Brian Reay wrote:
On 05/07/2015 21:17, wrote:
Roger Hayter wrote:
wrote:
snip
The impedance of a transmitter output will be nothing like 50 ohms
resistive, as this would result in an efficiency well below 50%, with
all the normal amplfier losses plus the actual RF power produced being
50% dissipated in the PA. This is why matching in the forward direction
coexists with a mjaor mismatch in the reverse direction. This is good
because if there is any reflected wave we don't want it to add yet more
to the PA dissipation. But it does explain what is happening, and why
there are increased losses in the feeder as well as the matching
networks.
The output impedance of an amateur transmitter IS approximately 50 Ohms
as is trivially shown by reading the specifications for the transmitter
which was designed and manufactured to match a 50 Ohm load.
They are designed to drive into a 50 ohm load, that doesn't mean they
have a 50 ohm source impedance. Otherwise efficiency would be rather
'disappointing'.
Nope. If they didn't have a 50 Ohm source impedance, the SWR with
50 Ohm coax and a 50 Ohm antenna would be high. It is not.
The PA stages are designed to operate safely with a load equivalent to a
SWR of (typically) 1.5:1 . Any higher, and it means the load is out of
spec, and the PA leaves its safe area of operation (assuming there is no
mechanism to reduce the power). This is were the myth of RF 'entering'
the PA came from - people thinking that a high SWR meant the reflected
RF was getting into the PA and causing damage. In fact, it 'sees' a
mismatch and therefore can't enter the PA.
You are mixing circuit theory with transmission line theory.
Designed to operate with a low load SWR means the output impedance is
designed to be about 50 Ohms, i.e. commonaly available coax.
--
Jim Pennino