> >I don't know what designs you are familiar with, but combiner losses
don't
> >usually impact significantly on efficiency. That's why they are used in
> >high-efficiency AM BC transmitters...
> >
> **High-efficiency BC-type designs are not suitable for applications where
> linear operation over a range of frequencies is required.
In terms of fractional bandwidth BC transmitters covering 500 - 1650kHz
aren't narrow-band! Of course BC transmitters are designed for AM, and
aren't linear, but there are people using class-D, E and F SSPAs for CW
operation on our bands, as a search of the Web will reveal.
As far as combiners are concerned, it's a matter of choosing the right
topology, of horses for courses.
One of the misconceptions that many people have about hybrid combiners is
that because they contain balancing resistors, which under fault conditions
might have to dissipate a substantial proportion of the power output of the
amp, they are inherently lossy.
That's not true.
Providing the combiners are properly designed, and the phase of the sources
being combined is similar, then they are low-loss devices, and can be
designed to cover several decades of frequency with good performance.
FWIW, I've direct experience of 180degree hybrids used in the FET output
stage of a nominal 10W (actually 20W over most of the band) 0.15 - 1000MHz
instrumentation power amplifier I designed a few years ago. The hybrids
covered from <100kHz to >1.5GHz with less than 0.1dB loss and with phase
balance of a couple of degrees. The major problem I had with that design
was getting the client to pay for suitably broadband blocking capacitors!
Vy 73
Chris
GW4DGU
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