal (ideal linear
parameters) issue, not a large signal issue. That is, the antenna/load are not
conjugately matched. What is said, is that a TX'er will deliver some given
power into, for example, 50 ohms. This says nothing about the output impedance
of the PA.

Power amplifiers are concerned with DC input power to RF output power
efficiency, thus they are load-line "matched," not impedance matched. The
concept of "output impedance" breaks down for large signal devices. For
example, what is the output impedance of a class C or D amp taken when the
transistor is on or off? I suppose one could consider the time-averaged
impedance, but I'm not sure of the utility (to be fair, the time-averaged
reactive output component is tuned out as best possible). The vague output
impedance is a problem even for large signal class A devices. Again, RF PA's
should be load-line matched. Output-Z is irrelevent.