Since returning to active amateur radio after a break of nearly two decades,
I've been a bit saddened to see that many hams seem to be shying away from
solid state amplifiers, and investing them with negative properties that
they don't necessarily have.
I think there are a couple of perceptions that need altering. Largely these
come about from the amateur insistence on running everything on 12V
supplies. While that's fine for mobile kit, it does nothing for linearity or
efficiency! Even moving to 28V makes a huge difference to both parameters:
50V devices are even better!
Another major problem is that hams tend to look at solid state devices as
though they are just another form of vacuum tube. They're not. Solid-state
devices have their own strengths and weaknesses, and from a systems
viewpoint they need to be treated differently to tubes to get the best
performance. Although it's possible to get quite decent power and linearity
from modern MOSFETS (big RF power bipolars are effectively obsolete) in an
open-loop amplifier - my 600W (saturated) FET amplifier for 144MHz ,
operating on 28V, does better than -30dB intermods up to about 530W pep -
the real prize comes when you start to apply linearisation techniques, most
of which are difficult (but not impossible!) to achieve with tubes. We can
use the low cost of complexity which solid-state brings to the game, to
improve the performance of equipment. The guys designing MF AM broadcast
transmitters using arrays of switch mode power amplifiers are commercially
driven. Modern transmitters in this field offer much greater efficiency, and
thus lower running costs, than traditional tube designs. And I'd be prepared
to bet that the modulation linearity is better, too!
I do most of my professional work on VHF-UHF transmitters, and I've recently
seen a demonstration of a linear UHF transmitter with the equivalent
of -60dB third-order IMD rejection, and a final amplifier efficiency well
into the high 70% region. The PA was running well into saturation while
producing this level of performance.....
If, as amateurs, we were to adopt techniques such as cartesian loop feedback
linearisation we could make much better ssb transmitters than are used
currently. -60dB intermods and <60% PA efficiency could be relatively easily
achieved, perhaps even on 12V! This isn't pie in the sky stuff: the
techniques are even employed in uhf digital handhelds (TETRA/APCO etc). The
major Japanese manufactures of ham equipment are well aware of the
techniques, and the additional hardware component costs to add CFBL
linearisation to a modern HF transciever would maybe be of the order of
twenty euros/dollars.
Perhaps a campaign for better transmitters could follow on that which led to
receiever performance improvements twenty years ago? One great advantage of
a properly designed linearised transmitter would be that it would be very
difficult to overdrive.........!
73
Chris
GW4DGU
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